Steric effects on the dynamics of electrolytes

The classical Poisson-Boltzmann (PB) theory of electrolytes assumes a dilute solution of point charges with mean-field electrostatics. Even for very dilute solutions, however, it predicts absurdly large ion concentrations (exceeding close packing) for surface potentials of only a few tenths of a volt. In this talk, we adopt a simple model for steric effects with only one parameter (the effective ion size), and we analyze the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage. A surprising prediction is that the differential capacitance varies non-monotonically with the applied voltage, and thus so does the response time of an electrolytic system. This effect is able to predict the reversal of AC electro-osmotic flow at high voltage and high frequency, as well as the increase of capacitance with temperature in molten salts.