Criticality and phase behavior in the restricted-primitive model electrolyte: Description of ion association

摘要

Ion association is incorporated into the restricted-primitive model electrolyte to account for the strong attraction between unlike ions. Two methods are investigated within the McMillan-Mayer framework: first is the binding mean-spherical approximation (BIMSA) based on the Wertheim Ornstein-Zernike integral equation formalism; and the second is the combination of the BIMSA with a simple integral equation formalism; and the second is the combination of the BIMSA with a simple interpolation scheme based on the Wertheim thermodynamic perturbation theory. The latter gives a better description. Four different association constants are used to calculate the degree of dissociation, the critical point, and the vapor-liquid coexistence curve. An increase in the association constant leads to a lower critical temperature and a higher critical density, and better agreement with computer simulations. When unlike ions are fully paired, corresponding to charged hard dumbbell system, we obtain the best agreement with the most recent computer simulations of the RPM electrolyte.