Effect of nonideality of solution on the diffuse layer properties near thepoint of zero charge in a nonsymmetrical electrolyte

The diffuse layer equations are analyzed near the point of zero charge in a nonsymmetrical electrolyte at any concentration dependence of the ionic ac tivity coefficients. The standard equality of two thermodynamic expressions for the Esin-Markov coefficients is shown to turn into an identity only if the equality (partial derivative mu(+)/partial derivative n(-))(n+) = (par tial derivative mu(-)/partial derivative n(+))(n_) is fulfilled. Here, mu() and mu(-) are chemical potentials of cations and anions, while n(+) and n _ denote the ratios of concentrations of cations and anions to the solvent concentration. Our conclusion is that, in the first order by the electrode charge, the differential of the Gibbs thermodynamic potential within the di ffuse layer may be written in a form that formally corresponds to neutral, rather than charged, species. In the point of zero charge, an expression is found for the diffuse layer capacitance that represents a generalization o f the Gouy-Chapman theory for the case of an actual nonsymmetrical electrol yte. Comparing with the experimental data on the differential capacitance o f the mercury electrode in sodium and lanthanum sulfate solutions shows tha t the deviation of the measured capacitance from that calculated with the G ouy-Chapman theory is caused largely by a strong dependence of the ionic ac tivity coefficient on the oppositely charged ion's concentration. Partial d erivatives of the ionic activity coefficients with respect to the concentra tion of the oppositely charged ion are found with an experiment.