INTEGRAL-EQUATION THEORY OF IONIC-SOLUTIONS

A new closure to the Ornstein-Zernike equation is proposed for ionic liquids such as electrolytes. The closure is investigated numerically for a model electrolyte consisting of charged soft spheres in a uniform dielectric medium. The new closure, which we call the ionic Percus-Yevick (IPY) closure, may be viewed as a prescription for the so-called "bridge function," which is approximated by zero in the well-known hypernetted-chain (HNC) closure. Compared to the results of Monte Carlo simulations, the pair correlation functions predicted from this new closure for 2:2 electrolytes at low concentrations are in much better agreement than those predicted by the HNC closure. In particular, whereas the HNC approximation predicts incorrectly a peak in the pair correlation functions for like charges at an interionic separation of about two diameters at low concentrations, the new IPY closure predicts correctly no such peak. At higher concentrations, both the HNC and IPY closures yield correlation functions which are close to those calculated from Monte Carlo simulations, with the new IPY closure being more accurate for unlike charges and the HNC closure being slightly more accurate for like charges.