DISCOVERING AND MODULATING LO NG RANGE INDUCED CHARGE DENSITY GRADIENTS IN ROOM TEMPERATURE IONIC LIQUIDS B y Ke Ma A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Chemistry Doct or of Philosophy 2019 ABSTRACT DISCOVERING AND MODULATING LONG RANGE INDUCED CHARGE DENSITY GRADIENTS IN ROOM TEMPERATURE IONIC LIQUIDS B y Ke Ma Room temperature ionic liquids (RTILs) are salts that are liquid at or below room temperature. The unique pro perties of RTILs such as low melting point, low vapor pressure, non - flammability and large electrochemical potential window have made them very promising in applications ranging from solvents for organic synthesis and electrolytes for energy storage device s to potentially novel electro - optic materials. Despite the broad utility of RTILs, understanding the fundamental interactions betwee n constituents at the molecular scale, and the existen ce of long - range organization in these systems remains limited. The refore, i t is important to characterize the length scale of organization in RTILs because such order will bring with it the de velopment of a variety of novel applications. We used picosecond laser technologies and time resolved spectroscopic approaches t o gain insight into the existence and length scale of molecular organization in RTILs. W e have used time correlated single photon counting (TCSPC) detection with a confocal microscope for spatially resolved excitation and time - resolved emission collection t o measure the rotational diffusion dynamics of three structurally similar chromophores (anionic, cationic and neutral) as a function of di stance from the silica support. The results reflected the existence of a charge density gradient induced in the RT IL by the charge present on the silica surface. Control experiments were also performed for 1) i dentical measurements in ethylene glycol and 2) c apping the silica support with Me 2 SiCl 2 to prove this property is unique to RTILs in contact with charged surface s. Second, w e have used fluorescence anisotropy decay imaging (FADI) with a confocal microscope to measure the rotational diffusion dynamics of cationic chromophore cresyl violet as a function of distance from the conductiv e oxide support, FTO (fluorine d oped tin oxide) or ITO (indium doped tin oxide) . Usin g this experimental configuration, control over the bias and current applied to the support can be achieved , allowing the reorientation dynamics of the charged chromophores to vary with the potential di fference between and the current across the FTO or ITO support. T he effect s of water on the induce d charge density gradient in the RTILs were also explored . Data from those measurements showed that when ca. 2 5,000 ppm or m ore of water is added to the RTI L, the induced charge density gradient persists but with apparently diminished amplitude. The results of this work have demonstrated a novel experimental method to study the local organization in RTILs. These findings represent an initial step for charact erizing and modulating the long range order in RTILs, which will result in using this family of materials most effectively and provid ing a practical framework to better understand ionic liquids. Copyright by KE MA 2019 v To my pa rents, Yuling Du and Dianchun Ma, and my beloved wife, Weijing Liu vi ACKNOWLEDGMENTS Twenty years ago, I remember ed my primary school teacher asked everyone what they want ed to be when they grew up, and my choice was scientist because that title was so shin y for a 6 - year - old kid. Time flies , my happy journey in graduate school has come to an end and the I would sincerely thank everyone who has inspired and supported me during the past five years. First and foremost, I would express my strongest gratitude to my advisor Dr. Gary Blanchard, for his support, optimism, generosity and leading me into the door of scientific research (and beer). He mentored me to be a scientist with critical thinking instead of being a lab technicia n who just follow protocols. He is also a great mentor in scientific writing and presentation. I am never an optimistic person, and there were so many times Gary pulled me back from the edge of giving up when I was stuck in research or frustrated in life . You can hear ter no matter where you are on the third floor in the Chemistry building and his laughter can always cheer you up. I am really grateful to everything I learned from Gary both in research and life. I would like to thank my com mittee members, Dr. Greg Swain , Dr. Marcos Dantus and Dr. Dana Spence, for s erving on my dissertation advisory committee , and their guidance and advice towards my dissertation work. Thanks to Dr. Romana Jarosova and Dr. Greg Swain for their insight and su ggestions, and providing me with countless amounts of fresh ionic liquid sample. Many thanks to everyone in Blanchard group, both former and current members: Dr. Chen Qiu, Dr. Stephen Baumler , Dr. Krystyna Kijewska , Dr. Xiaoran Zhang, Hannah Mize, Barrack vii Stubbs, Briana Capistran , Masroor Hossain, and Yufeng Wang. We have had so many wonderful memories together. I would like to thank all of you for your advice and support in both research and life. In addition, I would like to express my gratitude to the faculty members tha t have helped me on my courses, research, and teaching. Thanks to Dr. A. Daniel Jones for helping me prepare for my job interviews. Thanks to D r. Kathryn Severin for helping me with various analytical instruments in the Anal ytical labs in the Chemistry building. Last but not least, I would like to thank all of my friends that encouraged me and supported me throughout my life journey in graduate school and the happy time together. I really appreciate Sheryl Blanchard and Dr. Gary Blanch ard for being my wedding witnesses and arranging the dinner after that. I want to express my deepest gratitude to my parents Yuling Du and Dianchun Ma for their unconditional love and support, and providing me the opportunity to explore the world. Finall y, I want to thank my dearest wife, Dr. Weijing Liu. You bring happiness and love to my life and I cannot even imagine how the life would be without you. Thank you for making home the warmest and sweetest place for me to go every day after work and I am so grateful to have you in my life . viii TABLE OF CONTENTS LIST OF TABLES ................................ ................................ ................................ .......................... x LIST OF FIGURES ................................ ................................ ................................ ....................... xi CHAPTER 1: BACKGROUND AND MOTIVATION ................................ ................................ . 1 1.1 INTRODUCTION ................................ ................................ ................................ ............ 2 1.2 MOLECULAR SCALE ORGANIZATIONS IN IONIC LIQUIDS ............................... 6 1.3 OBJECTIVES OF THE DISSERTATION ................................ ................................ .... 12 REFERENCES ................................ ................................ ................................ .......................... 15 CHAPTER 2: EXPERIMENTAL TECH NIQUES FOR EXAMING LOCAL ORGANIZATION IN ROOM TEMPERATURE IONIC LIQUIDS ................................ ................................ .......... 19 2.1 INTRUMENTAL METHODS ................................ ................................ ....................... 20 2.2 TIME RESOVLED FLUORESENCE SP ECTROSCPY ................................ .............. 23 REFERENCES ................................ ................................ ................................ .......................... 31 CHAPTER 3: CHARGE - INDUCED LONG - RANGE ORDER IN A ROOM - TEMPERATURE IONIC LIQUID ................................ ................................ ................................ ............................. 35 3.1 ABSTRACT ................................ ................................ ................................ ................... 36 3.2 INTRODUCTION ................................ ................................ ................................ .......... 36 3.3 MATERIALS AND METHODS ................................ ................................ ................... 38 3.4 RESULTS AND DISCUSSION ................................ ................................ .................... 40 3.5 CONCLUSIONS ................................ ................................ ................................ ............ 55 REFERENCES ................................ ................................ ................................ .......................... 56 CHAPTER 4: MODULATION OF AN INDUCED CHARGE DENSITY GRADIENT IN THE ROOM - TEMPERATURE IONIC LIQUID BMIM + BF 4 ................................ ............................ 60 4.1 ABSTRACT ................................ ................................ ................................ ................... 61 4.2 INTRODUCTION ................................ ................................ ................................ .......... 61 4.3 MATERIALS AND METHODS ................................ ................................ ................... 63 4.4 RESULTS AND DISCUSSION ................................ ................................ .................... 66 4.5 CONCLUSION ................................ ................................ ................................ .............. 78 REFERENCES ................................ ................................ ................................ .......................... 80 CHAPTER 5: EFFECTS OF WATER ON THE INDUCED CHARGE DENSITY GRADIENT IN THE ROOM TEMPERAT URE IONIC LIQUID BMIM + BF 4 ................................ .............. 84 5.1 ABSTRACT ................................ ................................ ................................ ................... 85 5.2 INTRODUCTION ................................ ................................ ................................ .......... 86 5.3 MAT ERIALS AND METHODS ................................ ................................ ................... 88 5.4 RESULTS AND DISCUSSION ................................ ................................ .................... 90 5.5 CONCLUSIONS AND FUTURE DIRECTIONS ................................ ......................... 98 REFERENCES ................................ ................................ ................................ ........................ 101 ix CHAPTER 6: SUMMARY AND FUTURE WORK ................................ ................................ . 105 6.1 SUMMARY OF THE DISSERTATION WORK ................................ ....................... 106 6.2 FUTURE WORK ................................ ................................ ................................ ......... 111 REFERENCES ................................ ................................ ................................ ........................ 114 x LIST OF TABLES Table 3.1 Hydrodynamic volumes of system constituen ts. 19 ................................ ....................... 50 Table 4.1 Relationship between current and voltage across the bottom support plate thin film (ITO) and the change in temperature of ethylene glycol in the cell indicated in Fig 4.1b. .......... 75 xi LIST OF FIGURES Figure 1.1 Structures of commonly used RTIL cation (left) and anion (right) species. ................ 4 Figure 1.2 Gouy - Chapman - Stern model for electrical double layer. 19 The graph is adapted from ................................ ................................ ................................ ................................ ......................... 7 Figure 1.3 Two - dimensional simplified solid - state model of 1,3 - dialkyl imidazolium ionic liquids hydrogen bonds network between the imidazolium cation(C + ) and the anions (A ) (one cation is surrounded by three anions and vice - versa). The graph is adapted from Dupont, J., On the solid, liquid and solution structural organization of imidazolium ionic liquids. J Brazil Chem ................................ ................................ ................................ ................................ ......................... 9 Figure 2.1 Schematic o f (a) prolate rotator and (b) oblate rotator. 34 The graph is adapted from 25 Figure 2.2 Schematic of the Fluorescence Anisotropy Decay Depth Profiling Instrument. ........ 27 Figure 2.3 Illustration of a fa st stopwatch with two inputs mechanism of TCSPC detection ..... 30 Figure 3.1 Structures of the RTIL constituents BMIM + and BF 4 (top), resorufin, nile red, and 38 Figure 3.2 (a) Variation of resorufin (anion) anisotropy decay time constant with the distance from a planar silica support. (b) Variation of cresyl violet (cation) anisotropy decay time co nstant with the distance from a planar silica support. For both data sets, the data acquired over ................................ ................................ ................................ ................................ ....................... 42 Figure 3.3 (a) Variation of anisotropy decay time constant with the distance from a silica support for resorufin (anion, red) , cresyl violet (cation, blue), and nile red (neutral, black). (b) Variation of anisotropy decay time constant with the distance from a silica support treated with Me 2 SiCl 2 ................................ ................................ ................................ ................................ ....................... 44 Figure 3.4 Depth dependence of the anisotropy deca y time constant for resorufin (anion, red), cresyl violet (cation, blue), and nile red (neutral, black) in ethylene glycol. Data points at zero depth are displaced somewhat as a result of the interaction of the chromophores with the silica 46 Figure 3.5 (a) Variation of fluorescence lifetime with the distance from a silica support for resorufin (anion, red), cresyl violet (cation, blue), and nile red (neutral, black). (b) Variation of fluorescence lifetime with the distance from a silica support treated with Me 2 SiCl 2 for resorufin ................................ ................................ ................................ ................................ ....................... 54 Figure 4.1 (a) Schematic of cell holding RTILs configured as a capacitive device. The conducting coating is either ITO or FTO. For this cell, d = 1 mm. (b) Schematic of c ell holding RTILs with the lower support configured as a resistive device. The conducting coating is either ................................ ................................ ................................ ................................ ....................... 64 Figure 4.2 Depth dependent reorientation time constant, OR , of CV + as a function of distance from the lower support ing FTO - coated plate. Data were acquired with the cell configured as shown in Fig 4.1a, capacitive mode. Voltages applied (V bottom V top ) are (a) from 0 mV to - 1000 ................................ ................................ ................................ ................................ ....................... 69 xii Figure 4.3 Depth dependent reorientation time constant, OR , of CV + as a function of distance from the lower supporting plate. Data were acquired with the cell configured as shown in Fig 4.1b, resistive mode. Currents applied (a) ranged from 0 mA to 200 mA using FTO as the conducting film on the support plate and (b) ranged from 0 mA to 420 mA using ITO as the .... 73 Figure 4.4 (a) Dependence of CV + reorientation time constant, OR , in ethylene glycol on current applied to the lower support plate conducting film. ITO was used as the t hin film conductor for these measurements. Currents applied are as indicated in the legend and as shown in Table 4.1. (b) Dependence of CV + reorientation time constant, OR , in ethylene glycol as a function of ..... 77 Figure 4.5 T emperature of the cell as a function of power applied. Red box indicates the power region and consequent maximum temperature change over which the sign of the free charge ... 78 Figure 5.1 Structures of BMIM + BF 4 and Cresyl violet (cation). ................................ ................ 88 Figure 5.2 Schematic of the self - designed cell holding BMIM + BF 4 with the lower support configured as a resistive device. The conducting coating is either ITO or FTO on a silica support. ................................ ................................ ................................ ................................ ....................... 89 Figure 5.3 Depth - OR , of CV + as a function of distance from the lower supporting FTO electrode. Data were acquired with the cell configured as shown in Fig 5.2. Currents applied ranged from 0 mA to 200 mA usin g FTO as the conducting film on ................................ ................................ ................................ ................................ ....................... 93 1 CHAPTER 1: BACKGROUND AND MOTIVATION 2 1.1 INTRODUCTION efficient use of renewable energy. To cater to the needs of a new generation of electric vehicles and large scale storage of electrical energy, technologies must be developed that can deliver increased energy density (the amount of energy stored in a given mass) and power density (controlled by the speed of energy release) with better safety, lower cost and without adverse effects on the environment . Electrochemical supercapacitors (E S s) are characterized by high power density and long cycle life (exceeding 1 million cycles 1 ). Such devices are already in use as an integral part of electric vehi cles and back - up power supplies. 