2+ Cu -SELECTIVE FACILITATED ION TRANSPORT THROUGH PDCMAA/PAH MULTILAYER FILMS By Chunjuan Sheng A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Chemistry - Master of Science 2013 ABSTRACT 2+ Cu -SELECTIVE FACILITATED TRANSPORT THROUGH PDCMAA/PAH MULTILAYER FILMS By Chunjuan Sheng adsorption Layer-by-layer of poly[(N,N’-dicarboxymethyl) allylamine] (PDCMAA)/poly(allylamine hydrochloride) (PAH) films at low pH yields a thin film 2+ with abundant Cu -binding sites. When deposited on a porous alumina substrate, 2+ 2+ (PDCMAA/PAH)n films show average Cu /Mg diffusion dialysis selectivities of 50 and 80 for PAH-capped and PDCMAA-capped films, respectively. PDCMAA/PAH 2+ membranes also exhibit Cu /Ni 2+ 2+ 2+ and Cu /Ca 2+ 2+ selectivities. The high Cu /Mg selectivity despite similar aqueous diffusion coefficients and equal charge for the two ions suggests a facilitated transport mechanism. In contrast, PAA/PAH and PSS/PAH 2+ 2+ films show Cu /Mg selectivities <10. With PDCMAA/PAH films, Cu 2+ flux increases nonlinearly with increasing CuCl2 concentration in the feed. Sorption isotherms show that PDCMAA/PAH films contain both strong and weak binding sites, and the nonlinear increases in flux with increasing feed concentration likely represent hopping between weak binding sites, probably the amine groups of PAH. Strong binding of Cu 2+ to PDCMAA binding sites may displace ionic cross-links in the film and free amine groups for facilitated transport. Additionally, Cu 2+ 2+ binding to the film suppresses Mg transport, either through electrostatic exclusion or removal of hopping sites. To my family, for all the love and support iii ACKNOWLEDGEMENTS First I would like to thank my advisor, Dr. Merlin Bruening, for his patience and kindness throughout my graduate study. He was not only an insightful researcher, but also a great advisor. I learned a lot from him, about research, about writing, and also about being a nice person. I really appreciate all the encouragements and support. I would also like to express my gratitude to my fellow group members and friends in the department, for their help through all the qualification and research problems. With their companionship, all the time spent here has become such a delightful memory. I feel very lucky for all the love and trust from my family for every decision I’ve made through years. I’m especially grateful for the continuous support during the stressful times from my husband, Wen. Last but not least, many thanks to the readers of this thesis. iv TABLE OF CONTENTS LIST OF TABLES ......................................................................................................... vii LIST OF FIGURES ...................................................................................................... viii KEY TO ABBREVIATIONS ..........................................................................................x Chapter 1: Introduction and Background.......................................................................1 1.1 Layer by layer assembly of polyelectrolyte multilayer films ........................................1 1.1.1 Factors influencing polyelectrolyte multilayer growth ............................................2 1.1.2 Factors influencing polyelectrolyte multilayer film permeability ...........................4 1.1.3 Applications of polyelectrolyte multilayer films .....................................................8 1.2 Facilitated transport in membrane-based separations ...................................................9 1.2.1 Facilitated transport mechanism ..............................................................................9 1.2.2 Facilitated transport membranes ............................................................................10 1.3 Thesis outline ...............................................................................................................14 REFERENCES ..................................................................................................................16 Chapter 2: Facilitated Ion Transport through Polyelectrolyte Multilayer Films Containing Metal-binding Ligands ...............................................................................24 2.1 Introduction .................................................................................................................24 2.2 Experimental ...............................................................................................................26 2.2.1 Materials ...............................................................................................................26 2.2.2 Film preparation and characterization ..................................................................26 2.2.3 Diffusion dialysis ..................................................................................................28 2.2.4 Sorption studies .....................................................................................................29 2.3 Results and discussion ................................................................................................30 2+ 2.3.1 Preparation and characterization of Cu -binding PDCMAA/PAH films ...........30 2+ 2.3.2 Selective Cu transport through (PDCMAA/PAH)n films adsorbed on porous alumina ..................................................................................................................32 2.3.3 Comparison of fluxes and selectivities in mixed and single-salt diffusion through several types of polyelectrolyte multilayer films ..................................................35 2.3.4 PEM thickness and swelling .................................................................................40 2.3.5 Flux as a function of feed concentration ...............................................................42 2+ 2.3.6 Isotherm for Sorption of Cu in (PDCMAA/PAH)10-modified nanoparticles ...44 2.4 Conclusions .................................................................................................................46 REFERENCES ..................................................................................................................48 Chapter 3: Summary and Future Work .......................................................................53 3.1 Summary .....................................................................................................................53 v 3.2 Future work .................................................................................................................53 REFERENCES ..................................................................................................................57 vi LIST OF TABLES 50 * Table 2.1: Ion diffusion coefficients and equilibrium constants for formation of 51 ligand-metal ion complexes. ..........................................................................................33 2+ 2+ -10 -2 -1 2+ 2+ Table 2.2: Cu and Mg fluxes (10 mol cm s ) and Cu /Mg selectivities in diffusion dialysis through bare and PEM-modified alumina membranes. Dialysis a employed either single- or mixed-salt solutions in the feed. ...........................................37 2+ -10 -2 -1 2+ -10 -2 -1 a Table 2.3: Mg fluxes (10 mol cm s ) as a function of feed pH during diffusion dialysis through PEM-modified porous alumina membranes. ..........................................39 Table 2.4: Mg fluxes (10 mol cm s ) in sequential diffusion dialysis with single and mixed salts. .................................................................................................................40 vii LIST OF FIGURES Figure 1.1: Schematic illustration of layer-by-layer deposition of oppositely charged polyelectrolytes on a planar substrate. A) Experimental procedure for layer-by-layer deposition, B) cartoon of polyelectrolyte film growth. (Used by permission of American Association for the Advancement of Science from Science, 1997, 277, 1232-1237). For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this thesis. .................................................................................2 Figure 1.2: Cross-sectional FESEM image of porous alumina substrates before (A) and after (B) deposition of 10 (PAH/poly (sodium styrene sulfonate) (PSS)) bilayers. (Used by permission of the American Chemical Society from Chem. Mater. 