Gauge cell method for simulation studies of phase transitions in confined systems

A method for Monte Carlo studies of phase equilibrium in confined systems i s presented using an example of vapor-liquid equilibrium (capillary condens ation and evaporation) in cylindrical pores. The method, named the gauge ce ll method, allows one to construct the full phase diagram of a confined flu id in the form of a van der Waals loop, which includes stable, metastable, and unstable equilibrium states. The phase coexistence is then determined b y thermodynamic integration along the metastable and unstable regions of th e phase diagram employing Maxwell's rule of equal areas. The simulation res ults agree with experimental data on the capillary condensation of nitrogen at its boiling temperature on mesoporous molecular sieves. The method can be applied to other phase transitions in confined systems such as fluid-flu id separation, layering, and freezing.