2 Interfaces and surfaces are the regions where important events happen: energy storage, catalysis, molecular recognition, charge transfer, polymerization, and many other critical processes take place at the bound ary between one medium and another. 3 Central to the function of an interface is the structure and dynamics occurring in the interfacial region. The chemical composition of the interface region and the molecular arrangement at the interface can be very different from that in the bulk medium. Charge storage in E S s occurs through the formation of an electric double layer (EDL) at the electrode electrolyte interfaces. This device stores energy physically rather than electrochemically, and the ch emical reactions that operate in rechargeable batteries during charging and discharging are not relevant. Compared to rechargeable batteries, the electric double layer capacitor (EDLC) has a remarkably long cy cle - life and high power density. 4 However, due to the narrow electrochemical window of water (ca. 1.23 V), the energy density of EDLCs in aqueous electrolytes is generally low (5 10 Wh kg - 1 ). 5 Therefore, in recent years, 3 developing non - aqueous EDLC s has become a matter of great interest for their comparatively wide potential wi ndow and higher power density. For these reasons, r oom temperature ionic liquids (RTILs) have been identified as a useful family of materials. RTILs are a class of compounds that can be described as salts that me lt at room temperature or lower. 6 Most salts adopt a crystalline lattice structure wher e the ionic constituents reside at specific positions in a three - dimensional assembly. The stabilization energy of the resulting cry stalline structures is well in excess of k B T at ro om temperature. Typical melting points of salt crystals can b e multiple hundreds of degrees. Such a structural motif is possi ble because the charged species are comparatively compact and the ionic charge is not shielded ext ensively . For systems characterized by larger and/or irregularly shaped ionic sp ecies, the electrostatic forces between the charged species s till operate but their physical extent prevents close, ordere d packing, resulting in a fluid latti ce structure, wh ich is termed room temperature ionic liquid (RTIL). The structures of so me of the more widely - used RTIL cations, typically organic, and a nions, typically inorganic, are shown in Fig 1.1 . 4 Figure 1 . 1 Structu res of commonly used RTIL cation ( left) and anion (right) species . Because of their unique properties, RTILs hold much promise for applications in areas ranging from electrolytes in supercapacitors 7 - 8 , novel solvents for organic synthesis 9 , green solvents for separations 10 , and pot entially as novel electro - optic materials . Despite the wide use of these materials, much remains to be understood about their properties and the underlying chemical and physical principles that account for them. There are severa l characteristics of RT ILs that are important for determining their properties: (1) l ow melting point so they exist as liquids at ambient temperature and have wide usable tempe rature range, (2) e xtremely low volatility as RT ILs are thermally stable , do not have a significant vapor pressure 11 and are typically nonflammable, (3) RT ILs have high ionic conductivity, and (4) o rganic ions allowing 5 for a broad range of struc tures and combinations. 2 RTILs having a wide electrochemical potential window (ca. 3.0 5.0 V) are presently being c onsidered as promising electrolytes for developing high energy density supercapacitors. 2 The time scale of the format ion and relaxation of electric double layers is ca. 10 - 8 s, giving E S a high power density. 12 RT ILs are used as green electrolytes for those electrochemical supercapacitors because of their low volatility and high ionic conductivity, which are importan t properties for electrolyte solutions to exhibit. It is clear from the above discussion that a key issue in t he use of RTILs in energy storage applications is the nature and spatial extent of their organ ization at electrode surfaces. Macroscopic properti es of interfaces, such as surface tension at liquid - solid interfaces, have been studied widely, however, the microscopic properties of interfaces have bee n more difficult to ascertain. 13 - 15 Interrogating such orga nization poses an experimental challenge s ince the relevant length scale of such organization is presently not known . For solution phase electrolytes it is known that the electric double layer has a very limited spatial extent, but it is not clear whether this situation is relevant for ionic liquids based on their high charge density. One way to evaluate such organization is through the dynamics of specific probe molecules incorporated into the RT IL, and the determination of how such organization changes wi th distance from the charged support on a macroscopic ( ) level. A detailed molecular understanding of RT IL interfaces and any transition from interfacial to bulk organization and dynamics is critical to understanding how RT ILs behave macroscopically. 16 Fluorescence spectroscopy is sensitive to microenvironments in liquid solutions and, therefore, is particularly useful for the study of solvation phenomena. Rotational diffusion measurements performed with time - resolved fluorescence spectroscopic techniques provide a means of measuring the motional properties of a probe molecule and elucidating information on 6 local organizat ion in the medium. Through the use of optical spectroscopic techniques, a molecular scale understanding of RT IL - chromophore and RT IL constituent interactions is sought to increase our knowledge of RT ILs as a means of achieving greater control over chemica l processes like energy storage. 1.2 MOLECULAR SCALE ORGANIZATIONS IN IONIC LIQUIDS Room - temperature ionic liquids ( RTILs) hold promise as solvent - free electrolytes for supercapacitors, solar cells and batteries. For such applications, it is crucial to unders tand the structure of the RTIL - electrode interfacial region. The classical Gouy - Chapman - Stern model for dilute electrolytes has been used to interpret RTIL capacitance data. 17 The Gouy Chapman Stern model predicts that as the electrode becomes more highly charged, the diffuse layer in aqueous solutions becomes more compact and its differential capacitance grows. In this theory, ions outside the compact la yer are treated as point charges that occupy no volume. A consequence of this assumption is that the model is accurate only for dilute solutions and at potentials close to the potential of zero charge (PZC) . 18 7 Figure 1 . 2 Gouy - Chapman - Stern model for electrical double layer . 19 The graph is adapted from Ref. 19. However, the ions that comprise ionic liquids are often large, non - rigid , highly polarizable and chemically complex . Moreover, the relatively low melting point of ionic liquids also means a number of interionic forces , in addition to Coulombic forces , may act to affect the structure of these liquids. These include dispersion forces, dipole dipole interactions and hy drogen 8 bonding. 18 The Gouy Chapman Stern model is still not comprehensive enough for explaining the EDLs at the interfacial region for RT ILs. Experimental and theoretical investigations of RTILs have revealed four length scales over which organization of some type has been observed. These are hydro gen bonded n etwork organization - defined, however, and the relationship between the phenomena with different characteristic length scales remains to be understood fully. Hydrogen Bonded Network O rganization . Dr. Jairton Dupont developed the co ncept in his review that pure 1, 3 - dialkylimidazolium ionic liquids can be described as polymeric hydrogen - bonded supramolecules. 20 H ighly - ordered , hydrogen - bonded n etworks in the form of ( [(DAI) x (X) x - n )] n+ [(DAI) x - n (X) x )] n - ) n can be formed , where DAI is the 1,3 - dialkylimidazolium cation and X the anion. This structural motif is a general trend for the s olid phase and is maintained in the liquid phase to a significant extent and even in the gas phase. 20 X - ray studies on the structure of 1,3 - dialkylimidazo l ium salts was reported by the Cambridge Crystallographic Da ta Center . 21 One imidazolium cation surrounded by at least three anions and each anion is surrounded by at least three imidazolium cations in turn comprises the monomeric unit of t he salts. There is a trend of such structures forming in the solid state , an extended network of cations and anions connec ted together by hydrogen bonds. T he structural trend of one imidazolium cation hydrogen bonded to at least three anions and one anio n hydrogen bonded to at least three cations is a general trend in imidazolium salts. However, the number of anions that surround the cation (and vice - versa) can change depending upon the anion size and type of the N - alkyl imidazolium substituents. 9 Figure 1 . 3 Two - dimensional simplified solid - state model of 1,3 - dialkyl imidazolium ionic liquids hydrogen bonds network between the imidazolium cation(C + ) and the anions (A ) (one cation is surrounded by thr ee anio ns and vice - versa). The graph is adapted from Dupont, J., On the solid, liquid and solution structural organization of imidazolium ionic liquids. J Brazil Chem Soc 2004 , 15 (3), 341 - 350. (Ref. 20 ) . By neutron diffraction analysis of 1,3 - dimethylimidazoliu mchloride (DMI M + Cl ) and its hexafluorophosphate analogue (DMI M + PF 6 ) in both the solid and liquid phase, the Soper Grou p also provided strong evidence that the hydrogen - bonded network al so existed in the liquid phase. 22 Their model derived from the obtained data used Empirical Potential Structure Refinement indicated that significant charge ordering is present in the liquid phase and that the local order in these liquids resembles those found in the solid phase. 10 Moreover, the Hamaguchi group performed Raman spectroscopic analysis of both crystalline and liquid state of 1 - Butyl - 3 - methylimidazolium chloride ( BMI M + Cl ), 23 - 24 with similar results , indicating that three dimensional structure found in the solid state 25 is maintained in the liquid phase. 10 nm - Scale Organization . In an effort to understand the role of electrostatic screeni ng in RTILs, the Israelachvili group has produced an elegant body of work where temperature - dependent surface force measurements 26 were used to infer information on the force exerted on a charged surface by a RTIL. 27 - 28 This means of measuring forces of interaction has revealed the spatial extent of interaction between a RTIL layer and a charged (mica) plate. The result is that the characteristic effective ) - 1 , is on the order of 6 nm for 1 - E thyl - 3 - methylimidazolium bis(trifluoromethylsulfonyl)imide ( EMIM + NTf 2 ) and 1 - Propyl - 3 - methylimidazolium bis(trifluoromethylsulfonyl)imide ( PMIM + NTf 2 ) , corresponding to an ion number density of n* ~ 10 15 - 10 16 cm - 3 . Th e physical origin of order over this length scale derives from the formation of organization analogous to the electric double layer in the RTIL due to its contact with the charged mica surface. Within th e framework of the GCS model or some variant , 29 organization on t he order of 10 nm requires a very low free ion density. Increased dissociation results in the higher ion density and thus the greater extent of charge screening. Limited dissociation leads to longer length scale effects due to the field(s) associated wit h quasi - point charges. The picture that emerges from this work is that the dominant form of the RTIL is Bjerrum pairs with a relatively small fraction of free ions. 30 The Israelachvili group reported no longer ra nge organization. 1 - Scale Organization . The Shaw group has identified orientational order in RTIL films over distances 31 - 33 Using thin RTIL films, they measure the evolution of order 11 through changes i n the vibrational spectra of the RTILs and, using second harmonic generation measurements, they make a compelling case for the evolution of (non - centrosymmetric) order developing throughout the film over a timescale of tens of minutes at room temperature. They also note that the extent of order depends on the identity of the anionic component of the RTIL. Taken collectively, these data imply that the dominant species present in RTILs is the dipolar contact ion pair. If the RTIL constituents existed as dis crete ions, the resulting structure would be, necessarily, centrosymmetric in the bulk and therefore no evolution of second harmonic signal would be observed. We note that this finding is in conceptual agreement with the report of the Israelachvili group, albeit with a differen ce in length scale, which could be a consequence of the different techniques used by the two groups. It is also important to note that this work provides direct experimental evidence for second order nonlinear optical effects being operative in RTILs, a property we will exploit ( vide infra ). While the Shaw group has provided a detailed interpretation of the vibrational spectra, which gives some insight into the structure of the organized media, a firm understanding of either the micr oscopic organization in these systems or the driving forces for the evolution of said organization is still lacking . The time - scale of organizational evolution in these RTIL films strongly suggests an annealing process to form an organized arrangement of dipoles where the k B T . In contrast to the order dipoles in such a way as to produce a non - centrosymmetric structure. One way to reconcile the while the Israelachvili group senses the free ionic species. There is no reason a priori why these two di fferent entities should be correlated in the characteristic length scale of their organization. 12 1 00 - Scale Organization. We have identified and characterized organization in RTILs upport. 34 We also demonstrate the ab ility to control the induced organization . 35 F luoresc ence anisotropy decay of the charged chromophores is measured using time correlated single photon counting system in the RTIL as a function of distance from the charged support. 36 D epth resolution is achieved with an inverted confocal microscope through mechanical control over the microscope stage position. The chromophores exhibit depth - dependent reorientation dynamics that vary with their ionic charge. The occurrence of spatially varying chromophore dynamics re quires the RTIL to be in contact with the (charged) silica support. W e have c onstructed a closed cell that confines the RTIL between two transparent conductive surf aces (indium doped tin oxide (ITO) or fluorine doped tin oxide (FTO)) i n order to gain contr ol over the surface charge density . The influences of water dilution of the RTIL on the long range induced charge gradient density is also studied. The details of our work will be described in the following chapters. 1.3 OBJECTIVES OF THE DISSERTATION The f undamental interactions between RTIL constituents, and the existence of long - range organization in RTILs remains to be explored fully. One fundamental obstacle to understanding RTILs rests on the unresolved issue of the extent to which ionic liquids exist as ion - paired dipolar species and as dissociated ions, and the co nsequences of this equilibrium. Most attempts to model RTIL properties start from the premise that they can be treated as a fluid dielectric medium. Such a starting point cannot be fully a ccurate because of dissociation in RTILs and the dissociated ions function as carriers. RTILs, however, cannot be treated effectively as conductors. The essential questions of free ion density, the dielectric response of the ion - pairs, and the consequent screening effect(s) in RTIL media are not well understood. 13 The work contained in this dissertation has been focused on discovering and modulating the induced long - range charge density gradient in room temperature ionic liquid BMIM + BF 4 , and the possible theory and explanations behind the charge density gradient. Time Correlated Single Photon Counting (TCSPC) Microscopy and Fluorescence Anisotropy Decay Imaging (FADI) were employed as measurement techniques to examine the rotational d iffusion dynamics of probe molecules in RTIL|electrode systems, and provide a detailed description of the measurement science in Chapter 2. In Chapter 3, we report on the rotational diffusion behavior of three probe molecules cresyl violet (cationic), res orufin (anionic) and nile red (neutral) separately in the RTIL|silica system using FADI. O pposite trends of reorientation time c onstants OR ) vs. distance from glass substrate surface for cresyl violet and resorufin were found , and this gradient persists for distances up to 100 µm from the silica support. It was also found that for OR is invariant with the distance from the support surface. Two sets of control experiments were done separately to further help us understand the reasons for our observations. In order to modulate th e induced charge density gradient in the RTIL , BMIM + BF 4 , a sealed electr ochemical cell wa s designed using indium doped tin oxide (ITO) or f luorine doped t in o xide (FTO) as the working electrode . T he RTIL i s confined between the two electrodes in a sandwich structure. Two different methods were used to change the surface charge density o n the electrode in an effort to control the induced , counter balancing charge density gradient within the RTIL. T he detail ed result s of the experiments are describe d in Chapter 4. In Chapter 5, the effects of water concentration in the RTIL sample on the induce d charge density gradient using FADI and Karl - Fisher titration are reported . The use of optical techniques to examine the physical properties o f the RTIL|electrode system provides an opportunity to study the complex 14 dynamic properties of RTILs in the inte rfacial region in ways that have not been reported previously. On physical grounds, ionic interactions and the extent of the field(s) associated with them cannot account for the long - range order seen in RTILs. 