2000, 12, 19411946). ..................................................................................................................................6 Figure 1.3: Illustration of a simplified model of ion transport through a PEM. The film consists of two layers, a highly charged surface layer and a mostly charge compensated film bulk layer. The line represents a hypothetical concentration profile for the excluded ion. (Reproduced with permission of the American Chemical Society from Macromolecules 2002, 35, 3171-3178). .............................................................................7 Figure 1.4: Cartoon of facilitated transport through a (left) mobile-carrier liquid membrane and (right) fixed-carrier polymer membrane. The orange smiley faces stand for facilitated species selected by the carriers, and green circles stand for nonbinding species. (Adapted with permission of Elsevier from J. Membr. Sci. 2001, 181, 97-110)...10 Figure 1.5: Schematic drawing of layer-by-layer assembly of anionic macrocycles with cationic polyelectrolyte (top) and cationic macrocycles with anionic polyeletrolytes (bottom) on a porous support. (Reproduced with permission of Elsevier from Thin Solid Films 2008, 516 (24), 8814-8820). ...................................................................................14 Figure 2.1: Structures of the polymers employed to prepare PEMs. ................................30 2+ Figure 2.2: UV-VIS spectra of 1 mM Cu in water or aqueous solutions containing various polyelectrolytes. The concentration of the polyelectrolyte repeat units was 10 mM, and the solution pH was 3.6. ....................................................................................31 Figure 2.3: Reflectance FT-IR spectra of a (PAH/PDCMAA)3.5 film on Au wafter before and after immersion in 0.1 M CuCl2 solution (pH=3.6), and after subsequent immersion in 0.1 M EDTA solution (pH=6.4). .................................................................32 viii Figure 2.4: Evolution of permeate concentrations with time during diffusion dialysis of 0.1 M CuCl2, 0.1 M MgCl2 solutions through (PDCMAA/PAH)n-modified porous alumina membranes. The permeate initially contained deionized water. Filled and open symbols represent dialysis through (PDCMAA/PAH)4and (PDCMAA/PAH)3PDCMAA-modified membranes, respectively. .................................34 Figure 2.5: Evolution of permeate concentrations with time during diffusion dialysis of 0.1 M CuCl2, 0.1 M MCl2 (M=Mg, Ca or Ni) solutions through (PDCMAA/PAH)4modified porous alumina membranes. The permeate initially contained deionized water. 2+ 2+ This membrane was the least selective (Cu /Mg = 20) of those we examined. .........35 Figure 2.6: Ellipsometric thicknesses (Å) of PEMs adsorbed on Au wafers modified with MPA monolayers. The columns in each series stand for (from left to right): dry thickness in air, thickness under deionized water after a 1-h or a 2.5-h immersion (pH=6.4), thickness under deionized water after a 1-h immersion in 0.1 M CuCl2 (pH=3.6) followed by rinsing with deionized water, and finally the thickness under deionized water after a subsequent 1-h immersion in 0.1 M EDTA (pH=6.4) followed by rinsing with water. ...42 2+ Figure 2.7 (Left) Cu fluxes through a (PDCMAA/PAH)4 film deposited on porous 2+ 2+ alumina. (Right) Cu and Mg fluxes for 3 (PDCMAA/PAH)4 films deposited on 2+ 2+ porous alumina. A fourth membrane showed much lower fluxes for both Cu and Mg , but the flux trends remained the same. .............................................................................43 2+ 2+ Figure 2.8: Cu and Mg diffusion dialysis fluxes through porous alumina coated with (PAA/PAH)4 (Left) and (PSS/PAH)4 (Right) films. Large error bars in the flux arise from the defects in manufactured skin layer of commercial alumina membranes which 58 may lead to variation in fluxes through the PEM film. However, each individual membrane exhibited similar evolution of flux with feed concentration. ..........................44 2+ Figure 2.9: Adsorption isotherm for Cu binding to (PDCMAA/PAH)10-modified nanoparticles. The inset is an expansion of the lower concentration range. Adsorption incubation time was 14 hours at each concentration, and each point represents a fresh set of modified nanoparticles. ................................................................................................46 Figure 3.1 EDC-NHS amine coupling for modification of carboxylic acid-containing polyelectrolytes. ................................................................................................................54 Figure 3.2 Cu(I) catalyzed alkyne-azide cycloaddition. Use of the chemistry for PEM modification will require polyelectrolyte with azide or alkyne groups. ...........................55 Figure 3.3 Structures of lanthanide-binding compounds. ................................................56 ix KEY TO ABBREVIATIONS PEM polyelectrolyte multilayer LBL layer by layer PAH poly(allylamine hydrochloride) PAA poly(acrylic acid) FESEM field-emission scanning electron microscopy PSS poly(sodium 4-styrenesulfonate) PVS poly(vinylammonium) PVA poly(vinylsulphate) NF nanofiltration RO reverse osmosis PDCMAA poly[(N,N’-dicarboxymethyl) allylamine] IDA iminodiacetic acid MPA 3-mercaptopropionic acid ICP-OES inductively coupled plasma optical emission spectroscopy AAS atomic absorption spectroscopy EDTA ethylenediaminetetraacetate acid UV-Vis ultraviolet-visible FT-IR fourier transform infrared spectroscopy en ethylenediamine x Chapter 1 Introduction and Background This thesis describes a metal-binding membrane prepared by layer-by-layer deposition of polyelectrolytes on a porous substrate and demonstrates facilitated ion transport through this membrane using Cu 2+ and other divalent cations as probes. These studies build on a large body of research on polyelectrolyte multilayer films and facilitated transport membranes. To put this work in perspective, this introduction describes layer-by-layer film formation and the factors influencing film growth and permeability, and then reviews the applications of such films in various fields, especially ion separations. A subsequent section contains an overview of facilitated transport mechanisms in systems ranging from liquid membranes to polymer inclusion membranes. Finally, I present an outline of the thesis. 1.1 Layer by layer assembly of polyelectrolyte multilayer films Decher et al. introduced layer by layer (LBL) deposition of complementary polymers in the early 1990’s, 1,2 and this technique has become one of the most attractive strategies for synthesizing functional thin films. In one of its simplest forms, the deposition procedure features a dip-and-rinse process, during which the selected substrate undergoes alternating immersions in polycation or polyanion solutions, with solvent rinsing to remove excessive polymer after each immersion (Figure 1.1). The substrates suitable for film deposition include planar supports, nanoparticles. 9-11 3-5 porous membranes, 6-8 and Moreover, the constituents of the multilayers can range from the most 1 common polyelectrolytes to other charged species such as proteins, nanoparticles, 15-18 and dyes. 19-21 12-14 colloidal In addition to electrostatic interactions between polycations and polyanions, other interactions that may facilitate LBL film formation include hydrophobic interactions, and covalent bonding 8,22,23 hydrogen bonding, 24-26 27,28 π-π interactions 29-37 . Importantly, the thickness and permeability of LBL films depend on the film constituents, number of layers, and deposition conditions. 38-41 A 1 2 3 4 Substrate B 1. Polycation 3. Polyanion 2. Wash 4. Wash Figure 1.1 Schematic illustration of layer-by-layer deposition of oppositely charged polyelectrolytes on a planar substrate. A) Experimental procedure for layer-by-layer deposition, B) cartoon of polyelectrolyte film growth. (Used by permission of American Association for the Advancement of Science from Science, 1997, 277, 1232-1237). For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this thesis. 1.1.1 Factors influencing polyelectrolyte multilayer growth 2 The growth of polyelectrolyte multilayer films relies in many cases on charge overcompensation that reverses the substrate’s surface charge after each adsorption step. 42-44 The extent of charge overcompensation depends on deposition conditions such as the type and molecular weight of polyelectrolyte, density, 49 supporting electrolyte species and concentration, 45-48 50,51 polyelectrolyte charge dipping solution pH 41,52 and adsorption time. Adjustment of deposition conditions can tailor film properties such as swelling, 50 thickness, 52 48 and permeability for different applications. Supporting electrolytes affect the conformations of polyelectrolytes in both the dipping solution and the film to alter film structure. 45,53,54 In the absence of salt polyelectrolytes extend to minimize the electrostatic repulsion between the charged groups. In contrast, high salt concentrations screen charges on the polyelectrolyte, such that the polymer adopts conformations with loops and tails. 43,45 The variations in conformations of polyelectrolytes lead to a difference in the degree of surface-charge overcompensation and the interdiffusion of polymers, and finally result in dramatic differences in PEM film thicknesses. 55-57 The choice of salt for the supporting electrolyte also affects the film thickness and permeability. 45,58 Even with similar concentrations of supporting electrolyte, the morphology, thickness, and permeability of PEMs vary dramatically with the composition of the constituent polyelectrolytes. For example, high charge density on a given polyelectrolyte leads to extended polymers and high densities of ionic cross-links to give thin films with low permeabilities. 