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 [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 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 of 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.