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R.; Yethiraj, A., First - Principles, Physically Motivated Force Field for the Ionic Liquid [BMIM][BF4]. The Journal of Physical Chemistry Letters 2014, 5 (15), 2670 - 2674. 17 14. Kislenko, S. A.; Samoylov, I. S.; Amirov, R. H., Molecular dynamics simulatio n of the electrochemical interface between a graphite surface and the ionic liquid [BMIM][PF6]. Physical Chemistry Chemical Physics 2009, 11 (27), 5584 - 5590. 15. Son, C. Y.; McDaniel, J. G.; Schmidt, J. R.; Cui, Q.; Yethiraj, A., First - Principles United At om Force Field for the Ionic Liquid BMIM+BF4 : An Alternative to Charge Scaling. The Journal of Physical Chemistry B 2016, 120 (14), 3560 - 3568. 16. Steinrück, H. - P., Recent developments in the study of ionic liquid interfaces using X - ray photoelectron spec troscopy and potential future directions. Physical Chemistry Chemical Physics 2012, 14 (15), 5010 - 5029. 17. Bazant, M. Z.; Storey, B. D.; Kornyshev, A. A., Double Layer in Ionic Liquids: Overscreening versus Crowding. Physical Review Letters 2011, 106 (4), 046102. 18. Lockett, V.; Horne, M.; Sedev, R.; Rodopoulos, T.; Ralston, J., Differential capacitance of the double layer at the electrode/ionic liquids interface. Physical Chemistry Chemical Physics 2010, 12 (39), 12499 - 12512. 19. Electric Double Layer. https://web.nmsu.edu/~snsm/classes/chem435/Lab14/double_layer.html (accessed 06/17/2019). 20. Dupont, J., On the solid, liquid and solution structural organization of imidazo lium ionic liquids. J Brazil Chem Soc 2004, 15 (3), 341 - 350. 21. Siegel, W. R., Nanophase Materials, Encyclopedia of Applied Physics . Weinheim: 1994; Vol. 11. 22. Hardacre, C.; Holbrey, J. D.; McMath, S. E. J.; Bowron, D. T.; Soper, A. K., Structure of mol ten 1,3 - dimethylimidazolium chloride using neutron diffraction. J Chem Phys 2003, 118 (1), 273 - 278. 23. Ozawa, R.; Hayashi, S.; Saha, S.; Kobayashi, A.; Hamaguchi, H., Rotational isomerism and structure of the 1 - butyl - 3 - methylimidazolium cation in the ioni c liquid state. Chem Lett 2003, 32 (10), 948 - 949. 24. Hayashi, S.; Ozawa, R.; Hamaguchi, H., Raman spectra, crystal polymorphism, and structure of a prototype ionic - liquid [bmim]Cl. Chem Lett 2003, 32 (6), 498 - 499. 25. Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H., Crystal structure of 1 - butyl - 3 - methylimidazolium chloride. A clue to the elucidation of the ionic liquid structure. Chem Lett 2003, 32 (8), 740 - 741. 26. Israelachvili, J.; Min, Y.; Akbulut, M.; Alig, A.; Carver, G.; Greene, W.; Kristiansen, K.; Meyer, E.; Pesika, N.; Rosenberg, K.; Zeng, H., Recent advances in the surface forces apparatus (SFA) technique. Rep Prog Phys 2010, 73 (3). 27. Gebbie, M. A.; Dobbs, H. A.; Valtiner, M.; Israelachvili, J. N., Long - range electrostatic screening in ion ic liquids. P Natl Acad Sci USA 2015, 112 (24), 7432 - 7437. 18 28. Gebbie, M. A.; Smith, A. M.; Dobbs, H. A.; Lee, A. A.; Warr, G. G.; Banquy, X.; Valtiner, M.; Rutland, M. W.; Israelachvili, J. N.; Perkin, S.; Atkin, R., Long range electrostatic forces in ion ic liquids. Chem Commun 2017, 53 (7), 1214 - 1224. 29. Oldham, K. B., A Gouy Chapman Stern model of the double layer at a (metal)/(ionic liquid) interface. Journal of Electroanalytical Chemistry 2008, 613 (2), 131 - 138. 30. Gebbie, M. A.; Valtiner, M.; Banquy , X.; Fox, E. T.; Henderson, W. A.; Israelachvili, J. N., Ionic liquids behave as dilute electrolyte solutions. P Natl Acad Sci USA 2013, 110 (24), 9674 - 9679. 31. Anaredy, R. S.; Shaw, S. K., Long - Range Ordering of Ionic Liquid Fluid Films. Langmuir 2016, 32 (20), 5147 - 5154. 32. Anaredy, R. S.; Shaw, S. K., Developing Distinct Chemical Environments in Ionic Liquid Films. J Phys Chem C 2018, 122 (34), 19731 - 19737. 33. Lucio, A. J.; Shaw, S. K.; Zhang, J.; Bond, A. 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Langmuir 2014, 30 (33), 9951 - 9961. 19 CHAPTER 2: EXPERIMENTAL TECHNIQUES FOR EXAMING LOCAL ORGANIZATION IN ROOM TEMPERATURE IONIC LIQUIDS 20 2.1 INTRUMENTAL METHODS Due to the growing use of RTILs, and the recognition of the need for a deeper understanding of their prope rties, there is a rich literature on the bulk and interfacial structure of RTILs, with both experimental methods and computer simulations being reported. Many applications of RTILs involve physical or chemical processes at interfaces, such as lubrication 1 - 2 , electrical energy storage devices 3 - 4 , and catalysis . 5 - 6 The interfacial structure of the RTILs|Support system influences or m ediates many of these processes, and detailed knowledge of local organization in this region will afford new opportunities. The structure and organization of these systems are complicated because of the potentially high ionic strength of the medium and th e unknown extent of dissociation. A major challenge is the need for in situ characterization of molecular and longer - scale organizatio n in the proximity of the RTIL|S upport interface. Gaining such information is hampered by the limited number of techniqu es available, the possible role of impurities in the ionic liquids, , and even a lack of consensus on what an ionic liquid is and what experimental probes that are able to interrogate this condensed - phase sy stem. 7 While not an exhaustive list, the utility and limitations of the most common and widely used methods for study of bulk and interfacial structure of RTILs will be discussed briefly below. Small - Angle Scattering (SAS). Small - angle X - ray scattering (SAXS) and Small - angle neutron scattering (SANS) are jointly referred to as SAS. SAXS is a non - destructive method that can reveal spatial correlations in RTILs on the order of tens of Angstroms. 8 This method is based on analyzing th e elastic scattering behavior of X - ra ys interacting with the sample by recording their scattering at small angles. Triolo et al. 8 - 10 reported experimental evidence for the existence of nanoscale segregation of th e alkyl chains of 1 - alkyl - 3 methylimidazolium 21 hexafluorophosphate (C n MIM + PF 6 , SAXS . The size of these structural heterogeneities was found to depend linearly on the alkyl chain length. However, when the alkyl ch ain was too short (n < 4), it was difficult to identify the analogous structural heterogeneities using SAXS because they appear as a small shoulder on a much more intense X - ray diffraction amorphous halo. 9 SANS is an experimental technique that uses elastic neutron scattering at small scattering angles to inves tigate the structure of various substances on a mesoscopic scale of 1 100 nm. Atkin et al. 11 demonstrated the existence of amphiphilic nanostructure for RTILs ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) using SANS. Their data appears to be the first experimental evidence of nanoscale heterogeneity in RTILs with alkyl chains less than C 4 . However, for SANS there remain some disadvantages, such as neutron sources that are very expensive to build and maintain. Fluorescence Correlation Spectroscopy (FCS). FCS is a correlation analysis of fluctuations of fluorescence intensity. It can monitor the motion of a small number o f chromophores by measurement of spontaneous fluorescence fluctuations caused by Brownian motion of the chromophores. The observation volume can be extremely small (femtoliters) and it is determined by the focus of a confocal microscope. 12 - 15 The technique is exquisitely sensitive, offering detection down to the single - molecule level. 15 Guo et al. 15 reported that heterogeneous liquid structures exist in N - alkyl - N - methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (C n MPy + Tf 2 N , n = 3,4,6,8 and 10). Rhodamine 6G (R6G) was used as the fluorescent probe molecule and FCS results revealed biphasic diffusion dynamics for all of the aliphatic chain lengths studied. The fast and slow diffusion rates for R6G are due to the diffusion wit hin non - aggregated regions and self - aggregated domains, 22 respectively. Despite the utility and sensitivity of this technique, there remain some limitations, including the appropriate treatment of experimental data under different conditions and the lack of models for data interpretation. 16 - 20 Second Harmonic Generation (SHG). SHG is a nonlinear spectroscopic technique in which two photons with the same frequency interact with a nonlinear material (in this case, RTI Ls), are "combined", and generate a new photon with twice the energy of the initial photons. The sum frequency signal intensity is related to the first hyperpolarizability and order of the material. The signal is zero for centrosymmetric systems but nonz ero at interfaces or in materials that do not possess a center of inversion. Thus SHG can be used to detect the presence of interfaces or ordered structures in bulk non - centrosymmetric materials. 21 - 22 Shaw et al. 21, 23 reported the transformation of ionic liquid films from isotropic bulk to a fluid - ordered state over micrometer length scales through the application of a shearing force, and they sensed the order induced in the RTIL with SHG measurements. One advantage of SHG measurements is that the generation of second harmonic light does not require the use of a chromophore introduced into the RTIL. Rather, SHG senses organization in RTILs directly. More consideration o f SHG measurements will be discussed in Chapters 5 and 6 of this dissertation. Computer Simulations. The two main families of simulation that have been applied to RTILs are molecular dynamics (MD) and Monte Carlo (MC) simulations. 24 MD is a method for studying the physical movements of atoms and molecules. This method is now routinely used to investigate the structure, dynamics and thermodynamics of a variety of systems. For different imidazolium - based RTILs, a first sharp diffraction peak (FSDP) at low frequency in the X - ray and neutron scatte ring spectra can be observed. 25 FSDP has often been experimentally interpreted as indicative of mesoscopic organization of those RTILs. 8, 10, 26 - 27 Annapureddy et 23 al. 25 combined detailed MD simulations with evidence from published experimental data to clarify the geometrical or igin of the FSDP in 1 - alkyl - 3 - methylimidazolium (C n MIM + , n=6, 8 and 10) based RTILs with spherical or pseudospherical anions. While the existence of complex morphologies is neither proved nor disproved by their study, they concluded that the geometric ani sotropy in the cation is a necessary but not a sufficient condition for the nanoscale segregation of the cation alkyl chains. T he FSDP can be explained by much simpler consideration of solvation shell asymmetry of the cations. MC techniques can be used to compute the equilibrium properties of classical many - body systems. 28 They are useful for simulating systems with many coupled degrees of freedom, such as fluids, disordered materials, strongly coupled solids, and cellular structures. Shah et al. 29 carried out MC simulations on the RTIL 1 - Butyl - 3 - methylimidazolium hexafluorophosphate (BMIM + PF 6 ), and reported that PF 6 anions preferentially cluster in two favorable regions near the cation. The highest probability location is in proximity to the cation C 2 carbon atom, both below and above the plane of the imidazole ring. These findings suggest but do not prove the existence of ion - paired species in RTILs. MC and MD simulations act as a bridge between microscopic length and time scales and the macroscopic world of the laboratory. They also act as a bridge in another sense: between theory and ex periment. 24 2.2 TIME RESOVLED FLUORESENCE SPEC TROSCPY Time - resolved fluorescence spectroscopy is a powerful method for investigations ranging from condensed matter physics and chemistry to the life sciences. 30 The measurement of time resolved anisotropy decay allows the study of molecular scale reorientation of chromophores in a 24 variety of matrices. In this section, information on fluorescence microscopy techniques used to examine the rotational diffusion behavior of a chromophore in a RTIL is provided. A brief explanation of the theory behind the rotational diffusion measurement techniques is described. The associated instrumenta l design is described in detail. Fluorescence Anisotropy Decay Measurements. Fluorescence anisotropy is the phenomenon where the light emitted by a fluorophore has unequal intensities ( I ) along different axes of polarization. When the polarization of an incident (resonant) electric field is parallel to the transition dipole moment of a chromophore, the molecule will absorb the photon and be excited to a higher energy state. The molecule will relax rapidly and non - radiatively to the v=0 S 1 state. Subsequ ent radiative relaxation from this state will proceed according to first order kinetics, with a characteristic time constant that is the reciprocal of the rate constant for radiative relaxation . This time constant is referred to as the fluorescence lifeti me. The emitted photon will have a specific polarization with respect to the molecule. By measuring the time - resolved intensity of fluorescence both parallel I || (t) and perpendicular I (t) to the initial excitation polarization , the fluorescence lifetim e (eqn. 2.1) and anisotropy decay function (eqn. 2.2) of the chromophore can be formulated. 31 [2.1] [2.2] The decay of R(t) is due to the rotational diffusion of the fluorescent probe molecule in the Chromophore|RTIL system. Although the R(t) function can contain up to five exponential components, for most systems only one or two component anisot ropy decays are observed . 32 - 33 Chuang and Eisenthal 31 reported that the functional form of R(t) depends on the shape of the 25 volume swept out by the reorienting chromophore molecule. A series of equations were derived to relate the decay of R(t) to the Cartesian components of the rotational diffusion constant. The shape of the volume swept o ut by the rotating chromophore molecule is modeled as an ellipsoid. Chuang and Eisenthal related the Cartesian components of the rotational diffusion constant to the rotor shape as follows. 31 - syst em of the planar chromophore is assigned as the xy plane, with the x - axis being the long in - plane axis and the y - axis being the short in - plane axis, the z - axis is normal to the xy p lane. For a prolate rotor, D x > D y = D z, and for a oblate rotor, D z > D x = D y. Figure 2 . 1 Schematic of (a) prolate rotator and (b) oblate rotator . 34 The graph is adapted from Ref. 34. For a p rolate rotator with the excited and emitting transition dipole moments parallel to the dominant (x) axis of rotation, R(t) decays as a single exponential (eqn. 2.3). For an oblate rotator with the transition dipole moments parallel and in the x - y plane, R (t) exhibits a two - component exponential decay (eqn. 2.4). R(0) is related to the angle between the excited and emitted transition dipole moments of the chromophore molecule. [2.3] 26 [2.4] The determination of rotational diffusion constant (D) is based on measuring its Cartesian components (eqn. 2.5). The information we extract from the function of R(t) is the angle between the excited and emitting transition dipole moments (R(0)) and the aniso tropy decay time constant(s) ( OR ). We focus on the decay time constant(s), which are inversely related to the Cartesian components of D. For a prolate rotor, where the only Cartesian component of D that is available is D z , OR = (6D z ) - 1 . For oblate systems, the decay time constan ts are related to the D z and D x components of D. Molecular motion is described by Debye - Stokes - Einstein (DSE) equation 35 - 37 (eqn. 2.6). [2.5] [2.6] olution, k B T is the thermal energy of the system, V is the hydrodynamic volume of the rotating entity, f is a boundary condition term used to describe the frictional contributions of the chromophore - solvent interactions, which can range from near zero in t he slip limit to one in the stick limit, 38 - 39 and S is a shape factor that accounts for the non - spherical shape of the rotating entity. 40 For the chromophores discussed in the following chapters, all of them can be modeled as a prolate rotator and R(t) can be fitted using a single exponential decay function. 32 - 33 Fluorescence Anisotropy Decay Depth Profiling Instrument. The instrument used to acquire fluorescence anisotropy decay data of the ch romophores in the RTIL BMIM + BF 4 is based on the combination of a time correlated single photon counting (TCSPC) laser system 27 coupled to an inverted confocal laser scanning microscope (CLSM) (Nikon Eclipse Ti - U), shown in Fig. 2.2. Figure 2 . 