59-61 growth patterns. Sun et al. Polyelectrolyte molecular weight also contributes to film 62 demonstrated an exponential growth (thickness increases 3 exponentially with the number of adsorption steps) of poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) films with low molecular weight PAA (7, 15 or 50 kDa) in contrast to linear film growth with high molecular weight PAA (90 kDa). Confocal microscopy combined with fluorescently labeled PAA showed that low molecular weight polyelectrolyte penetrates or diffuses into the film over large distances. However, low molecular weights can also lead to film instability. Whereas high molecular weight polyelectrolytes (100 kDa) may exhibit kinetic irreversibility, 42 low molecular weight polyelectrolytes can leach from films when exposed to oppositely charged polyelectrolytes during deposition. 63 Polyelectrolyte concentrations 64,65 generally have a relatively small effect on film growth. For weak polyelectrolytes such as PAA, film growth strongly depends on the dipping solution pH. In the early 1990’s Rubner et al. 66 found a 2-fold decrease in the thickness of (PAH/PAA)30 films when the deposition pH was 4.5 rather than 2.5. The decrease in film thickness stems from the increased ionization of PAA at pH 4.5. The higher charge density at the higher deposition pH results in more extended polymer chains and thinner films. More recent studies confirm these results. 41,52,67-69 Adsorption times less than those required for equilibration should also alter film properties. A few studies show that 95% of the adsorption typically occurs during the first 1 or 2 min of exposure to the polyelectrolyte solutions. 70,71 Thus the immersion time in this thesis is typically 5 min to minimize variations from incomplete equilibration. 1.1.2 Factors influencing polyelectrolyte multilayer film permeability 4 Membrane-based separations are attractive because of their operational convenience and low energy cost, but efficient separations of molecules or ions require membranes with both high selectivity and high permeability. Unfortunately, membranes that exhibit high selectivity usually have low permeability, and vice versa. To some extent, composite membranes overcome this tradeoff by employing ultrathin, selective layers that allow high flux due to their minimal thickness. A highly permeable support layer provides mechanical stability for the composite membrane. 72 Polyelectrolyte multilayer films can cover the surface of a highly permeable membrane support without filling the underlying pores to create the ultrathin skin of a composite membrane. The field-emission scanning electron microscopy (FESEM) images in Figure 1.2 clearly show the formation of a complete polyelectrolyte skin on a porous support. 60 Layer-by-layer deposition of polyelectrolytes can take place on a variety of supports with a number of different polyelectrolytes and deposition conditions to create a wide range of membrane properties. Additionally, changing the film growth conditions (see the previous section) can tailor membrane permeabilities and selectivities. 5 Figure 1.2 Cross-sectional FESEM image of porous alumina substrates before (A) and after (B) deposition of 10 (PAH/poly (sodium 4-styrenesulfonate) (PSS)) bilayers. (Used by permission of the American Chemical Society from Chem. Mater. 2000, 12, 19411946). As a result of the charge overcompensation during film growth, PEMs usually contain a highly charged surface layer in addition to a neutral bulk film covering the porous support (see Figure 1.3). 73 With their immobile surface charge that varies with polyelectrolyte type and deposition conditions, PEM films are particularly attractive for the separation of ions with different valences. Ion transport through the PEM depends not only on size exclusion from the film structure but also on the electrostatic potential due to the fixed surface charge. 61 Variations in the polyelectrolyte type and number of bilayers deposited dramatically change the ion permeability by changing the surface charge density, film thickness and swelling, and consequently the selectivity among different ions. 57,60,61,74 For example, changing the capping layer of a (PSS/PAH) n film from PSS to PAH gives significantly different salt rejections in nanofiltration, from 86 to 96% 2+ for Ca 2- and from 56 to 35% for SO4 , mainly because of the change in film surface 6 2- charge. Similarly, PAA-capped and PSS-capped films exhibit distinct SO4 rejections of 56% and 92% due to the different surface charge density. Concentration Porous Support Film Bulk 75 Surface Layer Permeate Feed Nernst-Planck Transport Figure 1.3 Illustration of a simplified model of ion transport through a PEM. The film consists of two layers, a highly charged surface layer and a mostly charge compensated film bulk layer. The line represents a hypothetical concentration profile for the excluded ion. (Reproduced with permission of the American Chemical Society from Macromolecules 2002, 35, 3171-3178). In addition to the optimization of ion separations by changing the polyelectrolyte type and number of bilayers in a PEM, post-deposition crosslinking and variation of fixed charge density can also control ion transport. Toutianoush et al. + Na /Mg 2+ 76 improved the transport selectivity through a poly(vinylammonium)/poly(vinylsulphate) (PVS/PVA)60 film by adsorbing Cu 2+ ions in the film, allegedly increasing the crosslink density in the membrane. Capping of a (PAH/PSS) 5 film with (PAA/PAH) layers and - subsequent heat-induced crosslinking enhanced the Cl /SO4 7 2- transport selectivity from 7 60 for a (PAH/PSS)5 film crosslinked at 115 C. 74 to as high as 360 for a (PAH/PSS)5(PAA/PAH)2.5 hybrid film Balachandra et al. 73,77 - 2- improved the Cl /SO4 selectivity of PAA/PAH films by increasing the film charge density through templating with Cu during deposition and subsequent removing the Cu 2+ 2+ in pH 3 water to create a negatively charged film. Similarly, Dai et al. partially modified polyelectrolytes with photolabile groups, and postdeposition removal of the protecting groups from PEMs led to charged films with increased selectivities. 1.1.3 Applications of polyelectrolyte multilayer films With their notable advantages of simple deposition, versatile functionality, and tunable thickness and permeability, PEMs have found a broad spectrum of potential applications in fields such as drug delivery, immobilization, membrane reactors, and ion separations. 59,74,94,95 84,85 fuel cells, 78-80 86-88 81,82 enzyme and liquid, 89-91 or catalyst 83 91-93 molecule PEMs are especially attractive in nanofiltration (NF) because of their nanometer-scale thickness and selective permeabilities for monovalent over multivalent ions. Nanofiltration is a pressure-driven process similar to reverse osmosis (RO), but it requires a lower operating pressure than RO and thus consumes less energy. For applications such as water softening that do not require high rejections of monovalent ions, NF is preferable to RO. Previous studies showed high transport selectivities for monovalent ions over multivalent ions with PEMs deposited on porous - supports. Selectivity can occur with both anions (Cl /SO4 - Cl /[Fe(CN)6] 3- as high as 310 60 2- 74 selectivity as high as 360 , + 2+ in diffusion dialysis) or cations (Na /Mg 8 NF 59 selectivity of 22 ). The selective removal of divalent ions from monovalent ions effectively reduces the osmotic pressure required in water softening process and thus decreases energy costs. 1.2 Facilitated transport in membrane-based separations Conventional methods for the removal and recovery of heavy metals from waste streams include chemical precipitation, exchange. 99 96 adsorption, 97 solvent extraction, 98 and ion Unfortunately, these methods often suffer from low efficiency, high capital costs and sensitive operating conditions. 100,101 Membrane-based processes such as reverse osmosis, nanofiltration, and electrodialysis can potentially provide much simpler remediation methods. 102,103 However, most membrane separations depend on size exclusion or electrostatic exclusion to achieve selectivity and are thus not particularly selective among similarly charged ions. Facilitated transport membranes have emerged as a promising technique for the highly selective separation of some specific ions such as copper, zinc, cobalt, nickel, gold, silver and lanthanides. 104,105 1.2.1 Facilitated transport mechanisms Facilitated transport membranes contain mobile or fixed carriers that selectively and reversibly interact with one ion, the facilitated species, in a mixture. The carriers facilitate transport of the binding species through the membrane to provide high selectivity if other transport mechanisms such as solution-diffusion are slow. 106 A variety of carriers can facilitate different separation processes, e.g. quaternary or tertiary 9 amines, pyridine and derivatives, hydroxyquinoline, carboxylic acids, phosphoric acid esters, crown ethers and calix arenes. 