Ion association and condensation in primitive models of electrolyte solutions

Monte Carlo simulations have been used to investigate condensation and ion association in primitive models of electrolyte solutions. We have examined models in which the ions have equal diameter, the cation charge is +Zq, and the anion charge is -q, with Z=1, Z=2, and Z=4. Z=1 corresponds to the familiar restricted primitive model. Rough estimates of the critical temperatures have been obtained. In all cases the vapor phase is highly associated, with the ions forming electroneutral pairs, triples, and quintuples for Z=1, Z=2, and Z=4, respectively. Ion association has been investigated using a nearest-neighbor distribution function. For each value of Z the onset of ion dissociation in the low-density vapor is shown to occur at a temperature very close to the critical temperature. This raises the possibility of ion association affecting the critical behavior of ionic fluids. We have compared the simulation results for the restricted primitive model with the predictions of Debye-Hückel-based theories and the pairing mean spherical approximation. All of the theories perform well in the vapor phase, but are less successful in the liquid phase.