2 Schematic of the Fluorescence Anisotropy Decay Depth Profilin g Instrument. The light source for this instrument is a synchronously pumped cavity dumped dye laser (Coherent 701 - 3) excited by the output of a passively mo de locked Nd:YVO 4 laser (Spectra Physics Vanguard). The pump laser produces 13 ps pulses at 80 MHz repetition rate at both 355 and 532 nm, with 2.5 W average power at each wavelength. The repetition rate of the dye laser is controlled by a cavity dumper. The output of the dye laser is characterized by ca. 5 ps pulses at a repetition rate typically of 4 MHz (it can range from 80 kHz to 80 MHz), and the average power at the sample is less than 0.5 mW. By changing the dye and optics used and the excitation wavelength, the output of the dye laser can be tuned from 430 nm to 850 nm. For the work 28 presented in this dissertation, the dye laser output is set to be 563 nm. The wavelength was selected based on the excitation spectra of the chromophores used in th is work and the bandpass filters used in the confocal scanning system. The pulsed excitation light is passed through a polarizer selected to an angle of 0 degrees (vertical polarization), and sent to the confocal scanning head for delivery to the sample. Collected emission light is sent to a set of long - wavelength pass and bandpass filters, separated into vertical and horizontal polarization components, and then sent to avalanche photodiode detectors. The confocal scanning device, connected to an invert ed optical microscope, provides the requisite high focal depth resolution required to characterize the local organization of the RTIL BMIM + BF 4 that is reported in this dissertation. In this configuration, which was designed initially for confocal imaging , the laser is focused on the sample at a selected position, and polarized emission transients are acquired at each laser position. Images are acquired pixel - by - pixel by stepping the laser spot position in the focal plane. For the work reported here, the sample is homogeneous in the x - y plane for any given depth in the sample, and the acquisition of multiple pixels in a given focal plane serves as an efficient means of signal averaging. Detection of time - resolved data is achieved with a polarized dual ch annel confocal scanning instrument (Becker & Hickl DCS - 120) attached to an output port of the microscope and controlled by a galvo - drive unit (Becker & Hickl GDA - 120). The confocal scanner is equipped with a polarizing beam splitter and two avalanche phot odiode detectors (APD) (ID - Quantique ID100) for the acquisition of fluorescence lifetime and anisotropy decay images. Polarized fluorescence transients are acquired using time - correlated single photon counting (TCSPC) detection electronics (Becker & Hickl SPC - 152, PHD - 400N reference diode). 29 The main components for TCSPC detec tion electronics are constant fraction discriminators (CFD), electrical delays (DEL), the Time - to - Amplitude Converter (TAC), Amplifier (between the TAC and ADC), Analog - to - Digital Conv erter (ADC) and digital memory. TCSPC is a statistical method requiring a highly repetitive light source to accumulate a sufficient number of photon emission events for the required statistical data precision. The principle of TCSPC is the detection of s ingle photons and the measurement of their arrival times in respect to a reference signal, usually the light source (in this work the picosecond laser source). TCSPC electronics can be compared to a fast stopwatch with two inputs (Fig. 2.3). The clock is started by the START signal pulse and stopped by the STOP signal pulse. The time measured for one START STOP sequence will be represented by an increase of a memory value in a histogram, in which the channels on the x - axis represent time. Millions of START STOP sequences can be measured in a short time with a high repetition rate laser source. The fluorescence intensity versus time can be represented through the resulting histogram counts versus channels. Thus fluorescence anisotropy decay function R(t) can be calculated through the resulting histogram. The photon rate is kept low in comparison to the rate of the exciting lamp (usually 5% or lower) in order to ensure that only one photon per light flash is detected, o therwise histogram statistics wi ll be affected and erroneous measurement results will be generated through multi - photon events. The TCSPC system in our lab is operated in reverse time mode. The electrical delay unit moves the reference pulse to come after the emitted photons from the s ample instead of turning on the TAC for every excitation pulse with the reference channel. The reason and advantage of reverse mode is that TAC unit runs much less often so that electronics can recover between individual TAC events, resulting in the time - to - amplitude conversion being linear in time. Thus, 30 start the TACs at statistically determined times before the (fixed) reference pulse. An instrument respo nse function of less than 100 ps FWHM can be achieved by the combination of the pulsed dye laser source and TCSPC electronics. Figure 2 . 3 Illustration of a fast stopwatch with two inputs mechanism of TCSPC detection system . 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New York: Chemical Catalog Co., Inc., Journal of the Society of Chemical Industry 1929, 48 (43), 1036 - 1037. 36. Edward, J. T., Molecular volumes and the Stokes - Einstein equation. Journal of Chemical Education 1970, 47 (4), 261. 37. G. Stok es, G., On the Effect of Internal Friction of Fluids on the Motion of Pendulums . 1850; Vol. 9. 38. Hu, C. M.; Zwanzig, R., Rotational friction coefficients for spheroids with the slipping boundary condition. The Journal of Chemical Physics 1974, 60 (11), 4 354 - 4357. 39. Youngren, G. K.; Acrivos, A., Rotational friction coefficients for ellipsoids and chemical molecules with the slip boundary condition. The Journal of Chemical Physics 1975, 63 (9), 3846 - 3848. 40. Perrin, F., Mouvement brownien d'un ellipsoide - I. Dispersion diélectrique pour des molécules ellipsoidales. J. Phys. Radium 1934, 5 (10), 497 - 511. 35 CHAPTER 3: CHARGE - INDUCED LONG - RANGE O RDER IN A ROOM - TEMPERATURE IONIC LIQUID Adapted with permission from: Ma, K.; Jarosova, R.; Swain, G. M. ; Blanchard, G. J., Charge - Induced Long - Range Order in a Room - Temperature Ionic Liquid. Langmuir 2016, 32 (37), 9507 - 9512. 36 3.1 ABSTRACT C harge - induced long range (ca. 100 µm) order in the room temperature ionic liquid ( RT IL) 1 - butyl - 3 - methylimidazolium tetrafl uoroborate (BMIM + BF 4 ), supported on a silica surface was examined . T he rotational diffusion dynamics of anionic, cationic and neutral chromophores as a function of distance from a silica surface wer e measured . The results reflect the excess charge density gradient induced in the RT IL by the (negative) charge present on the silica surface. Identical measurements in ethylene glycol reveal spatially invariant reorientation dynamics for all chromophores. Capping the silica support with Me 2 SiCl 2 results in spatially invariant reorientation dynamics in the RT IL. T hese data are understood in the context of the RT IL exhibiting a spatially - damped piezoelectric response mediated by RT IL fluidity and disorder. 3.2 INTRODUCTION Room - temperature ionic liquids are a well - established class of materials that hold promise for a host of applications ranging from solvents for organic synthesis 1 to electrolytes in supercapacitors. 2 - 3 The properties of RT ILs that make them attractive are the characteristically wide temperature range over which they exist in the liquid phase, their large redox window, and their ability to sol vate a wide variety of solutes. Despite the plethora of uses for RT ILs, there remains only a limited understanding of this family of materials at the molecular scale. 4 - 6 The primary reason for this situation is that RT ILs are characterized by a very high charge dens ity As a consequence, the treatment of intermolecular interactions in RT ILs has not been resolved fully, including issues such as whether these systems are best considered as ion pairs or as discrete ions. Also, because of the high charge densit y of RT (GCS) model is of limited value. 7 While there is significant literature pointing to the existence of ordering in RT ILs for distances somewhat in excess of that seen for the double layer of a 37 dilute solution, there remains no clear exp erimental means to resolve the actual length scale of any such organization. It is important to characterize the length scale of organization in RT ILs because such order will mediate the physical an d chemical properties of the RTIL. There have been many e x perimental 8 - 10 and theoretical/ computational 11 - 13 studies of organization in RTILs, and it is typically held that order near an interface persists over 5 nm or less. 4 - 7, 14 There is presently, however, no effective means of p robing the extent and nature of such organization. Experimental methods, such as X - ray diffraction (XRD), neutron diffraction, or atomic force microscopy (AFM ), ar e of limited utility because of the fluid nature of RT ILs, and to this point, the methods available to interrogate organization at have been limited. The spatially resolved spectroscopic results report ed on herein here reveal persistent orga This distance is quite long relative to the typical electric double layer seen in dilute solution (hundreds of nanometers at most). Our approach to elucidating long range order at RT IL - solid interfaces is based on the use of spatially - resolved spectroscopic measurement of dilute fluorescent probe molecules. It is known that the linear spectroscopic properties of fluorescent probes are sensitive to their immediate environment, and the e xistence of charged probes similar in size to the RT IL constituents provides an opportunity to interrogate the local environment(s) formed by RT ILs over a range of length scales. T hree chromophores were used for this purpose in this chapter : Resorufin (R , anion), Nile Red (NR, neutral) and Cr esyl Violet (CV, cation) (Fig 3.1 ). T he rotational diffusion dynamics of these three chromophores are measured in BMIM + BF 4 as a function of distance from a planar silica support using a confocal microscope equipped with time - resolved, polarized fluorescence detection gear. Our data reveal opposing trends in the depth - dependent 38 reorientation dynamics for R and CV + and depth - independent reorientation for NR in the RT IL over a distance of ca. 100 . For ethylene gly col, a viscous neutral solvent, chromophore reorientation dynamics are spatially invariant. When the (negatively charged) silica surface is capped with dimethyldichlorosilane (Me 2 SiCl 2 ) prior to the introduction of RT IL, the reorientation dynamics of all chromop hores are spatially invariant. We show in this chapter that the surface distance - dependence in the anisotropy decay dynamics for R and CV in the RT IL BMIM + BF 4 - reflects the free charge density gr adient, f , in the RT IL, induced by the surface charg e of the silica support. The mapping of f in the RT IL provides the first direct measure of the spatial extent of surface charge - induced organization in an RTIL. The fact that this induced order persists ov provides many potential opportunit ies for control of RT IL optical and electronic properties through externally controllable potential gradients. Figure 3 . 1 Structures of the RTIL constituents BMIM + and BF 4 (top), resorufin, nile red, and c resyl violet (bottom, from left to right). 3.3 MATERIALS AND METHODS Chemicals. Resorufin sodium salt and Nile Red were purchased from Sigma Aldrich and used as received. Cresyl violet perchlorate was purchased from Eastman Kodak Co. and used as received. E 39 as received and chloroform (99.0 - 99.4%, Sigma Aldrich) was used as received. Wate r used in these studies was purified in - house with a Milli - Q filtration system (Millipore). Purification of the RT IL. The preparation and purification of the RT IL has been described previously. 15 As - received BMIM + BF 4 (Sigma - Aldrich, neat concentration of 5.35 M) was first stored over activated carbon for 3 days. After this time, the RT IL was centrifuged to separate the carbon powder. Most of the BMIM + BF 4 sample (several milliliters) was then carefully removed and (ca. 0.5 mL) heated to 70 °C for 50 min while purging with ultrapure Ar (99.99 95%, Linde). This procedure was performed with the RT IL in an electrochemical cell in a N 2 - purged vinyl drybox. Preparation of ionic liquid solution. To prepare the chromop hore containing RTIL sample, a stock solution of chromophore ( ca . 5 × 10 - 4 mol L - 1 ) was prepared and used as follows . The chromophore (final concentration 5×10 - 5 mol L - 1 ) + BMIM + BF 4 solution was prepared by transferring 0.1 mL of the chromophore stock solution into a 1 mL volumetric flask and then evaporating to dryness in an oven at 100 °C for 1 h. The volumetric flask was then filled to the mark with the purified BMIM + BF 4 . This solution was then stirred for 12 h before use. All sample preparation procedures were performed inside a N 2 - purged vinyl dry box (Coy hygrometer. All purifie d BMIM + BF 4 were stored over activated (heat treated at 400 °C in a furnace) 5 Å molecular sieves in a glass - stoppered bottle and kept in the dry box. Fluorescence a nisotropy decay depth profiling. The instrument used to obtain fluorescence anisotropy dec ay dynamics as a function of distance from the silica support has been described in detail in Chapter 2. Briefly, a Nikon Eclipse Ti - U inverted microscope is equipped with a confocal 40 scanning head (B&H DCS - 120) equipped with two time - resolved, polarized d etection channels, each with an avalanche photodiode detection (ID Quantique ID100). The time - resolved data are processed using time - correlated single photon counting electronics (B&H SPC - 152) and ar e recorded using B&H software. The light source for the experiment is a cavity - dumped synchronously pumped dye laser (Coherent 702 - 2) operating at 563 nm (5 ps p ulses, 4 MHz repetition rate). This dye laser is excited by the second harmonic output of a passively mode locked Nd:YVO 4 laser (13 ps pulses, 80 MHz repetition rate, 2.5 W average power at 532 nm). 3.4 RESULTS AND DISCUSSION The primary purposes of this work described in this chapter are to characterize the spatial extent of persistent organization in RT ILs in contact with a charged surface and to offer a framework within which to interpret these results. This organization is orders of magnitude in excess of the length scale expected for dilute solutions, where traditional double layer models apply. The f luorescence anisotropy decay of selected chromophor es in the RT IL as a function of distance above the silica support are measured . 16 D epth resolution is achieved with a microscope stage and with the use of a confocal microscope to minimize the depth of focus for the individual measurements. For the chromophore concentrations use d in this chapter ( ca . 10 - 5 M), excitation transport contributes negligibly to the measured depolariz ation. The chromophore s (Fig 3.1 ) exhibit different depth - dependent reorientation dynamics depending on their formal charge. T he depth - dependent anisotropy decay time constants for Resorufin (anion, Fig 3.2a ) and Cresyl Violet (cation, Fig 3. 2b) as a function of distance from the silica support are showed in Fig 3.2. The most obvious features contained in these data are that the cationic and anionic chromophores exhibit opposing trends while the neutral NR chromophore exhibits a negligible depth - 41 dependence (Fig 3 .3 a). The sec ond feature of note is that the distance over which the anisotropy decay time constants change is ca . 100 m. T he functional form of the depth dependence and the physical basis for these results are examined following verification of the role of surface a nd RT IL charge in these findings. 42 Figure 3 . 2 (a) Variation of resorufin (anion) anisotropy decay time constant with the distance from a planar silica support. (b) Variation of cresyl violet (cation) anisot ropy decay time constant with the distance from a planar silica support. For both data sets, the data acquired over 43 Figure 3.2 with blue, green, and red d ata points representing individual depth profiles, and for the data focus). 44 Figure 3 . 3 (a) Variation of anisotro py decay time constant with the distance from a silica support for resorufin (anion, red), cresyl violet (cation, blue), and nile red (neutral, black). (b) Variation of anisotropy decay time constant with the distance from a silica support treated with Me 2 SiCl 2 45 Figure 3.3 for resorufin (anion, red), cresyl violet (cation, blue), and nile red (neutral, black). Given the opposing functional forms of the distance - dependent dynamics for the cationic and anionic chromophores, it is important to determi ne the contributions of RT IL and supporting surface charge to the data. C hromophore dynamics have been measured in ethylene glycol (EG) supported on the same silica surface . The results are found to be independent of the distance from the silica support ( Fig 3.4). To evaluate the role of surface charge, the surface silanol functionalities were capped using dimethyldichloro silane. Treating the silica surface with this reagent terminates the surface with neutral dimethylsilane functionalities, giving rise to depth - independent anisotropy decay dynamics for all chromophores (F ig 3.3b ) . The anomalous distance - dependent dynamics observed thus require the RT IL to be in contact with the (charged) silica support. We now turn to understanding the functional form of the data shown in Fig 3. 2. 46 Figure 3 . 4 Depth dependence of the anisotropy decay time constant for resorufin (anion, red), cresyl violet (cation, blue), and nile red (neutral, black) in ethylene glycol . Data points at zero depth are displaced somewhat as a result of the interaction of t he chromophores with the silica surface. The induced orientational anisotropy decay of an ensemble of chromophores is derived from the normalized difference between pol ar ized emission transients (eqn 3.1) to produce the function, R(t). [ 3. 1] Extracting chemical and physical information from R(t) has been treated extensively in the literature. 17 - 23 The modified Debye - Stokes - Einstein (DSE) model is useful for relating specific properties of the system to the observed experimental data . 