Graduation Semester and Year

Spring 2026

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry

Department

Chemistry and Biochemistry

First Advisor

Purnendu K. Dasgupta

Second Advisor

Daniel W Armstrong

Third Advisor

Saiful M Chowdhury

Fourth Advisor

Charles Phillip Shelor

Abstract

Cation exchange membranes (CEMs) are central to a wide range of electrochemical and analytical technologies due to their ability to selectively transport cations while excluding co-ions via electrostatic (Donnan) interactions. The presence of fixed anionic functional groups within the polymer matrix, such as sulfonate and carboxylate groups, enables this selective transport under concentration or electrical gradients. However, deviations from ideal selectivity are frequently observed in the form of forbidden ion transport (FIT), where co-ions permeate the membrane despite electrostatic repulsion. Although FIT is recognized in membrane science, its governing mechanisms remain incompletely understood, particularly across varying hydration states and membrane architectures. In this study, FIT was systematically evaluated across a range of commercial and custom-fabricated membranes. The results demonstrate that while FIT generally decreases with increasing ion exchange capacity (IEC), this relationship is not universal. In highly hydrated membranes, water content significantly alters the dielectric environment and reduces electrostatic exclusion, leading to enhanced co-ion transport. This indicates that hydration state can supersede IEC as the dominant factor governing FIT in certain xi regimes. Additionally, differences in transport behavior among strong acids suggest that permeation may occur partially via neutral molecular species rather than exclusively as fully dissociated ions, providing a more comprehensive explanation for observed transport anomalies. Building upon these transport phenomena, the conductive properties of CEMs were investigated using an alternating current (AC) two-electrode method capable of resolving both tangential (in-plane) and transverse (through-plane) conductivity. The measurements reveal strong anisotropy in ion transport, with tangential conductivity consistently exceeding transverse conductivity. This behavior is attributed to preferential alignment of hydrated ionic domains parallel to the membrane surface. Membrane pretreatment was found to significantly influence conductivity, where hydration via boiling in deionized water produced moderate enhancements, while acid treatment resulted in substantial increases due to proton exchange, removal of residual counter-ions, and improved connectivity of ionic channels. Furthermore, membrane thickness strongly influenced time-dependent equilibration, with ultrathin membranes reaching steady state rapidly, while thicker membranes exhibited diffusion-limited transport kinetics. Measurement parameters such as electrode geometry, applied electric field, and AC frequency were also shown to affect apparent conductivity primarily through interfacial polarization and current distribution effects, emphasizing that conductivity is not purely intrinsic but strongly measurement-dependent. These fundamental insights were subsequently applied to the development of a non-electrolytic, membrane-based hydroxide eluent generation system for ion chromatography. In this approach, neutral guanidine is generated in situ from a xii guanidinium nitrate and sodium hydroxide mixture and allowed to permeate across a polymeric membrane into a flowing stream of deionized water. Upon permeation, guanidine is protonated to produce guanidinium hydroxide, effectively generating hydroxide ions in solution without electrolysis. This strategy eliminates gas evolution, reduces system complexity, and enables continuous, reagent-free eluent production. Comparative evaluation of membrane materials demonstrated that permeability, chemical stability, and structural integrity are critical determinants of system performance. While porous membranes exhibited high permeation rates, they suffered from limited thermal and mechanical stability. In contrast, dense fluoropolymer membranes such as Teflon AF provided controlled transport with excellent durability under highly alkaline conditions, making them most suitable for sustained operation. The generated hydroxide eluent was successfully coupled with a cation exchange suppressor, achieving effective suppression of background conductivity and confirming compatibility with suppressed ion chromatography systems. Overall, this work establishes a unified framework linking ion transport behavior in CEMs to membrane structure, hydration, and experimental conditions, while simultaneously demonstrating a practical application in the form of a non-electrolytic hydroxide generation platform. These findings advance the fundamental understanding of membrane-mediated transport phenomena and provide a simplified, robust alternative to conventional electrolytic eluent generation systems in analytical chemistry.

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Available for download on Monday, May 15, 2028

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