Ion-ion correlations in electric double layers from Monte Carlo simulations and integral equation calculations Part 2. Case of added salt

Abstract

Anisotropic ion–ion distribution functions gij (R 1, R 2), where R 1 and R 2 are the positions of ions of species i and j respectively, and various other properties of electric double layer systems have been calculated by simulation and integral equation methods. The system is composed of a 1 : 1 or 2 : 1 electrolyte solution between two planar walls with rather high surface charge density (0·267 Cm-2) and in equilibrium with a bulk electrolyte solution with a certain concentration (1·0 M or 2·0 M). In the integral equation calculations, the hypernetted chain (HNC) or the reference hypernetted chain (RHNC) closure has been applied for the ion-ion distribution functions in the inhomogeneous electrolyte (the so-called anisotropic HNC or RHNC approximations). The Widom test particle technique has been used to calculate the ion–ion distribution functions in the simulations. The results of the anisotropic RHNC calculations and the simulations are in almost perfect agreement throughout (they virtually coincide). The HNC approximation is almost as good as the RHNC approximation for 2 : 1 electrolytes, but it is not as good for 1 : 1 electrolytes due to the higher packing density of the monovalent counterions near the surfaces. The excellent results for the pair distributions ensures that thermodynamic properties calculated in the RHNC approximation will be very good. The double layer interaction pressure, which is sensitive to details of the distributions, is indeed in excellent agreement with the simulations. The results also show that the Widom technique is successful for calculating pair distributions for inhomogeneous electrolytes by simulation.