Analysis of Liquid-Vapour Phase Transitions for Restricted Primitive Model of Ionic System in Mean Spherical Approximation

Abstract

Within the framework of MSA exact solution for charged hard spheres with equal diameters found by Waisman and Lebowitz for Coulomb part of free energy and Mansoori et al., approximation for hard sphere contribution characteristics of evaporation phase transitions in terms of the equation of state have been estimated. The phase diagram has been evaluated and the behaviour of thermodynamic properties studied in a wide range of temperature and pressure for the example close to KF system. The position of the critical point is in accordance with the earlier determination of Telo da Gama, Evans and Sluckin using generalised MSA. Numerical values of the coefficients in the expansion of the relative deviation in pressure in powers of temperature and order parameter have been simply estimated near the critical region. Temperature and pressure dependencies of entropy and isothermal compressibility are typical enough for fluid systems, notwithstanding the collective nature of ionic interactions. In addition, the known Kauzman entropy paradox arises at low temperatures.

Key Words:

• Restricted primitive model
• mean spherical approximation
• equation of state
• evaporation
• critical behaviour