INFLUENCE OF ASSOCIATION ON THE LIQUID-VAPOR PHASE COEXISTENCE OF SIMPLE SYSTEMS

The liquid-vapor phase diagram of an associating fluid interacting via a central force model potential is computed by means of the Gibbs ens emble Monte Carlo simulations. The Hamiltonian contains two components , a harmonic oscillator potential which allows for chemical associatio n of particles and a Lennard-Jones interaction. The bonding potential depends on three parameters, bending distance L, potential depth D-e, and force constant k(e). We have studied the influence of L on the pha se coexistence properties of the system. For small L the liquid phase shrinks and the results suggest that for short enough L, the stable li quid phase disappears. In addition to this, the coexistence curves exh ibit a large change in the coexistence densities as bonding distance i s shortened. The fitting of the coexistence data to scaling laws shows that a classical value for the critical exponent beta may be adequate to describe the phase boundaries of a system with short bonding dista nce whereas both classical and Ising values would be suitable to descr ibe the coexistence densities for a large L. Finally, the effect of as sociation on the asymmetry of the liquid-vapor coexistence curve is di scussed. (C) 1997 American Institute of Physics.