107 Figure 1.4 illustrates facilitated transport with both fixed and mobile carriers. The flux of the facilitated species depends on both the concentration gradient across the membrane and the carrier concentration. Flux increases with increasing feed concentration until the facilitated species saturate the carriers and facilitated transport reaches a maximum rate. In fixed-carrier membranes with different carrier concentrations, a percolation threshold in terms of carrier concentration may appear if the transport occurs only when two carriers are close enough to transfer the facilitated species. Feed Feed A Mobile Carrier Liquid 108-110 B Polymer Flux Flux Fixed Carrier Binding Species Non-binding Species Permeate Permeate Figure 1.4 Cartoon of facilitated transport through a (left) mobile-carrier liquid membrane and (right) fixed-carrier polymer membrane. The orange smiley faces stand for facilitated species selected by the carriers, and green circles stand for nonbinding species. (Adapted with permission of Elsevier from J. Membr. Sci. 2001, 181, 97-110). 1.2.2 Facilitated transport membranes 10 Li et al. 111 first reported the use of a liquid surfactant membrane for the separation of different hydrocarbons via facilitated transport. A liquid membrane contains a thin organic liquid film that separates aqueous feed and receiving phases. Carriers dissolved in the liquid-phase membrane control the permeation of different ions from the feed to the receiving side, enabling higher carrier diffusivity than in solid membranes. 106 The carrier interacts with specific ions and shuttles them through the membrane, as Figure 1.4A illustrates. Several types of liquid membranes, including emulsion liquid membranes, bulk liquid membranes, and supported liquid membranes, can facilitate the separation of metal ions. 112 In supported liquid membranes immobilization of the organic phase in the microporous structure of a supporting material through capillary forces increases stability, 113 however the loss of carriers and the organic phase remains the main obstacle in the technical implementation of liquid membranes. 106,113 Casting of carriers along with plasticizers in polymer films yields polymer inclusion membranes that may eliminate the challenges of carrier and organic phase loss in liquid membranes. Several studies reported extraction and recovery of ions such as zinc, cadmium, lead and copper using polymer inclusion membranes containing carriers such as crown ethers, and phosphoric acids. 107,114-116 However these membranes are usually much less permeable than liquid membranes because of the high viscosity and hydrophobicity of the membrane. Another approach to create stable, carrier-containing membranes employs covalent attachment of the carriers to the backbones of the matrix polymer. In these fixed-carrier membranes, transporting species hop through the membrane from site to site as a result of the thermal motions of the polymer chains (see 11 Figure 1.4B). 106 Yoshikawa et al. studied halogen ion transport through synthetic polymeric membranes containing pyridine carriers in the 1980s. 117 Other studies examined fixed-carrier transport of ions such as europium, iron, zinc, potassium and sodium with carriers such as phosphonate esters, phosphoric acids, phosphonate esters and pseudo crown ethers. 118-120 However the limited selectivity (<5) among different ions plagued the separation process. Molecular imprinting has emerged as an attractive strategy to form membranes with well-defined morphologies and pore structures tailored for highly specific separations. 121 An ultrathin, imprinted layer on a highly porous support should lead to increased fluxes with imprinted films. Deng et al. 122 prepared an ion-selective membrane by cross-linking a polyelectrolyte film (on an ultrafiltration support) in the presence of a template ion. The minimal thickness of the imprinted film afforded improved ion flux, 2+ 2+ however the selectivity for Cu /Zn 2+ through a Cu -imprinted membrane was below 5 and decreased with increasing feed concentration. Layer by layer deposition also provides a promising method for including specific metal-binding ligands in ultrathin membrane skins. Carrier incorporation can occur through electrostatic adsorption (in the case of charged carriers) or covalently bonding carriers to the polyelectrolyte backbone before deposition. Tieke et al. 40,123-126 studied selective ion transport through layer-by-layer assembled calixarene/polyelectrolyte membranes, as well as through similar membranes with other macrocyclic compounds such as azacrowns and cyclodextrins (see figure 1.5). Interestingly, they found retarded transport for ions that interact with the complexing agent. Ions that specifically interact 12 with different calixarenes showed as much as a 2.9-fold lower permeation rate through the calixarene/polyelectrolyte membrane compared to the all-electrolyte PSS/PVA + membrane. Examples of ions that experienced retarded transport include Li through calix4/PVA, Mg calix8/PVA. 40 2+ through calix6/PVA, and transition metal and lanthanide ions through However the displacement of calixarene polyanions by divalent sulfate ions occured, which could cause a loss in ion selectivity or film desorption over time. And the ions with same charge, either monovalent or divalent, exhibited selectivities limited to below 5. 13 B,A,B B,A,B A B Multilayer Film Multilayer Film A A: cationic polyelectrolyte B: anionic p-sulfonato-calix[n]arene A B A A: anionic polyelectrolyte B: cationic polyazacronw ether or aminocyclodextrin Figure 1.5 Schematic drawing of layer-by-layer assembly of anionic macrocycles with cationic polyelectrolyte (top) and cationic macrocycles with anionic polyeletrolytes (bottom) on a porous support. (Reproduced with permission of Elsevier from Thin Solid Films 2008, 516 (24), 8814-8820). 1.3 Thesis outline This thesis examines whether selective, facilitated transport can occur though polyelectrolyte multilayer nitrilotriacetate and amines. that contain metal-binding functionalities such as The minimal thickness (<50 nm) of PEMs on porous support should lead to high fluxes even with fixed carrier transport. Specifically, I first 14 describe the layer-by-layer deposition of poly[(N,N’-dicarboxymethyl) allylamine] (PDCMAA)/poly(allylamine hydrochloride) (PAH) films at low pH to give a thin film 2+ with abundant Cu -binding sites. Subsequent diffusion dialysis studies show that when deposited on a porous alumina substrate, (PDCMAA/PAH) n polyelectrolyte multilayer 2+ (PEM) films have average Cu /Mg 2+ selectivities of 50 and 80 for PAH-capped and PDCMAA-capped films, respectively. PDCMAA/PAH membranes also exhibit 2+ 2+ Cu /Ni 2+ 2+ and Cu /Ca 2+ 2+ selectivities. The high Cu /Mg selectivity despite similar aqueous diffusion coefficients and equal charge for the two ions suggests a facilitated transport mechanism. 2+ In contrast, PAA/PAH and PSS/PAH films show Cu /Mg selectivities <10. With PDCMAA/PAH films, Cu 2+ 2+ flux increases nonlinearly with increasing CuCl2 concentrations in the feed. In typical facilitated transport flux initially increases with feed ion concentration and then reaches a maximum value upon carrier saturation. To investigate the reasons behind the nonlinear relationship between flux and 2+ feed ion concentration, I present sorption isotherms for Cu . These isotherms show that PDCMAA/PAH films contain both strong and weak binding sites, and the nonlinear increases in flux with increasing feed concentration likely represents hopping between weak binding sites, probably the amine groups of PAH. Strong binding of Cu 2+ to PDCMAA binding sites may displace ionic cross-links in the film and free amine groups for facilitated transport. Additionally, Cu 2+ binding to the film suppresses Mg either through electrostatic exclusion or removal of hopping sites. 15 2+ transport, REFERENCES 16 REFERENCES (1) Decher, G. Science 1997, 277, 1232-1237. (2) 835. Decher, G.; Hong, J. D.; Schmitt, J. 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Previous studies investigated the transport properties of PEM-coated membranes in pervaporation, separations with dissolved ions. 19-22 11-16 gas separation, 16-18 23-25 In particular, in nanofiltration and various PEMs allow selective transport of monovalent ions over multivalent ions, which is important in water softening. 18,25 The monovalent/divalent ion selectivity may stem from differences in ion hydration or electrostatic exclusion. 2+ similar hydrated radii, e.g. Cu 26 However, for ions with the same charge and 2+ and Mg , PEMs will likely show minimal selectivity. Nevertheless with appropriate selectivity and permeability, membrane-based processes should be attractive for such separations due to their high efficiency and low energy cost 27,28 compared to the conventional methods such like precipitation, evaporation, 32 ion exchange, 33 34,35 adsorption or solvent extraction. 