Negative differential capacities in electrical double layers

Monte Carlo simulations of an electrical double layer in a primitive model of a 0.05 M 1:2 electrolyte are reported in which the total potential drop across the interface decreases as the surface charge is increased. This violates an alleged exact condition for such systems which has, however, recently been called into question by Attard et al. [J. Chem. Phys. 96, xxxx (1992)] and confirms their analysis that the surface potential need not be a monotone function of the surface charge.