Using a novel method the force between two charged surfaces with an intervening electrolyte solution has been determined from Monte Carlo simulations. We find large deviations from the standard Poisson–Boltzmann treatment of the so called double layer force for divalent counterions at high surface charge densities and at short separations. The deviations have two causes: (i) Due to the inclusion of the effect of ion–ion correlations the counterions concentrate more towards the charged wall reducing the overlap between the double layers; and (ii) correlated fluctuations in the ion clouds of the two surfaces lead to an attractive interaction of a van der Waals type. For some realistic values of the parameters the attraction overcomes the repulsive part and there is a net attractive force between similarly charged surfaces. This finding leads to a modification of our conceptual understanding of the interaction between charged particles and it shows that the DLVO theory is qualitatively deficient under some, realistic, conditions.

The electrostatic contribution to the force between hexagonally packed B-DNA double helices has been studied using different statistical mechanical descriptions and the Monte Carlo simulation method. The effects of small ion correlations and sizes are considered, and comparison with experimental results is made. It is found that for monovalent counterions, the mean field Poisson–Boltzmann theory can give a reasonable reproduction of the experimental data, even though the simulations show that its description of the electrostatic interaction can be qualitatively wrong. The chemical equilibrium of the ordered DNA phase with the surrounding bulk salt solution is found to be an important feature. Furthermore, the simulations show that the correlation between the ion clouds of different DNA polyions gives rise to a significant attractive contribution to the interaction when divalent cations are present. It is suggested that this electrostatic attraction is an important driving force for the experimentally observed condensation and collapse of DNA solutions to ordered systems, which can be induced by multivalent ions.

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The relation between the molecular properties of a surfactant and the phase equilibria in the corresponding surfactant -water system is investigated. The emphasis is on the qualitative features that emerge from previous quantitative studies. The systems discussed are: poly(ethyleneoxide)alkyl ether -water, dialkylphosphatidylcholine -water, alkanoate -water, and alkanoate -alcohol -water. While the interactions involving the alkyl chains are essentially the same in these systems, there are large differences in the nature of the interaction between the polar groups. For each system the dominating interactions involving the polar groups are identified. For the ionic surfactant this is the direct ion -ion interaction, while for the zwitterionic phosphatidylcholines the headgroup -headgroup and headgroup -solvent interactions are both important. It is argued that the hydrophilicity of the ethyleneoxide chains is mainly due to the entropy of mixing with the aqueous solvent, while there is an effective repulsive interaction between an ethyleneoxide unit and the water above room temperature. The repulsion increases strongly with increasing temperature which has important implications for the temperature dependence of phase equilibria.

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