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 ensemble Monte Carlo simulations. The Hamiltonian contains two components, a harmonic oscillator potential which allows for chemical association 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 phase coexistence properties of the system. For small L the liquid phase shrinks and the results suggest that for short enough L, the stable liquid phase disappears. In addition to this, the coexistence curves exhibit a large change in the coexistence densities as bonding distance is 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 distance whereas both classical and Ising values would be suitable to describe the coexistence densities for a large L. Finally, the effect of association on the asymmetry of the liquid-vapor coexistence curve is discussed. (C) 1997 American Institute of Physics.