MONTE-CARLO SIMULATIONS OF AN ELECTRIC DOUBLE-LAYER

Monte Carlo simulations of an electric double layer in an electrolyte solution have been performed in the canonical ensemble. The chemical potential of the electrolyte was evaluated with a modified Widom method. The effect of salt concentration, surface charge density, and ionic size on the ion distributions and on thermodynamic quantities was investigated. Particular emphasis was put on the evaluation of the osmotic pressure, and the results were compared with predictions from the Poisson-Boltzmann equation. The Poisson-Boltzmann results are in good agreement with simulation for systems containing monovalent counterions. In the presence of divalent counterions, the approximate theory fails and there is a qualitative difference between Poisson-Boltzmann and the exact results. For a 1:2 salt at sufficiently high concentration, there appears to be a negative net osmotic pressure as a result of the different correlation effects in the bulk and in the double layer. © 1990 American Chemical Society.