ION-ION CORRELATIONS IN ELECTRIC DOUBLE-LAYERS FROM MONTE-CARLO SIMULATIONS AND INTEGRAL-EQUATION CALCULATIONS

The anisotropic ion-ion correlation function g(R(1), R(2)), where R(1) and R(2) are the coordinates ofions 1 and 2, is calculated for double layer systems composed of counterions between two charged, planar wal ls. The primitive model of the electrolyte is used. The Widom formula for the excess chemical potential is applied in the neighbourhood of a confined particle in the double layer to obtain g(R(1), R(2)) from Mo nte Carlo (MC) simulations. The pair correlations are also calculated in the anisotropic RHNC approximation, where the reference hyper-nette d chain (RHNC) closure is applied at the pair (ion-ion) level and wher e the anisotropic bridge function is taken from an inhomogeneous refer ence system composed of a hard sphere fluid with the same density prof ile as the double-layer system. The results of the MC simulations and RHNC calculations are generally in very good agreement with each other . This demonstrates the fact that the anisotropic RHNC approximation i s a very accurate method for calculating the various double-layer prop erties. It also follows that the Widom technique is a very versatile t ool in simulation studies of double-layer systems. To obtain 15 points for the pair correlation function by simulation takes about 60 times longer than to obtain 200000 points by anisotropic RHNC calculations ( the latter takes a few minutes on an TBM RS-6000/590 workstation).