ATTRACTIVE DOUBLE-LAYER FORCES FROM HARD-CORE CORRELATIONS

The osmotic pressure in an electric double layer is studied by means o f Monte Carlo simulations. The net osmotic pressure is calculated as t he difference between the internal and external pressures calculated a t the same chemical potential. This can be obtained with sufficient ac curacy using the Widom insertion particle technique in the canonical e nsemble. The same technique can also be applied in the calculation of different pressure contributions. We find that the net osmotic pressur e 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 a ppears at high particle density or volume fraction in the double layer , for example, at short separations, high salt concentration, and/or h igh 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 c orresponding bulk solution at this chemical potential has a very high salt concentration and hence also a high osmotic pressure.