A closed loop of liquid-liquid immiscibility for a simple model binary symmetrical mixture of square-well
monomers with a single short-ranged interaction site has been recently observed using the Gibbs ensemble
Monte Carlo technique @L. A. Davies, G. Jackson, and L. F. Rull, Phys. Rev. Lett. 82, 5285 ~1999!#. This
model system has unfavorable mean-field interactions between unlike components which leads to phase separation
at intermediate temperatures; the addition of a directional bonding site leads to association and miscibilty
of the system at low temperatures. In this work we present a detailed study of the effect of a variation in
pressure and of the strength of the bonding interaction on the phase equilibria of such a model system by Gibbs
ensemble simulation. The phase diagram is dominated by regions of liquid-liquid immiscibility which are
bounded at high temperatures by an upper critical solution temperature and by a lower critical solution
temperature ~LCST! for specific values of the pressure and association strength. This closed-loop region is seen
to increase in size as the pressure of the system is increased. For weak bonding interaction strengths the system
does not possess a LCST and is seen to exhibit regions of two-phase vapor-liquid coexistence which are
separated from the region of liquid-liquid immiscibility by a three-phase line. The phase equilibria of the same
model system is also determined using the statistical associating fluid theory as adapted for potentials of
variable range; the theory provides a good description of the closed-loop immiscibility and other features of the
phase diagram
