Aspects of sorption and phase behavior of near-critical fluids confined to mesoporous media

The local structure of a Lennard-Jones fluid confined to a mesoscopic slit-pore with structureless (i.e., ''smooth'') walls is investigated by computing the local density rho(z) in grand canonical ensemble Monte Carlo (GCEMC) simulations. For noncritical thermodynamic states of a corresponding bulk fluid, a homogeneous core region exists in the pore characterized by the bulk density rho(bulk) Sufficiently close to the bulk critical point, however, the core is depleted, that is its local density rho(core)(z) is less than rho(bulk). Along a bulk near-critical isotherm T* = 1.36 similar or equal to T-c,T-bulk* GCEMC data are analyzed in terms of the equations of state mu(bulk)=mu(rho(bulk),T-c,T-bulk) and mu(core) = mu(<(rho)over bar>(mid),T-c,T-bulk), respectively, where <(rho)over bar>(mid) is the average midpoint density of the core fluid. At densities sufficiently above and below the bulk critical density thermodynamic states of con and bulk are the same, i.e. mu(bulk) = mu(core). However, if depletion is observed the density dependence of mu(bulk) and mu(core) is generally different. By applying a mean-field theory to mu(bulk) and mu(core) depletion is related to a reduced degree of criticality of the core fluid compared with the bulk phase in a self-contained way. (C) 1997 American Institute of Physics.