18, 21 - 22 47 [ 3.2 ] where OR is the decay time constant of R(t); is the viscosity of the medium; V is the hydrodynamic v olume of the reorienting entity; f is a fri ctional boundary condition term; k B T is the thermal energy ; and S is a shape factor to account for the ellipsoidal shape of the rotating species. The key q uestion is which quantity in eqn 3. 2 is responsible for the ob served charge - and depth - dependent behavior in RT ILs. The shape factors S of the chromophores are known to be on the order of 0.75 and the (rigid) chromophores will not change shape with distance from the support. 2 4 - 25 Likewise, k B T is uniform throughout the system. The frictional interaction term, f , is taken to be 1 for all measurements, which is typical for polar systems. The viscosity of the RT IL is on the order of 50 cP and there is neither evidence or prec edent to suggest this term varies with proximity to a silica surface; it is a bulk property of the RT IL. The hydrodynamic volume of the rotating entity, however, can be used to account for the observed functional form of the OR data (Fig 3.2 ). 19 For the cationic and anionic chromophores, there is an equilibri um between free and ion - paired species. The chromophore is present at low concentration so the relevant ion - pairing will be between Resorufin and BMIM + or between Cresyl Violet + and BF 4 ([BMIM + ] = [ BF 4 ] = 5.35 M). Scheme 1 48 Scheme 2 Because the p resence of a charged surface is required to produce the observed anisotropy decay gradient and this gradient is seen only in ionic liquids, it is assert ed that the charged surface induces a compensating counter ion excess in the RT IL near the surface. Thi s situation is analogous to that in the Gouy - Chapman - Stern model. The difference between an RT IL and a dilute solution is that for the RT IL, all molecules are charged and the movement of one charge ultimately requires the compensatory movement of an oppos ing charge to maintain bulk electro - neutrality. Because the (charged) chromophores exist in an equilibrium between free ionic species and complexes with the dominant counter ion, and the presence of a surface charge induces a gradient in RT IL constituent concentrations, there will consequently be a gradient in the amounts of the free and complexed chromophores, reflected in the anisotropy decay time constant gradient as long as the rate constants for chromophore - RT IL association and dissociation are faster than OR - 1 . For a negatively charged surface, excess BMIM + near the surface gives rise to an enrichment in resorufin - BMIM complex, leading to a relatively slow anisotropy decay time constant that becomes more rapid with distance from the surface. For th e cationic cresyl violet chromophore, the relatively low density of BF 4 near the interface yields a lower complex concentration, and the measured anisotropy decay time constant should be faster near the surface, slowing with distance from the surface. 49 Relationship of the Rotational Motion Gradient to the Free Charge Gradi ent. It is instructive to relate the property we measure, the gradient in reorientation time constant, OR , to the displaced excess charge in the system and the surface potential of the silica plate. We accomplish this through the hydrodynamic volume o f the reorienting moiety. [3.3] At any given distance from the silica surface there will be an equilibrium between free and complexed chromophore (scheme s 1 and 2) that is determined by the ( RT IL) counter ion concentration, [3.4] where the terms X are the mole fractio ns of free and complexed chromophores. V is thus related to the counter ion gradient through V=V free X free + V complex X complex . In a system where every constituent molecule is charged, such a concentration gradient defines the gradient in displaced excess charge, D [3.5] w here is a collection of constants, including those relating V to [excess counter ion]. The hydrodynamic volume of each spec ies present in the systems under investigation can be determined using the method of van der Waals increments (Table 3. 1). 19 50 Table 3 . 1 Hydrodynamic volumes of system constituents. 19 Species Structure V (Å 3 ) Resorufin 165 Cresyl Violet 217 Nile Red 250 BMIM + 139 Tetrafluoroborate 50 R - BMIM complex 304 CV - BF 4 complex 267 With f = 1, we estimate from the experimental anisotropy decay time constant data for resorufin, X complex = 0.46 and X free = 0.54 at the silica surface, while at 100 m distance from the surface, X complex = 0.21 and X free = 0.79. Determining the actual excess BMIM + concentration at the surface is complicated by the inability to quantitate the charge density at the silica surface. The 51 excess BMIM + at the surface necessarily corresponds to a deficit in BF 4 because all species prese nt in the RT IL are charged. Quantitation of this charge gradient will require control over the charge density at the surface(s) of the RT IL. In the RT IL, the gradient in D is related to the surface potential of the si [3.6] w here is th e dielectric constant of the RTIL; is the scalar electric potential field arising fro m the surface charge ; and f is the free charge gradient. The ability to characterize the free charge gradient in the RT IL provides direct insight into the spatial extent of the electric field induced by eqn 3.6 ). The distance over which the gradient is observed is not consistent with the predictions of the GCS model, and we are not aware of an existing theoretical framework in which to treat these data. RT ILs are expected to deviate from the predictions of the GCS model because of the high density of charged species present. RT ILs are also not crystalline materials because of the characteristic sterically induced disorder which leads to their existence in the liquid state at room temperature. Analogy to Pie zoelectric Behavior. The presence of the free charge gradient in the RT IL in response to the surface charge on the silica support is reminiscent of the response of piezoelectric materials to the presence of surface charge, except the spatial extent of the piezoelectric effect is damped in this instance by the molecular properties of the system. Piezoelectric materials manifest surface charge upon experiencing external stress. Lattice distortion results in the redistribution of charge within the material. The converse of this effect is that the application of charge to the surface of the material gives rise to a lattice distortion. These well - known effects 52 are seen in deformable crystals that possess an asymmetric unit cell. RT ILs are obviously not crys talline materials, but they are characterized by strong ionic interactions that result in high viscosity and low volatility. Our observation of a free charge density gradient over ca . 100 m can be considered analogous to that seen in a piezoelectric mate rial. We note that piezoelectric behavior is seen in a variety of biomaterials, including peptide nanotubes 26 - 27 and viruses, 28 underscoring the fact that the effect is not limited to inorganic crystals. The piezoelectric effect is described through the relationship between the application of a stress to a material and the resulting charge displacement, D 29 [3.7] w here is the diel ectric constant of the material; E is the electric field; is the matrix desc ribing th e piezoelectric effect; with t being the converse, relating application of an E field to the strain induced in the system. T is the stress applied, which produces a strain, S; with s , the compliance, characterizing the proportionality between stress and strain. It is instructive to consider the molecular properties responsible for the effect we observe. The electric field is a property of the support and in future experiments will be a quantifiable and controllable experimental parameter. The matrix is a material property and will not vary with distance from the support. Likewise, the compliance of a material, s , is not expected to exhibit spatial variation for a nominally homogeneous material. The quantity in eqn 3.7 related to the molecular proper ties of the RT IL is the dielectric constant, . Alt hough termed a constant, is frequency - dependent and related to the molecular polarizability through the Clausius - Mossotti relation. 30 Polarizability depends on molecula r structure and orientation, and when considered in the bulk sense, the susceptibility is 53 known to depend on the ordering within the material. For this reason, the potential gradient in the RT IL should manifest as a refractive index gradient ( = n 2 ). Mea surement of the dielectric response of a material can be challenging, especially when it changes over micrometer length scales. One way to evaluate the dielectric response of a material, at least qualitatively, is through the fluorescence lifetime of a pr obe molecule. The factors that contribute to the fluorescence lifetime of a chromophore are numerous, but it is well established that changes in the dielectric response of a material over short distances are reflected in changes in the fluorescence lifeti me of a chromophore. 31 - 33 A gradient in fl is observed for the chromophores examined here ( Fig 3.5a ), consistent with a gradient in the dielectric response of the RT IL. For the same RT IL systems supported on a s ilanized surface we do not observe the same fluorescence lifetime behavior ( Fig 3.5b ). We note that the lifetime depth - dependence and the anisotropy decay depth dependence are not identical, and hasten to note that there is no fundamental basis for the fl uorescence lifetime and the anisotropy decay time to correspond directly. In fact, R(t) is normalized for fluorescence lifetime, rendering molecular motion measurements independent of changes in fl . 54 Figure 3 . 5 (a) Variation of fluorescence lifetime with the distance from a silica support for resorufin (anion, red), cresyl violet (cation, blue), and nile red (neutral, black). (b) Variation of fluorescence lifetime with the distance from a silica support tre ated with Me 2 SiCl 2 for resorufin (anion, red), cresyl violet (cation, blue), and nile red (neutral, black). 55 3.5 CONCLUSIONS There are two immediate impli cations of the findings demonstrated in this chapter . The first is that by devising a means of characteriz ing f in RT ILs, we have demonstrated for the first time a level of organization that exceeds previously reported gradients in this family of materials. The ability to relate this gradient to applied surface charge brings with it the possibility of contro lling material properties such as refractive index gradients in RT ILs. Perhaps of more fundamental value is that this family of materials can be understood in the context of piezoelectric materials where the bulk properties damp the spatial extent of the potential gradient owing to the disorder intrinsic to any liquid phase molecular system. Evaluating the dependence of f on the identity of the RT IL will help to determine the range of utility for this effect. Finally, the results give rise to a number o f questions to consider about heterogeneous electron transfer of charged and neutral analytes in RT ILs and how the redox probe molecule - RT IL complex has to reorganize for the electron transfer to occur. We believe our results to be fully consistent with o bservations of slow relaxation of the electric double layer in other RT ILs. 34 56 REFERENCES 57 REFERENCES 1. Yadav, J. S.; Reddy, B. V. 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J., Modulation of an Induced Charge Density Gradient in the Room - Temperature Ionic Liquid BMIM + BF 4 - . J Phys Chem C 2018, 122 (13), 7361 - 7367. 61 4.1 ABSTRACT In Chapte r 3 , we described in detail the existence of an induced charge density gradient, f , in a room temperature ionic liquid (RTIL, BMIM + BF 4 ) normal to a charged planar silica surface. In this chapter , experimental control over the sign and magnitude of the g radient is demonstrated . The spatial extent of f can exceed 100 µm from the charged surface . f was characterized through the rotational diffusion time constant gradient of a cationic chromophore in the RTIL. The sign and magnitude o f f in BMIM + BF 4 i s linked directly to the surface charge density of the electrode, which can be controlled. T ransparent conductive electrodes ( fluorine doped tin oxide ( FTO ) and indium doped tin oxide ( ITO ) coated on glass) were used as supports and it was demonstrated th at control over the electrode surface charge carrier density can influence the magnitude and sign of f . There are limitations to this approach based on the FTO and ITO properties and these limits will be demonstrated experimentally in this chapter . 4.2 INTR ODUCTION Room temperature ionic liquids (RTILs) are a useful family of materials that have found application in a number of areas. Despite the utility of these materials, their physical and chemical properties have been challenging to understand, in large part because of the high charge density present in such systems (ionic strength 5 - 6 M), and the underlying inability to resolve the fundamental unit species, i.e. whether RTILs should be considered as discrete ions or as ion - paired dissociable molecules . 1 Recently, the Shaw group 2 (described in Chapter 2) and our group 3 (described in Chapter 3) have independently identified organization in RTILs that extends over macroscopic distances. The Shaw group demonstrated that organization evolved slowly in RTILs at either gas or solid interfaces, where the media would organize into crystal - like (liquid) 62 media over periods of minute s to days, and this structural order persisted at least one micron into the bulk RTIL. This order was seen using infrared spectroscopy (IRRAS) and second order nonlinear spectroscopy. Their findings demonstrated clearly the (slow) evolution of crystal - li ke order in RTILs. In Chapter 3, we discussed that exposing RTILs to a planar charged surface induced a charge density gradient in the ionic liquid normal to the charged surface plane, and the extent of the gradient was on the order of 100 µm. It is not c lear if the order found by the Shaw group is related directly to that which we have found theirs is structural and ours deals with a concentration gradient with no explicit structural order implied. These two separate and important findings, however, po int to the fact that ionic liquids behave very differently than either ionic crystals or dilute solutions, and their propensity for exhibiting organization over macroscopic length scales renders them a unique class of materials. It is an ongoing effort to determine the relationship between the structural organization observed by the Shaw group and the induced charge density gradient observed by our group. Regardless, it is of central importance to determine if the induced charge density gradient seen in R TILs can be controlled and, if it can, what the limits of this control are. In this chapter, a means devised by our group of controlling f in RTILs by controlling the excess surface charge density on FTO - or ITO - coated support surfaces in contact with the RTIL is introduced . We detail below our work demonstrating this control and describe how our approach to this problem differs from a capacitive approach to controlling surface charge density. 63 4.3 MATERIALS AND METHODS Chemicals. Cresyl violet perchlorate (C V + ) was acquired from Eastman Kodak Co. and used Purification of the ionic liquid. The preparati on and purification of BMIM + BF 4 - has been described elsewhere. 4 BMIM + BF 4 (Sigma Aldrich, neat concentration 5.35 M) was stored over activated carbon for three days. The mixture was then centrifuged to separate t he carbon powder from the RTIL. The majority of the BMIM + BF 4 (several mL) was then removed and ( ca . 0.5 mL) was heated to 70°C for 50 min while purging with ultrapure Ar (99.9995%, Linde). This procedure was performed with the RT IL in an electrochemical cell in a N 2 - purged glove box. Preparation of ionic liquid solutions. To prepare the RTIL sample containing CV + , a stock solution of CV + (5×10 - 4 mol L - 1 ) in ethanol was prepared. CV + (final concentration 5×10 - 5 mol L - 1 ) in BMIM + BF 4 was prepared by tran sferring 0.1 mL of the CV + stock solution into a 1 mL volumetric flask and evaporating it to dryness in an oven (100 °C, 1 hr). The cooled volumetric flask was then filled to the mark with purified BMIM + BF 4 and the solution was stirred for twelve hours p rior to use. The solution was mounted in a cell configured as schematized in Fig 4.1 . 64 Figure 4 . 1 (a) Schematic of cell holding RTILs configured as a capacitive device. The conducting coating is either ITO or FTO. For this cell, d = 1 mm. (b) Schematic of cell holding RTILs with the lower support configured as a resistive device. T he conducting coating is either ITO or FTO. For this cell, d = 1 mm. 65 All sample preparation procedures were performed inside a N 2 - purged vinyl dry box (Coy temperature, measured using a hygrometer. All purified BMIM + BF 4 and solutions were stored over activated 5 Å molecular sieve in a glass - stopper ed bottle and kept in the dry box. The molecular sieve was activated 400 °C. Water impurity levels after this pretreatment were estimated to be below 20 ppm, as measured by thermogravimetric analysis. Electrode and cell preparation. Both FTO (Solaronix, TCO22 - 7 , 7 /sq) and ITO (Nanocs Inc., IT10 - 111 - 25, 10 /sq ) coated supports were sonicated in water containing detergent (Fisherbrand, Sparkleen 1) , 18 M water (Milli - Q) and then in isopropanol (Macron, 99.50%) , for 15 min. each. The cleaned electrodes were removed from the isopropanol, washed with ethanol and oven - dried at 200 C for 30 min. After the electrodes were cooled, they were cleaned using a UV/ozone cleaner for 20 min. and connections were applied to the FTO or ITO surface using conductive ( silver - filled) epoxy (MG chemicals, 8331S - 15G) . The epoxy was cured at 120 C for 30 min. prior to assembly of the cell. The cell spacer ( ca . 