Influence of association on the liquid-vapor phase
coexistence of simple systems
Fernando Bresme, Enrique Lomba, and Jose L.F. Abascal
ABSTRACT
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-like potential which allows for chemical association of particles
and a Lennard-Jones interaction.
The bonding potential depends on three parameters, bonding 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.
