Studies on cation exchange membranes (perpmaplex-C-20)

Abstract

The investigations described in this thesis have been aimed at contributing to the understanding of equilibrium and non-equilibrium phenomena in ion-exchange membranes. Towards this end, Permaplex C-20 has been chosen as an example of a commercial, heterogeneous cation-exchange membrane, and a detailed study of the various equilibrium and transport properties has been carried out. The format of the thesis is as follows: Chapter 1 contains a brief survey of the literature on the particular membrane chosen for investigation, and in addition, outlines the scope of the investigations. Equilibrium properties, in particular the exchange capacity, the solvent content and electrolyte uptake, constitute the subject matter of Chapter 2. The chapter involves a detailed description of the procedures adopted for the determination of equilibrium properties and for minimization of heterogeneity by a rational choice of membrane pieces. After reporting the results obtained both with pure water and water-DMSO mixtures, the chapter contains a discussion with special reference to electrolyte uptake of calcium, strontium and alkali metal chlorides. A simple pore-filling model has been proposed to explain the deviations from Donnan uptake. Chapter 3 is devoted to the studies on the self-diffusion of THO (tritiated water), Calcium-45, Rubidium-86, Cesium-137 and Strontium-90 through Permaplex C-20. An account has been provided of the experimental methods adopted and the results obtained with the radiotracer determinations. Attention has been focused in the discussion part of the chapter on two main points: (1) the variation of the self-diffusion coefficient of calcium ions which has been related through the viscosity to the DMSO percentage of the external water-DMSO mixtures; (2) the dependence of the THO self-diffusion coefficient on the water content of the membrane has been found to be in accordance with Meares equation. Chapter 4 concerns the experimental methods, the results and a discussion of the conductance of Permaplex C-20 in various ionic forms. Chapter 5 deals with tracer transport through Permaplex C-20 under transient conditions. The transient analysis is directed towards (a) the current-dependent 'induction' or hold-up time which has been found to elapse before the 'appearance' of the tracer in the cold compartment, and (b) the profiles of tracer concentration within the membrane. A theoretical analysis has been developed based on a rigorous solution of a modified Pick's law consistent with the appropriate initial and boundary conditions. Digital computer calculations have been shown to be representative of the main trends of the experimental results. Chapter 6 is devoted to the studies of the transport numbers of calcium ions and water as a function of current density and external CaCl? concentration in water and water-DMSO mixtures. The transport number determinations showed that: (1) there is a maximum in the bell-shaped t? versus i and t versus i plots; (2) the maximum value of t? shows a maximum with the external concentration of CaCl? and a decrease with DMSO concentration; and (3) the membrane potentials are less in the mixed solvents than in pure water. A discussion of these results has been presented in the light of previous work. In particular, the initial rise in t? with i has been argued to be due, not to back-diffusion, but to the current-dependence of the tracer concentration profiles within the membrane, in contrast to the post-maximum fall in t? with i which has been ascribed to membrane polarization. The t versus i plots have been attributed to water transport which is correlated with ion transport.