ATTRACTIVE DOUBLE-LAYER FORCES FROM HARD-CORE CORRELATIONS

The osmotic pressure in an electric double layer is studied by means of Monte Carlo simulations. The net osmotic pressure is calculated as the difference between the internal and external pressures calculated at the same chemical potential. This can be obtained with sufficient accuracy using the Widom insertion particle technique in the canonical ensemble. The same technique can also be applied in the calculation of different pressure contributions. We find that the net osmotic pressure is always repulsive in a system with monovalent co- and counterions, which is in agreement with the Poisson-Boltzmann equation as well as with previous simulation studies. In asymmetric salts, when the coion is multivalent we find that the external pressure exceeds the internal pressure resulting in a net attractive interaction. This attraction appears at high particle density or volume fraction in the double layer, for example, at short separations, high salt concentration, and/or high surface charge density. The origin of the attraction seems mainly to be the hard-core interactions, which the ions experience in a dense double layer, which is manifested in a high chemical potential. The corresponding bulk solution at this chemical potential has a very high salt concentration and hence also a high osmotic pressure.