LIQUID-VAPOR EQUILIBRIA OF POLAR FLUIDS FROM A VAN DER WAALS-LIKE THEORY

A van der Waals-like theory of quadrupolar and dipolar linear fluids is presented. The reference system consists of a hard polar fluid, and attractive forces are considered through the mean field approximation. The effect of polar forces on liquid-vapor equilibria and on critical properties is analyzed for a number of molecular elongations. Trends as predicted by the theory are compared with computer simulations of linear polar fluids, and good agreement is found. Polar forces increase the critical temperature and acentric factor of a fluid. Quadrupole moment increases the critical density of a fluid. However, high dipole moments decrease critical densities. Deviations from the principle of corresponding states are analyzed. Polar forces and molecular elongation provoke a broadening of the coexistence curve and an increase of the slope of the vapor pressure curve when reduced by their critical magnitudes. The presented treatment, being quite simple, describes most of the main features of vapor-liquid equilibria of linear polar fluids.