VAPOR-LIQUID-EQUILIBRIUM OF A MULTIPOLAR SQUARE-WELL FLUID .1. EFFECT OF MULTIPOLAR STRENGTHS

A simple polar fluid is modelled by square-well particles with point dipoles and quadrupoles at their centers. An equation for the free energy is derived by perturbation theory, obtaining the explicit dependence on the potential parameters, and allowing to study the effects of the strength of the multipoles on the vapor-liquid (V-L) equilibrium. It is found that the critical temperature is the property most sensitive to the values of the dipolar and quadrupolar moments, mu and Q, increasing with increasing moments although the effect of Q is much stronger. The shape of the coexistence curve is mostly affected on the liquid side and the coexistence diameters are found to be far from rectilinear. Comparison with simulation results shows that a multipolar SW fluid, for an attractive range equal to 1.5 times the diameter, is remarkably similar to the multipolar Lennard-Jones fluid.