Effect of competition between Coulomb and dispersion forces on phase transitions in ionic systems

Abstract

A restricted primitive model (RPM) for ionic systems in which the Coulomb and hard-core interactions are supplemented with short-range (SR) interactions between all the components, including solvent particles, is introduced and studied within a mean-field approximation. Continuum-space as well as simple-cubic lattice systems are considered. A continuous and a first-order phase transition, separated by a tricritical point (tcp), are found between uniform and charge-ordered phases in all the systems considered. The position of the tcp as well as the slope of the line of the continuous transition depend on both the model and the SR interactions. For weak or vanishing SR interactions, at temperatures lower than the transition temperature, two oppositely charged sublattices are found on the simple-cubic lattice, whereas in the continuum case a lamellar structure consisting of charged layers of alternating sign occurs. For strong SR interactions the structure becomes incommensurate with the lattice in the lattice model. Both on the lattice and in the continuum a transition between uniform ion-poor and ion-rich phases occurs for sufficiently strong SR interactions. This critical point (scp) is not to be confused with the liquid–gas type critical point (cp) that already occurs in the continuum-space version of the RPM in the absence of SR interactions. The density at the scp is significantly higher than the density at the cp. The way this latter critical point is perturbed by the presence of SR interactions is not addressed in the analysis here. The SR interactions influence the charge ordering in such a way that the tcp can be located in the same range of densities as the stable critical point. For strong SR the tcp is located close to the scp, whereas for weak SR it is close to the cp.