Electric impedance of cellulose acetate membranes and a composite membrane at different salt concentrations

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Impedance measurements have been performed on dense and asymmetric cellulose acetate membranes cast in our laboratory. The frequency range was from 1 mHz to 65 kHz. Different concentrations of NaCl in water at 25°C were used. Two different potentiostats and different types of measurement cells were used and compared. The repeatability of the time constants was within ±7% except at extreme dilutions. A hybrid RC model is proposed, where the resistance ( R) is calculated from the Donnan distribution of the ions and the ratios of membrane diffusion coefficients found in earlier work. The diffusion constant of Na + is determined as a new parameter, taking the fixed charge of the membrane from previous data. D NA + is of the order of 10 −8–10 −9 cm 2/s. At low NaCl concentrations the diffusion coefficient of Na + decreases, probably due to electrostatic binding to the unshielded glucuronic acid groups. The capacity ( C) exhibited a marked increase with concentration, most drastically seen in the case of asymmetric membranes. This feature is explained using a theory of Trukhan combined with the integration method of Brüggemann. The effect arises from the dynamically created double layer generated by the applied field, when ions are confined in spherical alveoles. Alveolar sizes of about 70 ± 30 Å are determined for the dense membranes. For the asymmetric we determine 2000 ± 1000 Å, but the alveoles here seem to form a (fractal?) hierarchy of sizes. This is reflected in the lower value of the Cole-Cole α-parameter, especially at high NaCl concentrations. Heat curing of an asymmetric membrane only influenced the resistance, indicating a tightening of the pores in the skin layer. In contrast to the Trukhan-Brüggemann theory, the Maxwell-Wagner-Sillars theory is not able to account for the observed large variations in the membrane capacitances with concentration. A commercial composite membrane for seawater desalination (Fluid Systems) shows two relaxations corresponding to a skin layer and a support layer. The capacitance of the polysulfone support layer is determined by the Trukhan-Brüggemann theory, too.