29,30 flotation, 31 36,37 Facilitated transport through membranes can address the challenge of obtaining selectivity when separating ions with the same charge and similar hydrated radii. This type of transport relies on ion complexation in the membrane and subsequent transport 24 either by ligand diffusion across the membrane or ion hopping between immobile binding sites. 38-41 The development of facilitated transport membranes has progressed from 42 liquid membranes 45 membranes to supported liquid membranes, and molecularly imprinted membranes. 46 43,44 to polymer inclusion However, most membranes still suffer from limited stability, selectivity or permeability. PEM membranes may overcome some of the stability issues that plague liquid membranes, and the minimal thickness of these films will enhance permeance relative to thicker solid membranes. Development of facilitated transport through PEMs will require polyelectrolytes that bind metal ions and allow hopping between binding sites. Additionally, the rate of simple diffusion through these films must be much lower than the rate of facilitated transport. Tieke et al. 4,19,47-49 reported the formation of membranes through alternating adsorption of charged macrocyclic compounds, e.g. calixarenes, azacrowns and cyclodextrins, and oppositely charged polyelectrolytes. However, ions that specifically interacted with the macrocycles passed through these membranes more slowly than ions that did not interact with the macrocycles. Additionally, the selectivities among ions with the same charge, either monovalent or divalent, were <5. This paper reports the formation of membranes through layer-by-layer adsorption of poly[(N,N’-dicarboxymethyl) allylamine] (PDCMAA) and poly(allylamine hydrochloride) (PAH). PDCMAA contains many iminodiacetic acid (IDA) groups that 2+ should strongly bind Cu , and at suitably high concentrations, Cu 2+ may also interact 2+ with the amine groups of PAH. Remarkably, these membranes show Cu /Mg diffusion dialysis selectivities around 25 50. We compare transport 2+ through (PDCMAA/PAH)4, (poly (acrylic acid) (PAA)/PAH)4, and (poly (sodium 4-styrenesulfonate) (PSS)/PAH)4 films, and only films containing PDCMAA show selectivities >10. Measurements of transport rates as a function of feed concentration and Cu 2+ sorption isotherms suggest that facilitated transport occurs via the amine sites in (PDCMAA/PAH)4 films. 2.2 Experimental 2.2.1 Materials Poly(allylamine hydrochloride) (PAH, Mw=120,000~200,000 Da) and poly(acrylic acid) (PAA, Mw=100,000 Da) were purchased from Polysciences. Poly(sodium 4-styrenesulfonate) (PSS, Mw=70,000 Da), 3-mercaptopropionic acid (MPA) and were obtained from Sigma-Aldrich. Ethylenediaminetetraacetate acid disodium salt (EDTA-Na2) was purchased from Jade Scientific. All reagents were used without further purification. Deionized water (18 MΩ· Milli-Q) was used to prepare all the aqueous cm, solutions. Alumina membrane supports (Anodisc, pore diameter = 0.02 μm) were purchased from Whatman. Colloidal silica nanoparticles (70-100 nm, SNOWTEX-ZL) were purchased from Nissan Chemical Industries, Japan. 2.2.2 Film preparation and characterization PEM films were first deposited on Au-coated (200 nm of Au sputtered on 20 nm of Cr on Si(100)) wafers. The Au-coated wafer was cleaned with UV/ozone for 15 min before a 26 30-min immersion in MPA solution (5mM), followed by rinsing with ethanol and then water for 1 min each, and drying under a N2 stream. The MPA-modified wafer was immersed in a PAH solution for 5 min, rinsed with water from a squirt bottle for 1 min, immersed in a polyanion solution (PSS or PAA or PDCMAA) for 5 min, and rinsed again. The dip-and-rinse process was continued to deposit the desired number of polyelectrolyte bilayers. Adsorption of PEMs on porous alumina membranes followed essentially the same procedure starting with the polyanion. The alumina membrane was placed in a holder that exposed only the top of the membrane to the solutions. All polyelectrolyte solutions contained 0.01 M polymer repeating unit and 0.5 M NaCl. The pH of these solutions was adjusted to the desired pH of 3 with 0.1 M HCl or 0.1 M NaOH. In modification of silica nanoparticles, 500 mL of a 0.02 M PAH solution (pH=3) was added to 1 g of silica colloid suspension. The samples were sonicated for 15 min, and the adsorption solution was left to stand for a minimum time of 30 min with continuous stirring. The solution was then centrifuged for 30 min at 6000 rpm. After the supernatant was removed, 500 mL of water was added to the sample, and the solution was sonicated for 5 min. The supernatant was removed again to rinse the unabsorbed polyelectrolyte from the colloids. 500 mL of a 0.01 M PDCMAA solution (pH=3) was then added to the remaining colloidal solution. Similar adsorption and washing steps were performed until films of ten PAH-PDCMAA bilayers had been prepared. Thicknesses of films deposition on Au-coated wafers were determined using a rotating analyzer ellipsometer (J.A. Woollam model M-44), assuming a 1.5 refractive 27 index for the dry films. Films in water have a smaller refractive index ranging from 1.3 to 1.5. Fourier transform infrared spectroscopy (FT-IR) spectra of these films were obtained using a Thermo Scientific Nicolet 6700 FT-IR spectrometer (80ºincident angle in a Pike grazing angle holder) with a MCT detector. A UV/ozone cleaned Au-coated wafer served as a background. Ultraviolet-visible (UV-Vis) spectra were acquired with a PerkinElmer UV/VIS spectrophotometer (Lambda 25). 2.2.3 Diffusion dialysis Diffusion dialysis studies were carried out in a home-made apparatus that consists of feed and permeate chambers separated by a membrane with the PEM facing the feed 8 solution. Initially, 90 mL of salt solution and 90 ml of deionized water were added to the feed and permeate chambers, and the two solutions were stirred vigorously. For analyses, one-mL aliquots were removed from the permeate approximately every 5 min for 40~60 min. To balance the water level in the two chambers, one mL of feed solution was simultaneously removed from the feed. The permeate aliquots were diluted 10-fold with 2% nitric acid and analyzed by Inductively Coupled Plasma Optical Emission Spectroscopy (Axial ICP-OES, Varian 710-ES ). Over the course of the experiment, the salt concentration in the permeate is always small compared to that in the feed so the concentration gradient and the flux across the membrane should be constant. Thus the slope in a plot of permeate ion concentration versus time can be used to calculate the ion 2 flux, taking into account the permeate volume and membrane area (2.1 cm ). 28 2.2.4 Sorption studies Sorption studies were carried out with (PDCMAA/PAH)10-modified nanoparticles. The high surface area of the nanoparticles and large number of bilayers gives the high number of binding sites needed for these studies. The modified nanoparticles were dried and ground into a fine powder with a mortar and pestle (dried, modified nanoparticles tend to aggregate). Weighed amounts of nanoparticles (0.1 g to 0.4 g) were then mixed with fixed volumes (1 mL or 5 mL) of source solutions with varied Cu 2+ concentrations, and incubated overnight at room temperature. The mass of the nanoparticles and solution volume were chosen to achieve sorption of at least 20% of the Cu solution. Sorption of Cu 2+ 2+ in the loading on bare nanoparticles was 10-20% of that on the modified particles. The total sorption was determined from ICP-OES analysis of the Cu 2+ concentration in the source solution before and after sorption. After sorption, the residual solution was decanted and the nanoparticles were rinsed with 1.5 ml of deionized water three times to remove the remaining solution and the weakly adsorbed ions. These particles were dried under vacuum and subsequently mixed with 1 mL of 0.1 M EDTA (pH=6.4) and incubated overnight. The resulting eluate was diluted and analyzed by atomic absorption spectroscopy (AAS, Varian AA240) because EDTA may precipitate in acid and clog the ICP sampling system which typically employs 2% nitric acid as a solvent. The Cu 2+ adsorption calculated from either the loading or eluate solutions was normalized by the initial mass of nanoparticles and then plotted against the equilibrium (residual loading solution) concentration to give the sorption isotherm. 29 2.3 Results and discussion 2+ 2.3.1 Preparation and characterization of Cu -binding PDCMAA/PAH films Figure 2.1 Structures of the polymers employed to prepare PEMs. Figure 2.1 shows the structure of the polymers we employed to create thin polyelectolyte films on porous alumina supports. Partial deprotonation of PAA and 26 PDCMAA allows their adsorption as polyanions. Prior studies demonstrated adsorption of PEMs using PAH as a polycation and PSS, PAA, or PDCMAA as the polyanion. For both PAA and PDCMAA, film thickness varies with adsorption pH, and we chose to deposit these films at pH 3 both to achieve a relatively high thickness and to create free – COOH groups for subsequent metal-ion complexation. At this deposition pH, (PAH/PAA)4 and (PAH/PDCMAA)4 films adorbed on MPA-modified gold-coated wafers have dry thicknesses of 26 and 42 nm, respectively. The corresponding (PAH/PSS)4 films are only 11 nm thick. In-solution UV-Vis spectra (Figure 2.2) demonstrate that PSS, PAH, PAA, and PDCMAA have very different affinities for Cu approximate pH of a 0.1 M CuCl2 solution.) 