Computer Simulation Studies of a Square-Well Fluid in a Slit Pore. Spreading Pressure and Vapor-Liquid Phase Equilibria Using the Virtual-Parameter-Variation Method

We study model square-well fluids with well-width parameter λ = 1.5 confined to hard planar slits. We derive a general computer simulation method for numerically calculating an arbitrary first derivative of the canonical ensemble partition function with respect to a simulation parameter, which we call the Virtual-Parameter-Variation method. Two special cases of this approach are the Widom test-particle insertion method for calculating the excess chemical potential, and a method for calculating the pressure due to Eppenga and Frenkel [Molec. Phys. 52, 52, 1303 (1984)]. We use this approach to calculate the volume derivative parallel to the slit walls of the Helmholtz free energy in an (NV T ) Monte Carlo simulation, and show that this spreading pressure is numerically consistent with the thermodynamic pressure obtained by integration of the Gibbs-Duhem equation using the simulated chemical potentials of the confined fluid as a function density. We obtain new simulation results for the spreading pressure and the phase equilibrium properties of the confined square-well fluid, and we also estimate its critical point properties, observing a decrease of the critical temperature in comparison to the bulk fluid.