MOLECULAR-DYNAMICS AND HYPERNETTED-CHAIN EQUATION STUDIES OF SOFT-CORE REPULSIONS IN ELECTROLYTE-SOLUTIONS

Effects of the repulsive soft core pair potential r(-nu) in 1:1 electrolytes (1 molar concentration) are studied by molecular dynamics (MD) simulations and by the hypernetted chain (HNC) integral equations. Comparisons on the electrolyte structure and dynamical properties are made between a (a) soft ion (SI) model and a (b) soft ion in solution (SIS) model of electrolyte. In the SI model, there is no solvent, whereas the SIS model has ions and neutral solvent molecules at liquid density. Inclusion of the solvent imparts liquid-like structure to the ion-ion pair correlation functions and, if nu not equal infinity, gives rise to substantial back scattering oscillations in the velocity autocorrelation functions with a consequent reduction in the particle self diffusion coefficient. Larger values of nu reduce the back scattering;in the velocity autocorrelation functions. The dynamical behavior of the ions in the SI model is similar to that of the primitive model electrolyte (theta = infinity) when nu greater than or equal to 9. In the SIS fluid, all the pair correlation functions have a similar oscillatory structure with roughly the same first peak separation for every pair. Simulation results obtained with the minimum image method compare well with those obtained with the more expensive Ewald sum method. The HNC theory predicts pair correlation functions in good agreement with simulations of fluids with continuous repulsive forces but over-predicts pair correlations in hard sphere fluids.