2+ in solutions at pH 3.6. (This is the Although for the spectra in Figure 2.2 the polymer repeat unit is in a 10-fold excess with respect to the 1 mM CuCl2, the presence 30 of PAH or PSS does not significantly alter the UV-vis spectrum of the solution, showing that complexes between PAH or PSS and Cu formation of PAA-Cu in the Cu 2+ 2+ 2+ 2+ and PDCMAA-Cu do not form at this pH. In contrast, the complexes gives rise to dramatic changes UV-Vis spectrum. 0.05 2+ Cu2+ Cu Absorbance 0.04 2+ PDCMAA-Cu2+ PDCMAA-Cu 2+ PAA-Cu2+ PAA-Cu 2+ PAH-Cu PAH-Cu2+ 2+ PSS-Cu PSS-Cu2+ 0.03 0.02 0.01 0 400 500 600 700 800 Wavelength (nm) 900 1000 2+ Figure 2.2 UV-VIS spectra of 1 mM Cu in water or aqueous solutions containing various polyelectrolytes. The concentration of the polyelectrolyte repeat units was 10 mM, and the solution pH was 3.6. Reflectance FT-IR spectra of (PAH/PDCMAA) films on gold (Figure 2.3) provide evidence for Cu 2+ -1 around 1589 cm 2+ sorption in these films. After Cu -1 and 1645 cm - sorption, the split COO peak at shifts and merges into the peak observed around 1600 -1 -1 cm . The shoulder due to the -COOH carbonyl stretch (~ 1720 cm ) also decreases in intensity after Cu 2+ sorption. Elution of Cu 31 2+ from the film using an EDTA solution and subsequent equilibration in pH 3 water returns the spectrum to essentially its initial intensities. 0.08 0.07 0.06 Absorbance After deposition after deposition - COO 2+ After Cu2+ adsorption after Cu 0.05 after EDTA After EDTA elute 0.04 0.03 COOH 0.02 0.01 0 1800 1700 1600 1500 1400 1300 wavenumber (cm-1) Figure 2.3 Reflectance FT-IR spectra of a (PAH/PDCMAA)3.5 film on Au wafter before and after immersion in 0.1 M CuCl2 solution (pH=3.6), and after subsequent immersion in 0.1 M EDTA solution (pH=6.4). 2+ 2.3.2 Selective Cu transport through (PDCMAA/PAH)n films adsorbed on porous alumina 2+ As Table 2.1 shows, the equilibrium constant for formation of Cu -iminodiacetic acid (IDA) complexes is >7 orders of magnitude higher than the corresponding constant 2+ 2+ for Mg -IDA. Similarly, binding constants for formation of Cu -ethylenediamine (en) 2+ complexes are also many orders of magnitude higher than for Mg . Thus if ion transport through (PDCMAA/PAH) n films involves hopping between ion-binding sites, Cu 2+ should move through the film with the exclusion of Mg . 32 2+ Table 2.1 Ion diffusion coefficients 51 ligand-metal ion complexes. Ions Cu Diffusion Coefficient -5 2 (10 cm /s) Formation Constant 2+ (IDA-M , log K) Formation Constant 2+ (en-M , log K) 50 and equilibrium constants for formation* of 2+ Ni 2+ 2+ 2+ Ca Mg 0.714 0.661 0.792 0.706 10.63 8.19 2.59 2.94 10.71 7.47 - 0.37 2+ *Temperature (T) and ionic strength (I) are 20 º and 0.1 for IDA-M , 25 º and 0.5 for C C 2+ en-M . Figure 2.4 presents permeate ion concentrations as a function of time during diffusion dialysis through porous alumina membranes coated with (PDCMAA/PAH) 4 and (PDCMAA/PAH)3PDCMAA films. The feed solution contained 0.1 M CuCl2 and 0.1 M MgCl2, and the receiving phase was initially deionized water. Based on the slopes of 2+ linear fits to the data in Figure 2.4, Cu 64-fold faster than Mg 2+ diffuses through these membranes 43-fold and for (PDCMAA/PAH)4 and (PDCMAA/PAH)3PDCMAA, respectively. The aqueous diffusion coefficients of Cu 2+ 2+ this high Cu /Mg 2+ and Mg 2+ selectivity most likely reflects selective Cu 2+ differ by only 1%, so binding to functional groups in the film and not size-based selectivity. Regardless of whether films terminate 2+ with PDCMAA or PAH, they show high Cu /Mg 2+ selectivities. Hence, if facilitated transport is responsible for the high selectivity, it does not require a large excess of PDCMAA at the membrane surface. 33 Permeate Conc. (μM) 70 2+ 2+ Cu w/ Mg Cu w/ Mg 2+ 2+ Mg w/ Cu Mg w/Cu 2+ 2+ Cu w/ Mg Cu w/ Mg 2+ Mg w/ Cu2+ Mg w/ Cu 60 50 40 30 20 10 0 0 10 20 30 40 Time (min) 50 60 Figure 2.4 Evolution of permeate concentrations with time during diffusion dialysis of 0.1 M CuCl2, 0.1 M MgCl2 solutions through (PDCMAA/PAH)n-modified porous alumina membranes. The permeate initially contained deionized water. Filled and open symbols represent dialysis through (PDCMAA/PAH)4and (PDCMAA/PAH)3PDCMAA-modified membranes, respectively. The formation constants for IDA- and en-metal ion complexes also suggest that 2+ 2+ PDCMAA/PAH films should exhibit Cu /Ni 2+ 2+ and Cu /Ca selectivity in diffusion dialysis. As Figure 2.5 shows, a (PDCMAA/PAH)4-coated membrane with a Cu 2+ 2+ selectivity of 20 also shows Cu /Ca 2+ 2+ and Cu /Ni 2+/ Mg 2+ selectivities >5. Because replicate membranes show some variation in flux and selectivity, which may result from the variability of the alumina substrates, in Figure 2.5 we compared selectivities with a single 2+ 2+ membrane. Given that both the Cu -IDA and Cu -en formation constants are at least 250-fold greater than the formations constants of other ions, we think that passage of the 2+ other ions may include diffusion through imperfect regions of the film. The Ca 2+ higher than the Mg 2+ and Ni flux is fluxes, perhaps because of the higher aqueous diffusion 2+ 2+ 2+ 2+ 2+ coefficient (smaller hydrated ion size) for Ca . Among Cu , Ni , Ca , and Mg , 34 2+ Ni has the second highest affinity for IDA and amines, but its low rate of transport likely reflects a non-facilitated pathway. However, the Cu 2+ flux is lowest with the 2+ solution containing Ni , suggesting some competition for binding sites between Ni 2+ 2+ Conc. (mM) and Cu . 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 2+ 2+ Cu w/w/ Mg2+ Cu2+ Mg 2+ 2+ Cu w/w/Ca2+ Cu2+ Ca 2+ 2+ Cu w/w/ Ni2+ Cu2+ Ni 2+ Mg2+ Cu2+ Mg w/w/ Cu2+ 2+ Ca2+ Cu2+ Ca w/w/ Cu2+ 2+ Ni2+ Cu2+ Ni w/w/Cu2+ 0 10 20 30 40 Time (min) Figure 2.5 Evolution of permeate concentrations with time during diffusion dialysis of 0.1 M CuCl2, 0.1 M MCl2 (M=Mg, Ca or Ni) solutions through (PDCMAA/PAH)4-modified porous alumina membranes. The permeate initially 2+ 2+ contained deionized water. This membrane was the least selective (Cu /Mg = 20) of those we examined. 2.3.3 Comparison of fluxes and selectivities in mixed and single-salt diffusion through several types of polyelectrolyte multilayer films 2+ If Cu binding to coordination sites limits the transport of other ions through polyelectrolyte films, ion fluxes and selectivities based on diffusion dialysis with single and mixed salts should differ greatly. Table 2.2 compares single- and mixed-salt selectivities in diffusion dialysis through bare alumina membranes and alumina coated 35 with (PDCMAA/PAH)4, (PDCMAA/PAH)3PDCMAA, (PAA/PAH)4 and (PSS/PAH)4 films. In the control experiment with bare porous alumina, the Cu 2+ and Mg 2+ fluxes are the same within experimental uncertainty, regardless of whether the feed solutions contain single or mixed salts. Thus the transport selectivity through the PEM-modified membranes results exclusively from the PEMs. Membranes coated with any of the 2+ polyelectrolyte films show significant Cu /Mg Nevertheless, selectivities are much 2+ higher selectivities in mixed-salt solutions. for the (PDCMAA/PAH) 4 and (PDCMAA/PAH)3PDCMAA films relative to the other PEMs. Comparison of ion fluxes in single- and mixed-salt solutions provides further 2+ evidence for Cu complexation in (PDCMAA/PAH)4, (PDCMAA/PAH)3PDCMAA, and (PAA/PAH)4 films. For membranes coated with these films, Cu 2+ single-salt feed solutions differ by a factor less than 2, but the Mg fluxes with mixed and 2+ than an order of magnitude in the presence of CuCl2. Binding of Cu fluxes decrease more 2+ may remove Mg 2+ hopping transport pathways in the film, but it could also introduce positive charge that contributes to electrostatic exclusion of divalent cations from the membrane. In contrast to PDCMAA/PAH and PAA/PAH systems, ion transport through (PSS/PAH)4 films shows little difference between single- and mixed salt solutions. Presumably this reflects minimal Cu 2+ complexation by PSS. 36 2+ 2+ -10 -2 -1 2+ 2+ Table 2.2 Cu and Mg fluxes (10 mol cm s ) and Cu /Mg selectivities in diffusion dialysis through bare and PEM-modified alumina membranes. Dialysis a employed either single- or mixed-salt solutions in the feed. 2+ 2+ 2+ 2+ 2+ 2+ Film Composition Mg (single) Cu (single) Mg (mixed) Cu (mixed) Cu /Mg b Selectivity Bare Substrate 302± 46 273± 44 275± 21 264± 19 1.04± 0.02 (PDCMAA/PAH)4 8.8± 3.0 3.6± 0.9 0.16± 0.10 6.1± 0.7 51± 32 (PDCMAA/PAH)3.5 20.4± 4.9 3.1± 1.1 0.075± 0.0022 5.9± 1.9 82± 35 (PAA/PAH)4 164± 43 5.6± 3.0 1.0± 1.2 3.8± 1.8 6.9± 4.8 (PSS/PAH)4 9.3± 8.0 22.9± 2.4 6.6± 4.2 24.0± 8.5 4.2± 1.6 a The pH values of the feed solutions were 6.4, 3.6, and 3.6 for 0.1 M MgCl2, 0.1 M CuCl2, and a mixture containing 0.1 M MgCl2 and 0.1 M CuCl2, respectively. b Selectivity was calculated based on mixed-salt fluxes, and the uncertainty is the standard deviation of selectivities for 3 different membranes. 