1 mm thick) was cut from silicone rubber sheet (MSC Direct), and was sonicated in water containing detergent and r (Milli - Q) for 15 min each. The cleaned spacer was washed with ethanol and then dried in flowing N 2 and air for 2 h prior to assembly of the cell. Fluorescence anisotropy decay depth profiling. The instrument used to acquire depth - dependent f luorescence anisotropy decay dynamics has been described in Chapter 2 and we provide only the essential details here. A Nikon Eclipse Ti - U inverted microscope is equipped with a confocal scanning head (B&H DCS - 120) that has two time - resolved, polarized de tection channels, each with an avalanche photodiode (ID Quantique ID1 00). The fluorescence transients are processed electronically using commercial time - correlated single photon counting gear (B&H 66 SPC - 152) and B&H software is used to operate the instrumen t and store the data. The light source is a synchronously pumped cavity - dumped dye laser (Coherent 702 - 2) operating at 563 nm (5 ps pulses, 4 MHz repetition rate). The dye laser is excited by the second harmonic output of a passively mode locked Nd:YVO 4 laser (Spectra Physics Vanguard) producing 13 ps pulses at 80 MHz repetition rate with 2.5 W average power at 532 nm. 4.4 RESULTS AND DISCUSSION In Chapter 3 we demonstrated the existence of a free charge density gradient, f , in the RTIL, BMIM + BF 4 . The free charge density gradient existed over distances in excess of 100 µm from a planar charged surface (SiO x ) and was probed through measurement of the rotational diffusion time constant of charged chromophores as a function of distance from the (ch arged) surface. This unexpected finding lead naturally to the desire to control the free charge density gradient, for both fundamental reasons and because of the potential for practical applications stemming from such charge organization in RT ILs. In thi s chapter , we report on the ability to control the charge density gradient in BMIM + BF 4 . C ationic chromophore Cresyl Violet (CV + ) was used because of its favorable and well - characterized optical properties. Before discussing control over f , it is useful to consider the magnitude of its spatial extent relative to known models of ionic spec ies in the liquid phase. Most treatments of ionic species in solution at a charged surface are based on the G o uy - Chapman - Stern model 5 or a variant to understand the electric double layer (EDL). The EDL forms in dilute ionic solutions in response to the presence of a c harged interface, such as an electrode. While the details of the EDL have been the focus of a great deal of attention, the thickness of the counter - balancing solution layer in a concentrated electrolyte solution would be in the range of 1 - 100 nm depending on the excess 67 surface charge. All such treatments are based on the ionic species being present at relatively low concentrations, and for ionic liquids this is clearly not the case. 1, 6 - 7 Recent capacitive electro chemical measurements in BMIM + BF 4 - however, are consistent with GCS behavior as well as the behavior expected for a molten salt, 8 in contrast to theoretical predictions for RTILs. 7, 9 There have been a number of investigations of interfacial organization of RTILs, both experimental and theoretical, with a common finding that organization, as measured by scattering or diffraction methods has a persistence length of 10 nm at most. 5, 10 - 18 S uch findings are apparently at odds with recent reports of long - range organization in RTILs, which show that spatial gradients can and do exist over m to sub - mm length - scales. 2 - 3 It is instructive to a different property of the system. For example, the work of Anaredy and Shaw reports on molecular orientation using FTIR and second harmonic generation spectroscopies. 2 Other approaches, including electron diffraction, neutron diffraction, X - ray re flectivity and surface force measurements sense order at or near the RTIL surface. 1, 14, 19 - 20 In our case the organization sensed is in the form of a charge density gradient and there is no specific implication o f a spatial variation in the molecular orientation or organization of either cationic or anionic species, or any implication of quasi - lamellar structure as implied by scattering methods. The charge density gradient we report is, rather, a redistribution o f discrete ionic species that is a systemic response to the imposition of a force and we have drawn the analogy of the effect we observe to the piezoelectric effect previously. 3 In Chapter 3 , a sample holder geometry was used where the RTIL was in contact with a planar silica window. There w as no silica plate in contact with the top of the ca . 1 mm thick RT IL film (the RT IL was contained in a closed vessel). While this configuration was adequate 68 for demonstrating the existence of the induced charge density gradient, it was less than optimal if the goal is to demonstrate control over the direction and magnitude of the gradient. In order to achieve greater control we constructed a closed cell where both the top and bottom planar sheets that define the cell were glass coated with either ITO or FTO, transparent conductive oxides in contact with the RTIL (Fig 4.1) . As shown in the data presented in Fig 4. 2, the direction of f , as sensed by CV + demonstrates that these transparent conductive materials are characterized by positive potentials, c onsistent with literature reports for pH values lower than 6. 21 This finding is significant because of the implication that the pH experienced by the silica surface is below 6. The RTILs we use are anhydrous but there is expected to be an adlayer of water on the support surface owing to its hydrophilic nature. The thickness of this aqueous adlayer is unknown but small, and the extent to which this layer interacts preferentially with either the support (ITO or FTO) surface or the RTIL is not known. Our empirical data indicate that th e effective pH of this interface is below 6 but more detailed information remains to be elucidated. 69 Figure 4 . 2 Depth dependent reorientation time constant, OR , of CV + as a function of distance from the low er supporting FTO - coated plate. Data were acquired with the cell configured as shown in Fig 4.1a, capacitive mode. Voltages applied (V bottom V top ) are (a) from 0 mV to - 1000 mV as indicated in the legend, and (b) from 0 mV to +1000 mV as indicated in t he legend. Initial efforts to control the charge density gradient were aimed at controlling the surface charge density, , present on the electrodes on each side of the RTIL. Such a configuration is, 70 sc hematically, a capacitor (Fig 4.1 a) and a potentiosta t can be used to control the potential difference across the electrodes. Experimental implementation of this means of controlling f in RTILs does not, however, produce the desired result. Potential differences of 1V between the plates, regardless of the direction of the polarization, did not yield any change in the experimental signal in either the magnitude or direction of f (Fig 4.2 ). The reason for this apparent failure lies in the geometry of the cell used, which is itself a consequence of the lengt h scale of the induced gradient discussed in Chapter 3 . To understand the reason for our findings, it is instructive to estimate the amount of charge that can be stored in this configuration and determine how that induced charge compares to the ambient ch arge on the FTO (ITO) surfaces. The capacitance per unit are a of this cell is given by eqn 4.1 [4.1] w here C/A i s the capacitance per unit area; is the dielectric constant of BMIM + BF 4 ( = 11.7); 22 0 is the permittivity of free space (8.854x10 - 12 F/m) ; and d is the distance between plates. For the cell shown in Fig 4.1 a, d = 1 mm, yielding a capacitance of 10 pF/cm 2 for the 1 cm 2 ITO (FTO) plates used. This capacitance co rresponds to ~ 6x10 7 charge carriers per electrode for |V| = 1V. It is important to compare this achievable surface charge density to that present on an ambient silica surface, the support on which f was first found to exist. We assume that the charg e density on an ambient ITO (FTO) surface will be similar to that of silica, and even if this assumption is not quantitatively correct, it will be within an order of magnitude of the actual 71 value. For silica, the surface silanol group density is ca . 5 mo l/m 2 (3 10 14 O - /cm 2 ). 23 The silica surface has been studied extensively and the silanol groups are characterized by two pK a v alues, one at ca . 4.5 (19%) and the other at ca . 8.5 (81%). 24 - 25 For silica in contact with BMIM + BF 4 , it is not possible to estimate the pH of the surface. If we assume that the silanol groups characterized by a pK a of 4.5 are deprotonated while those with a pK a of 8.5 are fully protonated, the silica surface charge density is 6 10 13 O - /cm 2 . Using a tw o - plate model (Fig 4. 1a), with control over the potential applied to each plate, can thus be changed by ca . 1 ppm of its ambient value. It is for this reason that no change is seen in f with variations in the plate potentials. Among the important ques tions is whether or not this physical configuration could be used to control f if it were changed dimensionally. The operative equat ion is Q = CV. As noted above, |V| = 1V was used , and the value of C was 10 pF. Two quantities can be changed; these are the distance between plates and the potential difference applied. For practical reasons, the smallest value of d is on the order of 1 m, yielding C = 10 nF. The second variable quantity is V. Assuming a potential difference of 100 V between the plates , a change of = 6x10 12 /cm 2 can be achieved. Such values would approach, if not exceed, the limits of the device and would provide for a change in surface charge of only ca . 10% of the ambient value of . For this reason, it is necessary to identify oth er means of controlling the charge density on the support plate(s). There are other ways to control surface charge density. One of these is use of the individual ITO (FTO) - coated plates as resistors ( ca . 10 ) and to pass current across them ( not between them). This experimental configuration is shown in Fig 4.1 b. By controlling the current passed through the ITO, can be controlled in a range that is on the same order as the ambient surface 72 charge density of silica. V = (V + V - ) and R L (Fig 4.3 b) ca n be controlled to control the current I passing through the electrode . This approach controls the steady state density of charge carriers, , in the ITO (FTO). From the definition of current, I = Q/t, the appropriate values of V and R L required to cont rol I can be estimated and thus Q in a relevant regime. Based on showing a measurable OR gradient, we need to be able to modulate the charge on the support by ca . 10 14 e - /cm 2 s, corresponding to 1.6 10 - 5 C /cm 2 s. For a 1 cm 2 ITO - or FTO - 10 - 5 A. The applied voltage is under our experimental control, and we use V = 200 mV for illustrative purposes (we use a range of voltages in our experimental measurem ents, vide infra ). For V = 0.2 V, R L 12.5 k . The circuit indicated in Fig 4.1 b places the ITO - coated plate (10 ) in series with R L , producing a voltage drop across the ITO plate of 0.16 mV. This is feasible based on the typical dopant density of IT O. The units of I are C /s and the actual current required depends on the residence time of an e - on the ITO plate. The drift velocity of the electron is v d = (I/n e ) and for the parameters given above, v d = 1 cm/s. Thus, for a flux of 10 14 e - /s, the resi dence time of the e - on the 1 cm x 1 cm ITO - coated plate is 1 s, but actual material properties ( e.g. thickness, grain boundaries, dopant density) can and do affect this value substantially ( vide infra ) . 73 Figure 4 . 3 Depth dependent reorientation time constant, OR , of CV + as a function of distance from the lower supporting plate. Data were acquired with the cell configured as shown in Fig 4.1b, resistive mode. Currents applied (a) ranged from 0 mA to 200 mA using FTO as the conducting film on the support plate and (b) ranged from 0 mA to 420 mA using ITO as the conducting film. 74 A note is in order regard ing the comparison of these two ge density on a planar surface. From Gauss the E field between the plates is given by E = /2 0 , and for a planar conductor, the E field perpendicular to the plate is gi ven by E = / 0 . The E field experienced by the RTIL is t he same for both configurations (Fig 4.1 ), and f rom Chapter 3 the free charge density gradient in the RTIL f the ible using the two experimental configurations. For the capacitor (Fi o V/d , and for the resistor (Fig 4. = v d ) , with the factor ( v d ) being required in the latte r c ase because of current flow. As discussed above, owing to th e spatial extent of f , control ration shown in Fig 4. 1b. In Fig 4.3, the experimental OR gradients for CV + in BMIM + BF 4 as a function of current applied to the FTO ( Fig 4. 3a) and ITO ( Fig 4. 3b) supports (corresponding voltages and powers are given in Table 4. 1) are shown . There are several important points to note regarding these data. The first is that the gradient for both FTO and ITO electrodes depends on the current across the lower conducting plate, demonstrating the validity of this means of controlling f . It is significant that the direction of the gradient can be changed with the application of ca . 150 mA across the plate. The current required to effect this change in gradient is significantly larger than that predicted theoretically. The reason for this is the finite thickness of the FTO or ITO material. It is the surface charge that mediates f , but due to finite plate thickness and the materi als properties ( e.g. dopant level, grain boundaries, defects), only a fraction of the current passes across the surface. The majority of the current passage is mediated by structure ( e.g. grain boundaries, defects) that is buried within the oxide layer an d is thus screened. 75 Table 4 . 1 Relationship between current and voltage across the bottom support plate thin film (ITO) and the change in temperature of ethylene glycol in the cell indicated in Fig 4.1b. I (mA ) V (mV) Power (mW) T ( C) 0 0 0 0 50 700 35 0.5 100 1700 170 2.5 200 3700 740 10.5 400 6300 2520 30 For this experimental configuration, control over the steady state charge density on the ITO - coated plate is easily achievable using conditions that should produce little Joule heating. 26 - 27 It is important, however, to quantitate the extent to which Joule heating contributes to our experimental data. As can be seen in Fig 4. 3, for the higher applied cur rents the CV + anisotropy decay time is seen to decrease, with a loss of gradient. One way to gauge the temperature of the medium in which CV + is reorienting is by modeling its behavior using the modified D ebye - Stokes - Einstein equation ( e q n 4.2), 28 - 31 [ 4.2 ] w here is the viscosity of the medium; V is the h ydrodynamic vol ume of the probe; 32 f is a frictional term to account for intermolecular interactions ; 30 and S is a shape factor to account for the non - spherical shape of CV + . 31 These quantities are known for CV + ; S = 0.645, V = 216 Å 3 and f = 1 in polar media. 33 While there are data on the viscosity of BMIM + BF 4 available, and 76 information on its temperature dependence, 34 - 36 we can use other solvents with thermal conductivity similar to that of the RTIL where the temperature - dependence of the solvent viscosity is equally well known. 37 - 38 One such solvent is ethylene glycol, a comparatively viscous solvent whose temperature - dependent viscosity has been characterized in detail previously. 37 While these same measurements could be made directly using BMIM + BF 4 , we assert that using ethylene glycol is preferable because the rotati onal diffusion behavior of the CV + probe has been examined more extensively in ethylene glycol than in any RTIL, and temperature - dependent changes in f cannot contribute to the measured CV + rotational dynamics in ethylene glycol. By measuring the reorien tation time of CV + in the same sample cell used for the RT IL measurements as a function of current passed through the ITO (FTO) window, we can gauge the role of Joule heating in our measurements. Using the parameters for CV + given above, OR in ethylene gl ycol is calculated as a function of temperature using the temperature - dependent viscosity data from Bohne et al . 37 We com pare the experimental data (Fig 4. 4a) with the calculate d temperature - dependence (Fig 4.4 b) to determine the relationship between power applied an d temperature of the cell (Fig 4. 5). While it is possible to heat the sample signifi cantly by means of Joule heating, it is clear from these data that the temperature change is less than 5 over the current range relevant to controlling the sign and magnitude of the free charge density gradient. We anticipate that with the use of thinner conductive layers the contribution of Jo ule heating will be even less. 77 Figure 4 . 4 (a) Dependence of CV + reorientation time constant, OR , in ethylene glycol on current applied to the lower support plate conducting film. ITO was used as the thin film conductor for these measurements. Currents applied are as indicated in the legend and as shown in Table 4.1. (b) Dependence of CV + reorientation time constant, OR , in e thylene glycol as a function of temperature, as determined from temperature - dependent viscosity data and eqn 4.2. 