Characterization and application of the surface charge-induced long-range organization in room temperature ionic liquids

Room temperature ionic liquids (RTILs) are salts characterized by a melting point below room temperature. RTILs have a wide range of applications, in areas ranging from supercapacitor energy storage to sequestration of toxic gas phase species and use as reusable solvents for selected organic reactions. All these applications stem from their unique physical and chemical properties, which remain understood to a limited extent. Among the issues of greatest importance is the extent to which RTILs exist as dissociated ionic species, and the length scales over the organizations are seen to exist in them. Our group have reported previously on the existence of a surface charge-induced free charge density gradient in RTILs with a characteristic persistence length of ca. 50 Îơm. The existence of such a long-range organization in fluid medium is unusual. The overall goal of this work is to achieve a deeper understanding of this phenomenon, thereby providing an opportunity to better understand the local and long-range organization in RTILs and broad their potential applications which benefit from gaining such knowledge.The induced free charge density gradient (Ϲf) is probed by measuring the fluorescence anisotropy decay of a trace-level charged chromophore in the RTIL as a function of distance from the indium-doped tin oxide (ITO) support surface. In chapter 2, we characterize the structure-dependence of this charge-induced organization as a function of the RTIL constituent identity, and use these data to evaluate the magnitude of the induced free charge density gradient. The magnitude of this gradient is found to depend on the chemical structures of the cationic and anionic constituents of the RTIL used.In chapter 3, we characterize Ϲf in three different pyrrolidinium RTILs and two imidazolium RTILs, which aims to expand on prior results (chapter 2) on the chemical structure-dependence of Ϲf. Our measurements demonstrate that the magnitude of Ϲf depend on the alkyl chain length of RTIL cation. Ϲf is larger in the RTIL with longer cation alkyl chain. This dependence has been revealed in both pyrrolidinium and imidazolium ionic liquids.In chapter 4, we report on the existence of a surface charge-induced gradient in the RTIL refractive index (n) and evaluate the relationship between the gradient in n and Ϲf. Because Ϲf is uniaxial, the induced change in n is manifested as an induced birefringence. We characterize the Ϲf -dependent n of the RTIL with an apparatus that uses the RTIL as a lens. Ϲf is controlled by the surface charge density (ϳs) of the RTIL support. The far-field image of light passed through the RTIL lens as a function of ϳs is used to measure charge-induced changes in n of the RTIL. We demonstrate a significant modulation of the n with modest changes in ϳs of the RTIL support. This report places the relationship between Ϲf and RTIL dielectric response on a quantitative footing and suggests the utility of RTILs for electro-optic applications.In chapter 5, We report on the dependence of surface charge-induced birefringence in room temperature ionic liquids (RTILs) with different cation constituents. The induced birefringence is related to Ϲf in the RTIL. We find that in all cases the induced birefringence is proportional to the ϳs and, that the change in n nearest the ITO surface can be on the order of 30%. Our findings indicate that the induced birefringence depends more sensitively on the cation aliphatic substituent length than on the identity of the charge-carrying headgroup.