ION-ION CORRELATIONS IN ELECTRIC DOUBLE-LAYERS FROM MONTE-CARLO SIMULATIONS AND INTEGRAL-EQUATION CALCULATIONS .2. CASE OF ADDED SALT

Anisotropic ion-ion distribution functions g(ij)(R-1,R-2), where R-1 a nd R-2 are the positions of ions of species i and j respectively, and various other properties of electric double layer systems have been ca lculated by simulation and integral equation methods. The system is co mposed of a 1:1 or 2:1 electrolyte solution between two planar walls w ith rather high surface charge density (0.267 C m(-2)) and in equilibr ium with a bulk electrolyte solution with a certain concentration (1.0 M or 2.0 M). In the integral equation calculations, the hypernetted c hain (HNC) or the reference hypernetted chain (RHNC) closure has been applied for the ion-ion distribution functions in the inhomogeneous el ectrolyte (the so-called anisotropic HNC or RHNC approximations). The Widom test particle technique has been used to calculate the ion-ion d istribution functions in the simulations. The results of the anisotrop ic RHNC calculations and the simulations are in almost perfect agreeme nt throughout (they virtually coincide). The HNC approximation is almo st as good as the RHNC approximation for 2:1 electrolytes, but it is n ot as good for 1:1 electrolytes due to the higher packing density of t he monovalent counterions near the surfaces. The excellent results for the pair distributions ensures that thermodynamic properties calculat ed in the RHNC approximation will be very good. The double layer inter action pressure, which is sensitive to details of the distributions, i s indeed in excellent agreement with the simulations. The results also show that the Widom technique is successful for calculating pair dist ributions for inhomogeneous electrolytes by simulation.