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 i s 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 l inear polar fluids, and good agreement is found. Polar forces increase the critical temperature and acentric factor of a fluid. Quadrupole m oment 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 elongati on provoke a broadening of the coexistence curve and an increase of th e slope of the vapor pressure curve when reduced by their critical mag nitudes. The presented treatment, being quite simple, describes most o f the main features of vapor-liquid equilibria of linear polar fluids.