NEW INTEGRAL-EQUATION THEORY FOR PRIMITIVE MODEL IONIC LIQUIDS - FROMELECTROLYTES TO MOLTEN-SALTS

A new closure to the Ornstein-Zernike (OZ) equation is proposed for io nic liquids and is investigated for primitive models of high valency ( 2:2) aqueous electrolyte solutions and molten salts. The new closure, which is related to an earlier closure for the soft-sphere case propos ed by Ichiye and Haymet, may be viewed as a prescription for the so-ca lled ''bridge functions.'' These functions are approximated by zero in the hypernetted-chain (HNC) closure which is generally used for ionic systems. In both the new closure and the soft-sphere closure, the rec ognition that the unlike bridge function is opposite in sign from the like bridge function leads to an approximation for these missing graph s by adding (for the unlike case) or subtracting (for the like case) a set of graphs similar to those used in Percus-Yevick theory to the HN C equation. Compared to the HNC closure, the pair correlation function s predicted for primitive models by the new closure are generally in m uch better agreement with Monte Carlo (MC) simulations of molten salts and aqueous 2:2 electrolytes. The fundamental improvement of this pap er over the Ichiye-Haymet work is that the separation of long- and sho rt-range part of c(r) for the hard-sphere case is clearly defined, whe reas it was done numerically for the soft-sphere case. Moreover, the p resent theory is in better agreement with MC simulations both in the m olten salt as well as in the dilute solution regimes than the soft-sph ere case. Finally, a study was made of the transition of the like char ge pair correlation functions from monotonic behavior at low densities to a nonmonotonic behavior at high densities. The new closure clearly predicts such a transition region at concentrations near 0.02 M and t emperatures near 314 K. There is also a region below 0.02 M and 314 K where the new closure fails to converge. Compared to MC simulations, t he critical region predicted by the new closure appears to be a lower estimate. However, for the HNC closure there is only a remote possibil ity of such a transition region since the correlation functions are no nmonotonic even at lower concentrations, a feature which is corrected in the new theory.