2+ In addition to the formation of Cu complexes, decreases in the feed solution pH 2+ from 6.4 to 3.6 upon addition of 0.1 M CuCl2 might alter Mg fluxes. To test this possibility, we adjusted the pH of 0.1 M MgCl2 feed solutions with HCl and performed diffusion dialysis. As Table 2.3 shows, for (PDCMAA/PAH)4 and (PDCMAA/PAH)3PDCMAA films, the feed pH does not significantly change the Mg 2+ flux. This insensitivity of Mg PDCMAA. 2+ permeability to pH may reflect the pKa values of The pKa values for the –COOH groups of IDA are 2.6 and 1.8. Presuming that in PDCMAA/PAH films the –COOH pKa values are similar, they lie out of the range of the feed solution pH change (from 6.4 to 3.6). 37 The FTIR spectra in Figure 2.3 confirm that –COOH groups in PDCMAA/PAH films are nearly all deprotonated. for PDCMAA/PAH films the primary effect of CuCl2 on the Mg 2+ Thus, flux should stem from complexation. In contrast, PAA/PAH films show a 20- to 30-fold decrease in Mg 2+ flux after changing the feed solution pH from 6.4 to 3.6 (Table 2.3). A second exposure to 0.1 M MgCl2 at pH 6.4 restores nearly all of the flux. The decrease in flux at low pH likely reflects a structural change 52 induced by the protonation of PAA side chains (the pKa of PAA in solution is around 6.5 and is shifted to around 2.0 in PAA/PAH film 52,53 ). Unfortunately the large variation in fluxes through PSS/PAH membranes prevents a conclusion on the effect of pH (Table 2.3), but Table 2.2 shows that the Mg 2+ flux did not 2+ change significantly in the presence of Cu . The degree of protonation of either PAH 52,54 (pKa in the range of 8-9 55 ) or PSS (pKa of protonated PSS around 1.0 ) will not change greatly on going from pH 6.4 to pH 3.6. 38 2+ -10 -2 -1 a Table 2.3 Mg fluxes (10 mol cm s ) as a function of feed pH during diffusion dialysis through PEM-modified porous alumina membranes. Feed solution pH Film Composition (PSS/PAH) 6.4 4 8.8± 3.0 12.0± 4.2 7.1± 2.6 3.5 20.4± 4.9 24.4± 4.2 43.2± 9.7 5.4± 4.9 116± 56 9.3± 8.0 (PDCMAA/PAH) 3.6 164± 43 (PDCMAA/PAH) (PAA/PAH) b 6.4 8.1± 7.2 8.9± 7.9 4 4 a The feed solution contained 0.1 M MgCl2. b After exposure of the film to pH 3.6 feed solution. The effect of CuCl2 on the Mg 2+ flux through (PDCMAA/PAH)4, (PDCMAA/PAH)3PDCMAA, and (PAA/PAH)4 films is reversible, but only fully reversible after eluting Cu 2+ from the film. As Table 2.4 shows, after experiments with feed solutions containing 0.1 M CuCl2 and 0.1 M MgCl2, diffusion dialysis of just 0.1 M 2+ MgCl2 yields Mg fluxes lower than in the same experiment with a fresh membrane. Subsequent exposure of membranes to EDTA (pH=6.4) restores Mg dialysis, further suggesting that Cu 2+ adsorption inhibits Mg 2+ 2+ fluxes in diffusion flux. This kind of gate effect accompanying facilitated transport also occurs in molecularly imprinted facilitated transport membranes. 46 For PAA/PAH films, the Mg 2+ flux reduction with CuCl2 in the feed solution is on the same level as flux reduction at pH 3.6 (compare Tables 2.3 and 2.4). However, the effects of the CuCl2 solution did not dissipate until after EDTA elution of adsorbed Cu 2+ from the film. Complexation of Cu structure at low pH. 39 2+ may stabilize changes in film 2+ Table 2.4 Mg fluxes (10 and mixed salts. (PDCMAA/PAH) (PAA/PAH) (PSS/PAH) 4 4 -2 -1 mol cm s ) in sequential diffusion dialysis with single Before 2+ Cu a exposure Mixed 2+b with Cu After Cu c exposure After EDTA d elution 4 8.8± 3.0 0.16± 0.10 0.73± 0.27 12.5± 1.2 3.5 20.4± 4.9 0.075± 0.0022 2.4± 1.6 14.0± 9.0 164± 43 1.0± 1.2 9.6± 2.1 145± 35 9.3± 8.0 6.6± 4.2 7.0± 5.0 11.1± 9.7 Film Composition (PDCMAA/PAH) -10 2+ a Diffusion dialysis of 0.1 M MgCl2 with a freshly prepared membrane. Subsequent diffusion dialysis of 0.1 M MgCl2, 0.1 M CuCl2. c Diffusion dialysis of 0.1 M MgCl2 after dialysis of the mixed salt solution. d Diffusion dialysis of 0.1 M MgCl2 after immersion the same membrane in 0.1 M EDTA (pH=6.4) for 30 min and rinsing with deionized water. b 2.3.4 PEM thickness and swelling Assuming a constant diffusion coefficient through a polyelectrolyte film and minimal mass transport resistance in the alumina support, the diffusive flux through a PEM-coated membrane is inversely proportional to the PEM thickness. Figure 2.6 shows the ellipsometric thicknesses of PEMs after a series of different treatment. The PEMs were adsorbed on a modified gold-coated substrate to facilitate ellipsometric studies. Soaking the film in water for 1 hour and determination of the film thicknesses in water leads to an increase of ~40% in thickness compared to dry films for PSS/PAH and PDCMAA/PAH, and ~20% for PAA/PAH. Immersion of PEMs in 0.1 M CuCl2 and subsequent determination of the film thickness in deionized water leads to a small but not statistically 40 significant increase in film thickness. Thus the inhibition of Mg 2+ flux after Cu 2+ adsorption for PDCMAA/PAH and PAA/PAH likely does not stem from a change in film swelling. However, the thickness of PAA/PAH films in deionized water decreases about 50% after immersion in 0.1 M EDTA solution. The shrinkage may stem from deprotonation of carboxylic acid groups, which leads to film reconstruction and possible deswelling 56 as the charge density on the PAA chain increases. The swollen thickness of PAA/PAH films after exposure to EDTA was recoverable after prolonged soaking in pH 3 water. The dry thickness of the film and FT-IR peak intensity remained relatively constant after exposure to EDTA, which precludes the loss of PEM film upon elution. For the PDCMAA/PAH film, the pH of the EDTA solution was not high enough to trigger a large scale deprotonation on the PDCMAA chain (pKa of both COOH groups < 3), so that the film structure remained stable in the process. 41 Ellipsometric Thickness (Å) 1200 1000 800 dried Dried 1hrh water 1 water 2.5hr h water 2.5 water 2+ 1hrh Cu 1 Cu2+ 11 hEDTA hr EDTA 600 400 200 0 (PAH/PSS)4.5 (PAH/PSS)4.5 (PAH/PAA)4.5 (PAH/PAA)4.5 (PAH/PDCMAA)4.5 (PAH/PDCMAA)4.5 Film Composition Figure 2.6 Ellipsometric thicknesses (Å) of PEMs adsorbed on Au wafers modified with MPA monolayers. The columns in each series stand for (from left to right): dry thickness in air, thickness under deionized water after a 1-h or a 2.5-h immersion (pH=6.4), thickness under deionized water after a 1-h immersion in 0.1 M CuCl2 (pH=3.6) followed by rinsing with deionized water, and finally the thickness under deionized water after a subsequent 1-h immersion in 0.1 M EDTA (pH=6.4) followed by rinsing with water. 2.3.5 Flux as a function of feed concentration For ions undergoing facilitated transport, as their feed concentration increases, eventually their flux should plateau due to saturation of binding sites in the membrane. 38,39 2+ However, Figure 2.7 shows that Cu flux through (PDCMAA/PAH)4 films increases nonlinearly with increasing CuCl2 feed concentration. Moreover, the flux continues to increase at feed concentrations as high as 0.5 M (see Figure 2.7). The nonlinear increase suggests unsaturated binding sites and an increasing hopping rate or increasing number of sites at high Cu 2+ concentrations. Isotherms of Cu 2+ sorption in similar films confirm the presence of weak binding sites that fill only at high Cu 42 2+ concentrations (see below for further discussion). For (PAA/PAH)4 and (PSS/PAH)4 membranes, plots of Cu 2+ flux versus feed concentration show at most small deviations from linearity (Figure 2.8). 2+ Whereas the Cu -imprinted polyelectrolyte membrane reported by Deng et al. 57 exhibited decreasing selectivity with increasing feed concentration, PDCMAA/PAH films maintain their selectivity at a high feed concentration. The use of high feed 18 16 14 12 10 8 6 4 2 0 Flux (10-10 mol s-1 cm-2) Flux (10-10 mol s-1 cm-2) concentrations should enable high ion fluxes in separations. 0 100 200 300 400 500 12.00 2+ Cu Cu2+ 2+ Mg2+ Mg 10.00 8.00 6.00 4.00 2.00 0.00 0 Cu2+ Feed Conc. (mM) 2+ 0.05 0.1 Feed Concentration (M) Figure 2.7 (Left) Cu fluxes through a (PDCMAA/PAH)4 film deposited on porous 2+ 2+ alumina. (Right) Cu and Mg fluxes for 3 (PDCMAA/PAH)4 films deposited on 2+ 2+ porous alumina. A fourth membrane showed much lower fluxes for both Cu and Mg , but the flux trends remained the same. 43 Flux (10-10 mol cm-2s-1) Flux (10-10 mol cm-2s-1) 2.5 2+ Cu2+ Cu 2+ Mg2+ Mg 2.0 1.5 1.0 0.5 0.0 0 2+ 0.05 70 60 50 40 30 20 10 0 2+ Cu2+ Cu 2+ Mg2+ Mg 0 0.1 Feed Conc. (M) 2+ 0.05 0.1 Feed Conc. (M) Figure 2.8 Cu and Mg diffusion dialysis fluxes through porous alumina coated with (PAA/PAH)4 (Left) and (PSS/PAH)4 (Right) films. Large error bars in the flux arise from the defects in manufactured skin layer of commercial alumina membranes which may 58 lead to variation in fluxes through the PEM film. However, each individual membrane exhibited similar evolution of flux with feed concentration. 