Effects of the fluid–wall interaction on phase transitions in confined fluids.

Monte Carlo simulations of simple Lennard Jones fluids confined in slit-like pores were studied for the gas–liquid transition using weak and very attractive walls. Several fluid–wall potential strengths were simulated and pressure transitions were obtained at different temperatures. In the case of weak fluid–wall interaction typical results were found, the gas–liquid transition pressure increases with the temperature. When the fluid–wall becomes more attractive, the transition pressure increases significantly but decreases with the temperature. For a very attractive wall, i.e. the fluid–wall interaction is very strong, the transition pressure decreases with the temperature given a negative slope of the coexistence line in the P-T phase diagram. Analysis of the systems in terms of the energy shows that for weak walls the difference in the potential and kinetic energies drops at the transition pressure and the enthalpy of adsorption is negative whereas for strong walls the difference in energies increases at the transition and the adsorption enthalpy has positive values. [ABSTRACT FROM AUTHOR]