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The dielectric properties of dense cellulose acetate (CA) membranes have been determined using impedance spectroscopy. The impedance of the membranes in equilibrium with external salt solutions (10-4-1 mot dm-3) and at varying pH as a function of the frequency of the applied field was measured. From the data, and using classical Donnan theory together with the Nernst-Planck electrodiffusion equations and known dielectric theory, it is possible to find the concentration ratios of the ions between the membranes and the surrounding solution as a function of the external salt concentration, as well as their diffusion coefficients. In addition, the relative permittivities of the membrane matrix and of the solution present in the membrane alveoles, as well as the size of the latter, can be estimated. The results show (1) that there is no qualitative difference between NaCl and KCI.(2) The concentration ratios of the cations and the anions differ very much from unity at low mol dm-3) external salt concentration. The cation ratios attain values of several hundred, decreasing rapidly as the external concentration increases. ( 3) The (relative) permittivity of the solvent inside the membrane is ca. 20, indicating either that the rotational degrees of freedom of the water in the alveoles are, to a considerable extent, hindered compared with normal water, or that the temporary H-bond clusters in pore water are less extended than in bulk water, with a resulting lower effecitve dipole moment; and (4) the average radius of the alveoles is ca. 6.5 f 1.0 nm. Our results are at variance with measurements on similar membranes reported a few years ago by Wiggins and van Ryn, J. Macrornol. Sci., Part A , Chern., 1986, 23, 875, who found considerable qualitative differences in the behaviour of the two alkali-metal salts. On the other hand, the low relative permittivity estimated for the water in the membrane supports the idea of anomalous water in the membrane alveoles, however, this seems to be caused by the properties and size of the alveoles, rather than by the presence of a particular cat ion.

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Cellulose acetate membranes (2.5 acetyl groups per glucose unit) have been cast from acetone solutions, from acetone/formamide solutions and from acetone/water solutions with and without complete evaporation before swelling and with or without thermal curing. Then, the membranes have been characterized electrochemically by EMF-measurements. EMF measurements on dense cellulose acetate membranes (with complete evaporation) are repeatable to within + 5 mV, and all the conclusions drawn earlier from measurements on a single membrane specimen are generally valid for dense membranes. The variation of the parameters with the cation (Li-f, Na+ or K + ) found at pH(conc) = 5 is even more consistent than found earlier, since the pH was fixed exactly by dilution and not measured by a glass electrode. Predictions of EMF-values for mixtures of three electrolytes (LiCI, NaCl and HCI) are in aggreement with experimental values. With complete evaporation, the asymmetry may be kept below +1 mV for the dense membranes. The asymmetric membranes (incomplete evaporation) have a considerably larger EMF asymmetry (up to ± 7 mV), but the model used for dense membranes work well on the mean values. The asymmetric membranes are more swelled. Accordingly, a smaller value of the fixed charge density is found, and the ratios of diffusion coefficients are close to values in pure water. Except for the asymmetry, the skin layer does not seem to be of importance for the mean EMF-values of the membranes. Heat curing has no influence on EMF-values.