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 electr olytes (1 molar concentration) are studied by molecular dynamics (MD) simulations and by the hypernetted chain (HNC) integral equations. Com parisons on the electrolyte structure and dynamical properties are mad e between a (a) soft ion (SI) model and a (b) soft ion in solution (SI S) model of electrolyte. In the SI model, there is no solvent, whereas the SIS model has ions and neutral solvent molecules at liquid densit y. Inclusion of the solvent imparts liquid-like structure to the ion-i on pair correlation functions and, if nu not equal infinity, gives ris e to substantial back scattering oscillations in the velocity autocorr elation functions with a consequent reduction in the particle self dif fusion 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 electro lyte (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 sim ulations of fluids with continuous repulsive forces but over-predicts pair correlations in hard sphere fluids.