title: A study of transport processes in ion exchange membranes
Creators: name: Gardner, Colin R.
Publisher: name: University of Glasgow
Year: 1970

description: The major part of this thesis is concerned with the study of transport and diffusion processes in ion-exchange membranes. The ion-exchanger chosen for study was the AWF C60 cation-exchange membrane. Previous studies (1) (2) had suggested that this membrane was more homogeneous than most commercially available exchangers and that it exhibited a high water content and high electrical conductivity, all of which made it a suitable subject for study. It had also been reported that heat treatment of this membrane led to an irreversible expansion of the membrane matrix, producing a membrane with a higher water content and lower flow resistance. (3) The normal and expanded forms of the exchanger were studied in the sodium form in sodium chloride solutions, so that a comparison of the results from the two exchangers might yield information about the effect of expansion on the exchanger properties. In addition to the determination of the basic properties of the exchangers, e.g. water content, physical dimensions, capacity etc. , this study entailed the measurement of the following properties; (1) conductivity, (2) electro-osmotic flow, (3) diffusion of counter- and co-ions, (4) transport numbers of counter- and co-ions. These measurements were made for each membrane in 0.1M, 0.5M 1.0M and 2.0M sodium chloride solutions. Three further experiments were conducted on/ on both membranes under the influence of an electrolyte concentration gradient: salt flow, osmosis and emf measurements were made with concentration gradients of 0.05/0.15 and 0.5/1.5. The results of these experiments have been discussed in terms of the current theories of membrane transport processes. It has been shown that the Nernst-Planck equation, modified to include the effects of convection, can adequately describe the variations in conductivity of the membranes with changing external concentration and the tortuosity factor, 0 (= 1+v w/1-v w), and absolute rate theory, (4) have been used to explain, satisfactorily, the dependence of the ionic diffusion coefficients on the concentration of the external solution. Combining the results of all the transport experiments, it proved possible to carry out a complete analysis of the systems using the theory of non-equilibrium thermodynamics. (5) Examination of the results of this treatment reveals a number of important observations. (1) Where a linear relationship exists between the counter-ion and water transference numbers, as it does for the C60 membranes, a complete analysis of the system can be achieved using the data mentioned above. (2) For the counter-ions, isotope-isotope interaction is an important term and should not be omitted from the calculations as it has been frequently in previous studies. (5) (3)/(3) For the co-ions, the isotope term is not large enough to significantly affect the results, even when the electrolyte uptake is fairly great. (4) Comparison of the results for the two membranes shows that, particularly at 0.1M, the variations in the fluxes of the species, through the membranes, can be almost wholly attributed to the difference in tortuosity of the exchangers. In dilute solutions it has been possible to use some simplify- ing assumptions which have enabled accurate predictions of the salt flow through the membranes to be made. Using the electrolyte uptake data required for the treatment described above, a structural analysis of the membranes has been obtained by employing the method of Glueckauf. (6) This analysis reveals that, although the membranes exhibit inhomogeneity, the degree of heterogeneity is considerably less than that observed in most commonly used ion-exchange membranes. The structural parameters have also been used in conjunction with the co-ion diffusion data, to show that the most continuous regions of the exchangers are those where the fixed charge concentration is very low. The final chapter deals with a study of the inorganic ion-exchanger, hydrous zirconia, both in particle and membrane form. The main feature of this exchanger is the variation in ion- exchange capacity with pH of the external solution. In the chloride form, the counter-ion diffusion coefficients have been determined as a function of the capacity. The results reveal that the/ the diffusion coefficients increase with decreasing capacity, i.e. with increasing distance between the sites of minimum energy, as predicted by the absolute rate theory. (4) Although the agreement is not Quantitative owing, to the assumptions made in the calculations, these results confirm the observations made' for the C60 membranes, that absolute rate theory applied to diffusion processes can be used in the treatment of diffusion in ion-exchangers.