LOCAL ORGANIZATION AND THE PIEZOELECTRIC EFFECT IN ROOM TEMPERATURE IONIC LIQUIDS (RTILs)

Room temperature ionic liquids (RTILs) have been used in a variety of fields due to their unique physical and chemical properties which are rooted in their complex molecular arrangements and interactions. It is therefore important to shed light on the organization of RTILs to control the physical and chemical properties to maximize potential. The Blanchard group has previously observed an intriguing phenomenon of surface charge induced free charge density gradients (ρ_f) with a characteristic persistence length of ca. 50 μm. There is no existing theoretical framework for molecular liquid systems that can explain this unprecedented observation. We hypothesized that there is analogy to the piezoelectric effect with this long-range order. Therefore, the larger goal of this work is to determine if RTILs exhibit the piezoelectric effect and to elucidate the underlying molecular-level mechanism.The free charge density gradient (ρ_f) is sensed by measuring the induced orientational anisotropy decay of trace amounts of charged chromophores in the RTILs as a function of distance from the charged support (silica or ITO). In Chapter 2, we successfully demonstrated the piezoelectric effect in RTILs by measuring open circuit potential (OCP) as a function of applied pulsed pressure. For this measurement, we devised a cylinder-piston cell system which allows the acquisition of reproducible data over many measurements. In Chapter 3, we conducted a comprehensive investigation into the structure dependence of the piezoelectric effect with six different imidazolium and four different pyrrolidinium RTILs. Our findings revealed that the piezoelectric effect is influenced by the head group of RTILs. We explored chiral ionic liquids anticipating a stronger piezoelectric response due to their non-centrosymmetric nature, a prerequisite for the piezoelectric effect. We found that chiral ionic liquids exhibit the piezoelectric effect which is slightly larger than that of pyrrolidinium based ILs but is less than that of HMIMTFSI. The piezoelectric effect exhibited by RTILs is not solely a molecular-level phenomenon but, rather, the characteristic length scale of the relevant structural unit is larger. In Chapter 4 and Chapter 5, we aimed to investigate the impact of dilution on RTILs and explore the existence of nano-domains within RTILs. Our findings revealed that ρ_f persists up to a certain amount of dilution, maintaining its magnitude and characteristic length. However, beyond this dilution level, the free charge density gradients collapses. The functional form of the magnitude and characteristic length do not resemble that of a homogeneous solution, rather the binary mixtures of RTILs are highly heterogeneous (85% of dilution). We calculated the hydrodynamic volume of the reorienting entity in the bulk RTIL-solvent system at 200 μm from the charged support surface, where there is no active free charge density gradient. We found that RTILs aggregate and form nano-domains. Moreover, the size of the nano-domains (aggregates) increases with increasing dilution. In Chapter 6, we achieved a significant breakthrough by establishing a connection between the free charge density gradients (ρ_f) and molecular scale organization. We measured the rotational diffusion dynamics of two different chromophores, cresyl violet and perylene to investigate polar and non-polar regions, respectively. Both chromophores exhibited the characteristics of oblate rotors, allowing the extraction of the Cartesian components of their rotational diffusion constants, thereby gathering detailed information on the local environments of the rotating entities. This information revealed depth-dependent changes in the organization of the chromophore local environments that correlated with the existence of f. In Chapter 7, we summed up and suggested some future work to enhance our understanding of the piezoelectric of RTILs.