Ion association in model ionic fluids

Monte Carlo simulations are used to investigate the temperature and pressure (density) dependence of ion association in the restricted primitive model. It is shown that at temperatures below the critical temperature T(c) the vapor consists almost exclusively of strongly bound ion pairs at or near contact. Significant ion-pair dissociation begins at temperatures very near T(c). This raises the possibility that compositional fluctuations between strongly bound and free ions influence the critical behavior. We note qualitative similarities between the present results and the Kosterlitz-Thouless transition in the two-dimensional Coulomb gas.