78 Figure 4 . 5 Temperature of the cell as a function of power applied. Red box indicates the power region and consequent maximum temperature change over wh ich the sign of the free charge density gradient, f , changes sign. These data also demonstrate that a temperature gradient does not exist across the thin film sample because the anisotropy decay time constant for CV + is depth - independent at each temperature measured (Fig 4. 4). Because of the similar thermal conductivities of BMIM + BF 4 (0.19 W/m - K) 39 and ethylene glycol (0.25 W/m - K), 37 the data shown in Fig 4. 4 demonstrate that the f gradient we observe is not accounted for by a thermal gradient. 4.5 CONCLUSION In this chapter, the ability to control the free charge density gradient, f , in the RTIL BMIM + BF 4 is disc ussed . The spatial extent of f and the physical properties of the ITO (FTO) 79 layer on the conducting support plates preclude the ability to control f by capacitive means. S teady state carrier density on the conducting support plates can be controlled by passing controlled current through them. 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Chen, Q. - L.; Wu, K. - J.; He, C. - H., Thermal Conductivity of Ionic Liquids at Atmospheric Pressure: Database, Analysis, and Prediction Using a Topological Index Method. Industrial & Engineering Chemistry Research 2014, 53 (17), 7224 - 7232. 39. Valkenburg, M. E. V.; Vaughn, R. L.; Williams, M.; Wilkes, J. S., Thermochemistry of ionic liquid heat - transfer fluids. Thermochimica Acta 2005, 425 (1), 181 - 188. 84 CHAPTER 5: EFFECTS OF WATER ON THE INDUCED CHARGE DENSITY GRADIENT IN THE ROOM TEMPERATURE IONIC LIQUID BMIM + BF 4 85 5.1 ABSTRACT In previous chapters, an induced charge density gradient f in the RTIL , BMIM + BF 4 , was reported on when it is in contact with a charged planar silica surface. The f gradient is normal to the silica surface and persists over ca. 100 m . The sign and magnitude of f in BMIM + BF 4 is linked directly to the surface charge density of the electrode . By using indium doped tin oxide (ITO) or fluorine doped tin oxide (FTO) electrodes instead of silica as a support and passing current through (at least one of) the electrodes, we can control the s teady - state carrier density on the conducting support plate ( s ). Thus , contr ol was gained over the sign and magnitude of f in BMIM + BF 4 . In this chapter, the effect of water on f in BMIM + BF 4 is discussed Controlled amounts of water were added to the RTIL|electrode system order to achieve concentration s up to ca. 25,000 ppm (2.5% w/w). The magnitude of f decreases with increasing wat er concentration, but the characteristic length scale appears to remain constant. When the water concentration reaches ca. 25,000 ppm, f is no longer observed using rotational diffusion experiments. This finding is unexpected based on the assumption that the dilute solution limit of RTIL behavior is described by the Guoy - Chapman - Stern model. There are essentially two possible reasons for this finding. Either the RTIL exhibits a structural anomaly that has not been described to date, or the addition of w ater is affecting the means we use to characterize f . To resolve this matter, other me asurements can be performed in the future. One experimental method is s econd - harmonic generation measurements of BMIM + BF 4 to characterize depth - dependent order in the R TIL in the absence of added chromophores. A second means is to change the anion of the RTIL to a hydrophobic one, thus maintaining the use of reori entation dynamics as a probe of f . The details of these approaches will be discussed below. 86 5.2 INTRODUCTION Ro om - temperature ionic liquids (RTILs) are a useful class of material s with properties that include a larg e electrochemical potential window, 1 low vapor pressure, 2 nonfla mmability, and high thermal stability 3 . RT ILs can be used in a variety of practical applications ranging from electrolytes for energy storage devices 4 - 5 to novel solvent systems for organic synthesis 6 - 7 . Despite the potential of RTILs, understanding their properties on the molecular scale remains limited. 8 - 11 The reason for th is limitatio n is the high charge density of RTILs (ca. 5 - 6 M) and the uncertainty associated with how to treat RTILs , either as ion pairs or discrete dissociated ions. The Gouy - Chapman - Stern (GCS) model does not describe the behavior of RTILs . 12 There have been experimental 13 - 15 and theoretical studies 16 - 18 of organization in RTILs ranging characteristic length scale from nanometer s 19 - 21 to micrometer s. 8, 22 - 23 We have discussed in Chapter 3 that an induced charge density gradient f exists in RTILs in contac t with a charged (planar) silica surface. The charge density gradient f is normal to the surface and persis ts for ca. 100 m. In Chapter 4 , we discussed that the charge density gradient in RTILs can be modulated in both sign and magnitude by controlling the steady state carrier density in the ITO or FTO support material . These findings were predicted based on our initial ex periments and underscored the fact that RTILs behave differently than dilute solutions and are a unique type of material. Further research on the molecular organization of RTILs and intermolecular forces that are operative in these materials will provide a better fundamental physical and chemical understanding of this class of materials. that can be absorbed when the the RTIL is exposed to atmosphere. Certain room temper ature 87 ionic liquids are known to be hygroscopic. 24 - 27 It was found that even though many 1 - butyl - 3 - 1 - Butyl - 3 - methylimidazolium hexaflu orophosphate (BMIM + PF 6 ) and 1 - Butyl - 3 - methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM + TfN 2 ), they are still very hygroscopic and can absorb water quickly when they are exposed to a humid environment. 28 It is thought that the RTIL anions interact more strongly with water molecules than the RTIL cations. The dominant water - anion interaction is assumed to be hydrogen bond formation. In many RTILs it is difficult if not impossible to avoid at least trace amount s of water being present , and sometimes water is added deliberately to reduce RTIL viscosity, increase conductivity and lowe r the high price of RTILs . 29 At the present time, however, the fundamental nature of the interactions of water with the RTIL constituents is not well understood, and the relationship between RTIL component structure, amount of water present and macroscopic properties is not predictable. There are currently no theoretical models describi ng the thermodynamic p roperties over the entire compositio nal range from pure RTILs to infinitely dilute RTILs solutions. The primary reaso n for this l ack of models is the extremely complex molecular behavior of RTILs , both in their pure form and combined with water. 29 Understanding how H 2 O interacts with RTILs and how the properties of RTILs are affected by the presence of H 2 O is important in both scientific research and practical applications. In this chapter, we discuss the effect s of water in BMIM + BF 4 - on the induced charge gradient reported in Chapter 3 . The water concentrations studied range from ca. 500 ppm to 25,000 ppm H 2 O 0.006 to 0.031 in mole fraction). Reorientation data for Cresyl Violet (CV + ) show that the magnitude of f decreases with the increase of H 2 O H 2 O have a resolvable effect on the length scale of f . When H 2 O reach es 0.031, f was no longer 88 observed in the RTIL|H 2 O system. We describe below our results and consider possible reasons for the water - dependence we observe in these data. 5.3 MATERIALS AND METHODS Chemicals. Cresyl violet perchlorate (CV + ) was purchased fro m Eastman Ko d ak Co. and - Aldrich Co. and used as received. Water was purified in - lab with a Millipore Milli - Q filtration system. Purification of IL. The preparation and purification of the RTIL has been des cribed somewhere else. 30 BMIM + BF 4 (Sigma - Aldrich, neat concentration 5.35 M) was store d over activated carbon for 3 days prior to further processing . The mixture was centrifuged to separate the RTIL from the act ivated carbon. Then the majority of the RT IL (several mL) was removed purging during the process. The RT IL sample was housed in an electrochemical cell that was in a N 2 - purged glovebox during the purification process to minimize exposure to H 2 O . Figure 5 . 1 Structures of BMIM + BF 4 and Cresyl violet (cation). Electrode and C ell Preparation. FTO substrates (Solaronix, TC O22 - cleaned according to the following procedure. FTO substrates were sonicated in water containing detergent (Fisherbrand, Sparkleen 1) - Q) , a nd isopropanol 89 (Macron, 99.50%) for 15 min each. The cleaned substrates were removed from isopropanol and washed with ethanol and water, and oven - in air . After cooldown of the substrates, they were put in a UV/ozone cleaner and cleaned for 20 min. The copper wire connections were attached to the substra tes by silver conductive e poxy (MG chemicals, 8331S - 15G). The epoxy was bake d was about 1 mm thick and was cut from a sili cone rubber sheet (MSC Direct). The spacer was sonicated in water c ontaining detergent and - Q) for 15 min each. Then the clean spacer was washed with ethanol and water, and dried in flowing N 2 and air for ca. 2 h before assembling the cell. Figure 5 . 2 Sc hematic of the self - designed cell holding BMIM + BF 4 with the lower support configured as a resistive device. The conducting coating is either I TO or FTO on a silica support. For this cell, d = 1 mm. Fluorescence Anisotropy Decay Depth Profiling. The instr ument used for acquisition of depth - dependent fluorescence anisotropy decay data has been de scribed in Chapter 2. 31 Briefly, a Nikon Eclipse Ti - U inverted microscope is equipped with a confocal scanning head (B&H DCS - 120) which has two time - resolved, polari zed detection channels. Each channel has an avalanche ph oto diode (ID Quantique ID100). A commercial time - correlated single photon counting signal processing system (B&H SPC - 152) is used to acquire and process the time - resolved data. B&H software is used operate the instrument and rec ord the data. A 90 synchronously pumped dye laser (Coherent 702 - 2) operating at 563 nm (5 ps pulses , 4 MHz repetition rat e) is used as the light source. The dye lase r used in this experiment is excited by the second harmonic output of a passively mode - locked N d : YVO 4 la ser (Spectra Physics Vanguard). The source l aser produce s 13 ps pulses at 80 MHz repetition rate and produces 2.5 W average power at 532 nm. 5.4 RESULTS AND DISCUSSION In previous chapters , we have demonstrated the existence of an induced charge density gradient with a persistence length of ca. n the RTIL BMIM + BF 4 . We measured the rotational diffusion time constant of three different chromophores (cationic, anionic and neutral ) as a function of distance from the IL|Support interface. The existence of f brought the implication that control ove r support surface charge of the substra te could, in principle, provide control over the magnitude and sign of f . We used the cationic chromophore cresyl violet to probe the rotational diffusion time constant as a function of distance from the support sur face bec ause it is a well - characterized chromophore. We used ITO and FTO separately as the electrodes . Both thin film materials are conductive and transparent , which are two primary requirements for the experiments reported here. Current was passed throu gh the conductive film on the lower support to change the steady state surface charge density. The experimental results showed that the sign and magnitude of the induced charge gradient density can be changed in response to changes in surface charge densi ty . As noted above, i t is well - known that RTILs absorb water from the atmosphere . It is thought that RTIL organization and physical properties are affected significantly by the presence of water, e.g. the viscosity, conductivity and electrochemical window , even though the water concentration might be low . 28, 32 The data we present here demonstrate that the effects of water on the induced charge density gradient in 91 RTILs is not consistent with the predictions of th e Guoy - Chapman - Stern model (dilute ionic solution). The configuration of the surface potential - controlled cell was described in Chapter 4. T he cell is a sandwic h structure as shown in Fig 5. 2. The top and bottom support are both FTO electrodes. Between the two electrodes is the silicone spacer. C urrent was passed through the bottom FTO electrode. S ince I = dQ/dt, and the drift velocity of an electron in the FTO or ITO thin film electrode is on the order of 1 cm/s, the surfac e charge density of the supp ort will change with current. We measured the rotational diffusion dynamics of CV + from the RT IL|FTO interface through I || (t) and I (t), where the subscripts refer to the polarization of the emission relative to that of the excitation. Taking the normalizing difference be tween polarized emission transients produces the induced a nisotropy decay function R(t) , of CV + in the RTIL BMIM + BF 4 (eq n 5. 1). [ 5. 1] The chemical information of importance to this work is contained in the decay functionality of R(t), as has been described thoroughly in the literature. 33 - 37 In the RTIL BM IM + BF 4 - , CV + can be treated as a prolate rotator and R(t) is fitted using a single exponential decay function (eqn 5. 2). [5.2] In eq n. 5. OR is the decay time constant of R(t), and this quantity is related to system properties through the mo dified Debye - Stokes - Einstein (DSE) model (eq n 5.3). 34 - 37 92 [5.3] In eq n. 5. is the viscosity of the medium. V is the hydrodynamic volu me of the reorienting entity, f is a fri ctional boundary condition term. k B T is the thermal energy and S is a shape fact or to account for the non - spherical shape of the volume swept out by the reorienting entity. We discussed in Chapter 3 that V , the weighted average hydrodynamic volume of the free and complexed form of the chromophore is the quantity we extract from the experimental data. The OR in the data is actuall y the gradient of V, which is related to the induced charge gradient , f , as described in Chapter 3. The water concentration [C] H2O of the dry BMIM + BF 4 sample use d for this work was measured using a Quacounter AQ - 2100 Karl - Fischer coulometric analyzer . U sing this instrument, the BMIM + BF 4 - was found to have an ambient water concentration of ca. 500 ppm. A microsyringe was used to add specific amounts of water to the BMIM + BF 4 sample, to achieve [C] H2O of 1600 ppm, 3000 ppm, 5500 ppm, 7600 ppm, 10100 ppm, 12100 ppm and 24500 ppm. The rotational diffusion dynamics of CV + in these eight sample s are shown in F ig 5.3. From Fig 5. 3 we can see, for the dry sample ([C] H2O 500 ppm), the positive surface charge is meditated by electrons in the FTO thin film on the support, resulting in the sign and magnitude of f being controllable experimentally . This result agrees with results reported in Chapters 3 and 4 . When [C] H2O increases in the sample s , the magnitude of f decreases . However, the length scale of f is not affected by [C] H2O . f ceases to be observable for currents when the water concentration reache s 24500 ppm. 93 Figure 5 . 3 Depth - OR , of CV + as a function of d istance from the lower supporting FTO electrode. Data were acquired with the cell configured as shown in Fig 5.2. Currents applied ranged from 0 mA to 200 mA using FTO as the conducting film on the support plate with a water concentration of ca. (a) 500 ppm, (b) 1,600 ppm, (c) 3,000 ppm, (d) 5,500 ppm, (e) 7,600 ppm, (f) 10,100 ppm (g) 12,10 0 ppm and (h) 24,500 ppm in the 94 Figure 5.3 BMIM + BF 4 sample. 95 Figure 5.3 96 Figure 5.3 There is literature that consider how water interacts with RTILs, using experimental and computational ( Monte Carlo ) methods. Tran et al. 28 used near - infrared (NIR) spectroscopy to 97 show that interactions between water and RTILs are strongly d ependent on RTIL anion identity, for a given cation . W a ter appears to interact strongly with RTIL anions, leading to changes in the organization of the water . One consequence of this finding is that the organization of water within the RTIL will vary with RTIL iden tity . For BMIM + BF 4 , water interactions with BF 4 lead to stronger hydrogen bonds than are seen between water and bis(trifluoromethylsulfonyl)imide (Tf 2 N ) or h exafluorophosphate (PF 6 ). Guti é rrez et al. 38 carried out MD simulations f or binary systems of 1 - Ethyl - 3 - methylimidazolium tetrafluoroborate (E MIM + BF 4 ) and BMIM + BF 4 with several molecular solvents . Their computations showed that the molecular solvents interact preferentially with BF 4 whereas the interactions between the cation and the molecular solvent is less important. Zhang et al. 39 applied two - d imensional (2D) IR spectroscopy to investigate the dilution pr ocess of E MIM + BF 4 in H 2 O. Their data show ed that the interactions between RTIL cation and anion become wea ker with the additi on of H 2 O. During the dilution process, the three - dimensional (3D) n etwork structure of E MIM + BF 4 is gradually degraded into ion - cl usters, with t he ion - clusters then becoming predominantly ion - p airs surrounded by H 2 O for higher H 2 O concentrations . Even when H 2 O = 0.9, E MIM + BF 4 has not been fully dissociated into ions. Spickermann et al. 40 reported similar results using Molecular Dynamics (MD) simulations. Even for RTIL:H 2 O = 1:60 for 1 - Ethyl - 3 - methylimidazolium chloride (EMIM + Cl ), contact ion pair still e xist. In this Dissertation we have focused on the RTIL B MIM + BF 4 , where only a small fraction of the compact ion pairs are expected to dissociate. Dissociated BF 4 anions will bind with CV + to from the ion - paired complex CV + BF 4 . Induced charge density gradient f in the sample is thus the concentration gradient of free BF 4 . Based on the data we reported in this chapter, the increase in [C] H2O interferes with the abilit y of BF 4 - to form the complex with CV + , precluding the ability of 98 rotational diffusion experiments to sense a gradient in the fraction of free and complexed chromophore. This is not to say that the actual free charge density gradient has been affected by the presence of water (indeed, the experimental persistence length of the gradient is not seen to change), but the ability to sense the gradient is compromised. If the increased water concentration in the sample functioned as a simple diluent, the approac h to behavior described by the Gouy - Chapman - Stern model would manifest as a decrease in the characteristic length scale of the gradient, and this is not observed experimentally. It is thus important that another way to measure the charge density gradient be devised. 