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Abstract:
We studied electric relaxation across bi‐ionic (asymmetric) cells of the form electrode ‖ solution A ‖ membrane ‖ solution B ‖ electrode. Solutions A and B differ either in concentration or in chemical composition. Response curves were measured at 25 °C as a function of external current and voltage, membrane thickness, and various salts and solution concentration. Cellulose acetate films, between 3×10−4 and 21×10−4 cm thick, were used as membranes. External voltage changes the steady‐state ion concentration and the conductivity of the membranes. Conductivity increases with voltage of one polarity, but decreases with the opposite polarity. Ionic concentrations in the membrane require finite time to respond to voltage changes; consequently, the membrane exhibits time‐dependent rectification. This is clearly seen in experiments with sinusoidal waveforms. Input and output waveforms are superimposed at high frequencies, where ion redistribution rates are too slow to keep up with voltage oscillations: Here the membrane acts like a constant conductivity. As the ac frequency is reduced and ion redistribution can take place, we see waveform distortion and rectification in the output signal. We measured the voltage response of NaCl/MgCl2 cells to step‐current input and the current response of the same cells to step voltage. Our experiments are consistent with a nonlinear relaxation process, where the instantaneous conductivity Λ (t) relaxes toward a conductivity parameter G (V). G (V) is a function derived by Goldman, which relates the steady‐state conductivity to the voltage V across the membrane. Bi‐ionic cells exhibit both monotonic and oscillating response curves. Following numerical calculations by Cohen and Cooley, oscillating curves are thought to indicate two competing processes. External voltage increases the concentration of ions from one external solution in the membrane, while simultaneously decreasing the concentration of ions from the other solution. The first process increases membrane conductivity; the second lowers it. Superposition of a time‐dependent conductivity increase and a time‐dependent conductivity decrease produces maxima and minima in response curves. For comparison purposes, experiments on symmetric cells (solution A ‖ membrane ‖ solution A) are also reported.
