Counter-ion intradiffusion in sulphonic ion-exchange systems

Abstract

The intradiffusion data of the sodium counter-ion in different sulphonic ion-exchange systems in the sodium form (from linear polystyrene to commercial membranes of graft copolymers of styrene) are analysed as a function of q, the number of water molecules per ionic group. These data represent experimental results from a considerable number of workers during the last three decades. A general comprehensive picture is obtained, which shows how individual characteristics (water content, exchange capacity, crosslinking and inert material added) affect in a gradual and congruent way the value of the intradiffusion coefficient. The comparison with simple electrolyte systems such as aqueous sodium chloride and sodium p-ethylbenzene sulphonate shows not only a clear similarity, which shows that the transport mechanism is of the same type in all these systems, but also a clear differentiation owing to the polymeric and polyelectrolytic character of the ion-exchange systems. The complex nature of the obstruction to ion movement (tortuosity) is evidenced. The overall behaviour observed leads to the view that it is realistic to consider ion-exchange systems as homogeneous hydrated gels, since intermolecular interactions between charged sites, counter-ions and water are very important. Consequently, it is more convenient to study the transport properties as functions of the swelling variable q than in terms of the commonly used stoichiometric volume fraction of polymer. For several systems a linear relation between ln D^†_Na⁺ and 1/q holds, within the range 2 ⩽q⩽ 70. The effect of cross-linking and of inert material on these relations is discussed. Furthermore, a similar analysis leads to the verification of a previously obtained linear equation between ln D^†_Na and a₀, the thermodynamic activity of water.