2.3.6 Isotherm for Sorption of Cu 2+ in (PDCMAA/PAH)10-modified nanoparticles Sorption isotherms may help explain trends in diffusion dialysis fluxes as a function of feed concentration. We chose to examine sorption on PEM-modified nanoparticles because their large surface area enables quantitation of binding even with high (0.4 M) Cu 2+ concentrations in solution. The use of 10 rather than 4 polyelectrolyte bilayers also increases the total sorption. In these experiments, the decrease in the Cu 2+ concentration in a loading solution after equilibration with (PDCMAA/PAH) 10-modified nanoparticles allows an estimate of the amount of Cu 2+ adsorption per mass of particles. We also rinsed the loaded particles with deionized water and eluted the Cu eluted Cu 2+ may be less than the bound Cu 2+ 2+ with EDTA. The amount of if rinsing removes some of the Cu 2+ from the beads. Figure 2.9 shows the Cu 2+ sorption isotherms as calculated from both the decrease in 44 Cu 2+ concentration in the loading solution and the amount of Cu The isotherm based on eluted Cu 2+ 2+ eluted after rinsing. already approaches saturation at a 19 mM equilibrium concentration in the loading solution, and the maximum sorption is ~ 130 µmol/g. However, for total sorption (determined from the loading solution), Cu 2+ binding approximately doubles on increasing the equilibrium concentration from 19 mM to 376 mM. These data and the shape of the total sorption isotherm suggest two types of sorption 2+ sites, with different affinities for Cu . We speculate that the strong binding sites, which saturate at Cu 2+ concentrations around 20 mM, are the IDA functionalities of PDCMAA and the weak binding sites are amine groups of PAH. At sufficiently high concentrations, the Cu 2+ ions may effectively compete with protons and bind to the amines of PAH. If facilitated transport involves hopping between weak binding sites (amines), increased binding to these sites at high Cu 2+ concentrations should enhance flux. Moreover the flux should increase nonlinearly with the number of binding sites as more percolation pathways become available. However, if transport occurs via the amine sites, why do (PAA/PAH)4 and 2+ 2+ (PSS/PAH)4 films not show similarly high Cu /Mg shows that the Cu 2+ selectivities. In fact, Table 2.2 fluxes through (PSS/PAH)4 films are higher than those through (PDCMAA/PAH)4 films. Even when normalized to film thickness, the Cu 2+ permeability through (PSS/PAH)4 is higher than that through (PDCMAA/PAH)4 and (PAA/PAH)4. Nevertheless, fluxes of both Cu 2+ and Mg 2+ increase linearly with concentration for (PSS/PAH)4 films. Most likely, the sulfonate-ammonium groups in PSS/PAH remain ionically cross-linked even at Cu 2+ concentrations of 0.1 M. Because Cu 45 2+ binds weakly to amines and negligibly to sulfonates, the presence of 0.1 M CuCl2 does not affect Mg or Cu 2+ 2+ transport though these films and ion permeabilities are essentially independent of the feed concentration. With (PDCMAA/PAH)4, Cu 2+ binding to PDCMAA should break 2+ ionic cross-links and create free ammonium groups that may bind Cu . A similar effect may occur with (PAA/PAH)4, but these films are much more permeable to Mg 2+ 2+ than (PDCMAA/PAH)4 in the absence of Cu , and diffusional transport even after Cu 2+ binding may at least partially mask facilitated transport. 450 Total Adsorption Adsorption (μmol/g) 400 Eluted Adsorption 350 300 200 250 150 200 100 150 50 100 0 50 0 5 10 0 0 100 200 300 Cu2+ Conc. (mM) 400 2+ Figure 2.9 Adsorption isotherm for Cu binding to (PDCMAA/PAH)10-modified nanoparticles. The inset is an expansion of the lower concentration range. Adsorption incubation time was 14 hours at each concentration, and each point represents a fresh set of modified nanoparticles. 2.4 Conclusions PDCMAA/PAH films adsorbed on porous alumina allow selective diffusive transport of Cu 2+ 2+ 2+ 2+ over Mg , Ni , and Ca reaching values as high as 80. 2+ in mixed salt solutions, with Cu /Mg 2+ 2+ These high Cu /Mg 46 2+ selectivities selectivities do not occur with PSS/PAH and PAA/PAH films or in single-salt experiments with PDCMAA/PAH films. Binding of Cu 2+ to PDCMAA/PAH membranes greatly decreases the Mg by saturating hopping sites or inducing electrostatic exclusion. 2+ flux, either 2+ The high Cu /Mg 2+ selectivity of these films in mixed salt solutions suggests facilitated transport, as the aqueous diffusing coefficients of the two ions differ by only 1%. transport membranes the Cu 2+ Unlike most facilitated flux through PDCMAA/PAH membranes increases nonlinearly with the concentration of Cu 2+ in the feed solution. 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Natl. Acad. Sci. 2011, 108, 8577-8582. (56) Jaber, J. A.; Schlenoff, J. B. Langmuir 2006, 23, 896-901. (57) Deng, H.; Gao, L.; Zhang, S.; Yuan, J. Ind. Eng. Chem. Res 2012, 51, 14018-14025. (58) Cheng, C.; Yaroshchuk, A.; Bruening, M. L. Langmuir 2013, 29, 1885-1892. 52 Chapter 3 Summary and Future Work 3.1 Summary This thesis describes a simple and convenient method for preparing an ultrathin facilitated transport membrane with metal ion-binding groups. Chapter 1 discusses the versatility of the layer by layer (LBL) technique for synthesizing functional polymeric films with tunable thickness and permeability. These polyelectrolyte multilayers (PEMs) are especially attractive for a variety of ion-separation processes such as water softening and metal ion removal/recovery. Chapter 2 demonstrates the preparation of a facilitated transport membrane by LBL deposition of poly[(N,N’-dicarboxymethyl) allylamine] 2+ (PDCMAA)/poly(allylamine hydrochloride) (PAH) films at low pH. The abundant Cu binding sites in the film, either iminodiacetic acid groups from PDCMAA or amine groups from PAH, afford selective facilitated transport of Cu 2+ through the membrane. The rejections of other ions with identical charge and similar hydrated radii but much 2+ 2+ 2+ lower ligand binding constants (e.g. Mg , Ni , Ca ) are high compared to Cu 2+ 2+ rejection due to the preferential Cu -sorption in the film. Diffusion dialysis studies show 2+ 2+ average Cu /Mg selectivities of 50 and 80 with PAH-capped and PDCMAA-capped films, respectively. 3.2 Future Work This thesis shows the feasibility of synthesizing facilitated transport membranes using LBL deposition of functional polyelectrolyte. These membranes allow selective 53 Cu 2+ transport. Separation of other ions such as alkali metal ions, lanthanides and transition metals will require ion-binding groups tailored for different ions. Tieke et al. 1,2 incorporated various ion-binding compounds (e.g. calixarenes, cyclodextrins, and azacrowns) into the PEMs by electrostatic adsorption, but these films suffered from instability. Covalent bonding between the ion-binding groups and the polyelectrolyte backbone could potentially solve the problem. One method to prepare such films would employ polyelectrolytes partially modified with ion-binding groups prior to deposition. The unmodified ionic groups would ensure the PEM buildup and stability. For example, poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) can both be partially modified with ion-binding functional groups through amidation. Postdeposition modification of the PEM provides another way to covalently incorporate ion-binding groups. For example, PAA/PAH films deposited at low pH have an abundance of free carboxylic acid groups, which may serve as attachment sites for ion-binding groups. 3,4 The modification process could utilize reactions such as 1-ethyl-3- (3-dimethylaminopropyl) coupling 5 carbodiimide/N-hydroxysuccinimide (EDC/NHS) or copper(I)-catalyzed alkyne-azide cycloaddition (click chemistry) amine 6,7 for polyelectrolytes with different functionalities (see Figure 3.2.1 and 3.2.2). Figure 3.1 EDC-NHS amine coupling for modification of carboxylic acid-containing polyelectrolytes. 54 Figure 3.2 Cu(I) catalyzed alkyne-azide cycloaddition. Use of the chemistry for PEM modification will require polyelectrolyte with azide or alkyne groups. Separations of rare earth metal ions, lanthanides along with scandium and yttrium, through tailor-made facilitated transport membranes is especially attractive due to the similar radii of these ions and labor costs in conventional separation techniques such as cascading, fractional crystallization and solvent extraction. Suitable ion-binding groups for lanthanide separations vary from the iminodiacetic acid group used in the present study, to calixarenes, crown ethers, diazamacrocyles, cryptands, and also amines (see 8 Figure 3.2.3). Efficient separation will also require optimization of the PEM thickness, permeability and charge density to achieve high flux for the selected ion as well as high rejection for the other ions. 55 Figure 3.3 Structures of lanthanide-binding compounds 56 REFERENCES 57 REFERENCES (1) Tieke, B.; El-Hashani, A.; Toutianoush, A.; Fendt, A. Thin Solid Films 2008, 516, 8814-8820. (2) 131. Tieke, B.; Toutianoush, A.; Jin, W. Q. Adv. 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