5.5 CONCLUSIONS AND FUTURE DIRECTIONS We report the effect s of water dilution on the induce d charge density gradient f in the RTIL B MIM + BF 4 by measuring the rotation al diffusion dynamics of CV + in the RTIL. The results showed the magnitude of the induced charge density gradient f is sen sitive to the presence of water, but the characteristic persistence length does not exhibit a water conce ntration - dependence over the concentration range examined (500 ppm 25,000 ppm). There are two possible reasons for this finding: Water molecules interact strongly with BF 4 interfering with the + . It is also pos sible that the addition of water decreases the viscosity of the RTIL, making the solution behaves more like dilute electrolyte solutions. This latter explanation is not consistent with the observed absence of a persistence length - dependence on water conce ntration. In order to determine the effects of water on the persistence length of the free charge density gradient, an alternative means of measuring the gradient is thus required. As noted in previous chapters, there is an analogy to be made between th e persistence length of the free charge density 99 gradient seen in RTILs and the existence of piezoelectric behavior in a solid material. While this analogy remains to be explored, one attribute of piezoelectric materials is that the unit cell does not poss ess a center of inversion and the material thus possesses a second order nonlinear optical response in the bulk. This is a property that can be measured experimentally, either through induced birefringence or the generation of second harmonic light. A po ssible alternative means for measuring the magnitude and persistence length of the induced charge density gradient in RTILs is to measure the intensity of second harmonic light produced as a function of system properties, such as charge density on the supp ort plate, identity of the RTIL, amount of water present in the RTIL, polarization of the incident electric field and orientation of the (charged) support plane with respect to the direction of the incident electric field. An immediate benefit to this app roach, if successful, is that the measurement of second harmonic light would not require the use of a chromophore in the RTIL. Rather, the measurement would sense organization in the RTIL directly. Miller has showed that t he second order polarization coe fficients that describe the nonlin ear interactions of laser beams with piezoelectric crystals vary from one material to another by several orders of magnitude. 41 By performing SHG measurements w e can further prove the existence of f and the effects of water concentration in the RTIL directly. Another approach to measurement of the free charge density gradient in RTILs is to use the established rotational diffusion experimental method, but with a different RTIL which has a hydrophobic anion (e.g. TfN 2 or PF 6 ). The idea behind this approach is to reduce water interference with the formation of the CV + chromophore with the RTIL anion. At some level, this approach would be limited by the interactions of water with either CV + or the RTIL constituents, and for this reason the demonstration of second harmonic generation as a means of characterizing f would be preferable. 100 101 REFERENCES 102 REFERENCE S 1. 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Applied Physics Letters 1964, 5 (1), 17 - 19. 105 CHAPTER 6: SUMMARY AND FUTURE WORK 106 6.1 SUMMARY OF THE DISSERTATION WORK The ultimate goal of this research is to und erstand dynamics, o rganization and the response t o external forces of room temperature ionic liquids (RTILs) . In previous chapters, we have demonstrated that the RTIL BMIM + BF 4 in contact with a charged surface ca n exhibit a free charge density gradient f Other experimental and theoretical investigations of RTILs have elucidated organization on ca. 10 While the 10 nm 1 - 3 4 - 5 organization have precedent in electrochemical and liquid crystal literature, the m in an ionic liquid is without precedent and needs to be understood. On physical grounds, ion ic interactions and the spatial extent of the electrostatic for ces associated with them cannot account for the longer - range order seen in RTILs. This situation, combine d with the knowledge that it can take minutes to hours for dipolar annealing to occur in thin RTIL films 4 , leads to the conclusi on t hat long - range induced order in RTILs bears a mechanis tic similarity to field - induced distorti ons in piezoelectric materials. We focus in this dissertation on what is u nderstood about organization in RTILs over the longest length scale because that is the most difficult to reconcile with the current understanding of RTILs and it also persists over distances that could be deterministic to how this family of materials is u sed in future applications. The spatial extent and functional f orm of the observed free charge density gradient f provides in sight into possible reasons for its existence. The se experiments are presently in progress and the ab ility to characterize induced f in RTILs will bring with it the development of a variety of novel application s such as e lectronically t unable i onic l iquid o ptics . 107 In C hapter 2 of this disser tation, we described the instrumental method to measure the rotational diffusion dynamics of fluorescent probe molecules and thus to examine local organization in RTILs. The f luorescence anisotropy decay depth profiling instrument is introduced as a means to spatially resolve variations in the fluorescence anisotr opy decay dynamics of the chromophores in RTILs . This instrument combines a time correlated single photon counting (TCSPC) laser system and an inverted confocal laser scanning microscope (CLSM), and provides ca. 50 ps time resolution , limited by the avalanche diode detectors and the TCSPC detection electronics. The time - resolved data generated from this instrument is related to the local environment of probe chromophore in RTILs . In Chapter 3, we report ed long range charge density gradient f in RTILs over a range of ca. 6 T he fluorescence anisotropy decay of the chromophores resorufin (anion), cresyl violet (cation) and nile red (neutral) was measured in the RTIL BMIM + BF 4 as a function of distance from the charged support. The instrument is equipped with an inverted confocal microscope thus d epth resolution can be achieved t hrough mechanical control over the microscope stage position. The chromophores exhibit depth - depe ndent reorientation dynamics th at depend on their ionic charge. For resorufin, OR ) vs. the distance from the silica support and the gradient is inverted for cresyl violet. And for nile re OR is independent of OR is the decay time constant of the anisotropy decay function R(t), which is related inversely to the rotational diffusion constant, D ROT . The observed distance - dependent dynamics differ ac cording to chromophore charge, implicating charge as a contributing factor. The chromophore dynamics in ethylene glycol (a viscous but non - ionic solvent) supported on the same charged silica surface are also measured as control experiments 108 and no depth - de pendence in those data is found . To evaluate the role of surface charge, the surface silanol functionalities is capped using dichlorodimethylsilane. Reacting the silica surface in this manner terminates surface silanol functionalities with neutral dimeth ylsilane groups, resulting in depth - independent anisotropy decay dynamics for all of the chromophores . The occurrence of spatially varying chromophore dynamics is thus unique to RTIL and is induced by the surface charges of silica support. The quantity in Debye - Stokes - Einstein (DSE) model 7 - 9 responsible for the observed charg e - and depth - OR in RTIL is the chromophore hydrodynamic volume ( V ). 8 Since the silica surface is negatively charged, there will be an excess of (dissociated) BMIM + in the RTIL near the surface compensating for the surface negative charge. This situation is somewhat analogous to the formation of an electric double layer in dilute solution except that for an RTIL there is no solvent medium and all constituents are either charged or are Bjerrum pairs capable of dissoc iation. The existence of one charge requires a compensatory opposing charge to maintain bulk electro - neutrality. The presence of a surface charge induces a gradient in RTIL dissociated constituent concentrations, giving rise to a gradient in the concentr ations of the free and complexed chromophores ( BMIM + Resorufin and Cresyl violet + BF 4 ) since t he chromophore is present at low concentration (typically ca. 10 - 5 M) , and this gradient is manifested as a gradient in the anisotropy decay time constant. With this understanding of the long range induced charge density gradient f in RTIL, we reported the applicable methods to modulate f in Chapter 4. 10 In order to gain control over the surface charge density we have constructed a closed cell that confines the RTIL between two transparent conductive surf aces ( fl uorine doped tin oxide (FTO) or indium doped tin oxide (ITO) ). Initial attempts to control surface charge density were through the potential difference 109 between the two conducting surfaces, based on the relationship Q = CV . However, depth - dependence of f does not change as a function of applied potential difference between the FTO - or ITO - coated plates . The reason is that only ca. 1 ppm of the ambient charge is changed with the application of a potential across the plates. Another way to control surface charge density is to use the individual FTO - or ITO - them ( I = Q/t ) . By controlling the current passed through the FTO or ITO , the charge density can be controlled in a range that is comparable to the native surface charge density of silic a. Thus conclusively control over f in the RTIL can be achieved. An important consideration in controlling f in this manner is understanding th e role that Joule heating plays in the observed effect. By measuring the temperature - dependent r otational di ffusion dynamics of cresyl violet ( CV + ) in ethylene glycol (EG) as a function of current applied to the ITO plate , the contribution of heating to the data can be quantitated. 11 - 12 There is no depth - OR for CV + for a given applied current and t here is a current - OR for CV + which is related directly to the change in temperature of the system through the known temperature - dependence of ethylene glycol visco sity. 13 The control experiments showed that Joule heating does not induce a thermal gradient and the temperature is constant at a given current across the ca. 1 mm film. The work described in Chapter 5 investigated the effects of water on the induced charge density gradient f in the RTIL BMIM + BF 4 . B y measuring the rotational diffusion dynamics of CV + in the RTIL using the resistive device cell schematic (Fig 4.1b), we showed that the magnitude of f is sen sitive to the presence of water. However, the characteristic persistence length does not exhibit a water concentration - dependence o ver the water concentration range of 500 ppm 2 5,000 ppm. The concentration of water in the RTIL sample is measured by Karl - Fisher titration method. Th e possible explanation for this finding is that w ater molecules interact 110 strongly with BF 4 + . The change of the viscosity of the RTIL is also considered since adding water decreases the viscosity of the RTIL, making the solution behave more like dilute electrolyte solutions. Howev er, this explanation contradicts the observed absence of a persistence length - de pendence on water concentration. As introduced in Chapter 1, four length scales of organization in RTILs have been revealed by e xperimental and theoretical methods. These are hydrogen bonded network , ca. 10 nm, ca. 1 . still a vague definition here. For the hydrogen bonded network organization, it is mainly formed between cations and anions of RTILs in the solid state, and it is maintained in the liquid phase to a significant extent and even in the gas phase . 14 roughly analogous to the electric double layer, present when RTILs are in co ntact with a charged surface. 1 - 3 4 gradient. 6, 10 The presence of any organization over such a range of length scales is highly unusual for a fluid medi um. electrochemical 15 and liquid crystal 16 - 17 literature, the existence of a free charge density gradient ithout precedent and requires a deeper understanding. The main focus of this work is necessarily on the long est range organization in RTILs because it is the most difficult to explain in the context of known models. The fact that there is a harge density gradient, however, cannot be accounted for in the framework of current models. The existence of such long - range order implies that the treatment of RTILs as 111 Th e work described in this dissertation provide s a ne w insight and a deeper understanding of RTILs, which is a prerequisite for using this family of materials most effectively and, more importantly, will provide a new way to think about ionic liquids and similar materials in future studies . 6.2 FUTURE WORK The w ork we reported in this dissertation shows promise in studying the induced long range charge density gradient in RTILs. Future studies can be carried out in different directions to complement the knowledge we have at this point. Second - Harmonic Generation (SHG) Measurements of Pure RTILs. As introduced in previous chapters, t he creation of f in the RTIL in response to the surface charge density , s , on the silica support is reminiscent of the response of piezoelectric materials to the presence of externa l force. For a material to exhibit piezoelectric behavior, it must be non - centrosymmetric. The application of an electric field across a piezoelectric material induces stress in the material, resulting in strain, wh ich is an induced polarization. Indu ced polarization can be measured optically, and the lowest order optical response that requires a non - centrosymmetric medium is second order optical nonlinearities. I t has already been established that RTILs exhibit a bulk second order nonlinear optical r esponse based on work by the Shaw group. 4 - 5 In analogy, we can measure the intensity of produced second harmonic light of RTILs as a function of system properties , such as the surface charge density etc. If this approach is applicable and successful , then the measurement of second harmonic light can be done without the us e of a chromophore in the RTIL, a nd organization in the RTIL can be sense d directly. 112 Piezoelectric Response of RTILs. A key assertion of this di ssertation is that the ca. 100 order seen in RTILs is not consistent with established models for free charge density gradients or any other type of order in liquid media. The existence of long - range organization that is influenced by the presence of in terfacial charge is described in the context of the piezoelectric effect. If an effect analogous to the piezoelectric effect is operative in RTILs, however, it should also exist when the RTIL is in the solid phase. Arguably the most direct means of measur ing the piezoelectric effect is with atomic force microscopy, a technique that can measure dimensional changes in (solid) materials as a result of a potential difference applied across the material. Temperature - dependent AFM is well - established and the in strumentation is available in the MSU Composite Materials and Structures Center (CMSC) surface science facility. By performing SHG measurements, the calibration for the piezoelectric response by AFM measurements and how this effect changes in magnitude an d spatial extent for temperatures above the RTIL melting point can be connected. The suite of measurements described above provide a direct means of evaluating the relative efficiency of the piezoelectric behavior of RTILs. This property is a direct cons equence of the ability to induce a free charge density gradient, f , in the RTIL. Dependence of f on RTILs Structures. The dependence of f on the chemical structure(s) of the RTIL anions and cations will depend on several factors. T he magnitude of f w ill be determined by the extent of dissociation of the RTIL, while the spatial extent of f will be determined by the mobility of the ionic species in the RTIL medium. The manner in which the identities of the RTIL cation and anion will influence f is thr ough the extent to which each RTIL ion pair dissociates. 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