Three-phase osmotic equilibria using the Gibbs ensemble simulation method

The Gibbs Ensemble Monte Carlo method has been used to simulate osmotic equilibria for Lennard-Jones mixtures. When the simulations are performed with two independent boxes, one containing solvent and the other a mixture of solute and solvent significantly negative osmotic pressures (Pi) develop. Following a sugestion of Powles et al. (1997, Molec. Phys., 90, 665), we have extended these simulations to include a third box and the possibility of modelling three coexisting phases. The simulations show that the two phase equilibria with negative values of Pi are metastable and that the system spontaneously separates into three phases: pure solvent, dilute solute-solvent and dense solute-solvent with a resulting osmotic pressure that is normally small and positive.