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 lambda=1.5 conf ined to hard planar slits. We derive a general computer simulation method f or numerically calculating an arbitrary first derivative of the canonical e nsemble partition function with respect to a simulation parameter, which we call the virtual-parameter-variation method. Two special cases of this app roach are the Widom test-particle insertion method for calculating the exce ss chemical potential, and a method for calculating the pressure due to Epp enga and Frenkel [Mol. 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 (N,V,T) Monte Carlo simulation, and show that this sprea ding pressure is numerically consistent with the thermodynamic pressure obt ained by integration of the Gibbs-Duhem equation using the simulated chemic al potentials of the confined fluid as a function of density. We obtain new simulation results for the spreading pressure and the phase equilibrium pr operties of the confined square-well fluid, and we also estimate its critic al point properties, observing a decrease of the critical temperature in co mparison to the bulk fluid. (C) 2000 American Institute of Physics. [S0021- 9606(00)50510-8].