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 Mont e Carlo (GCEMC) simulations. For noncritical thermodynamic states of a corresponding bulk fluid, a homogeneous core region exists in the por e characterized by the bulk density rho(bulk) Sufficiently close to th e bulk critical point, however, the core is depleted, that is its loca l density rho(core)(z) is less than rho(bulk). Along a bulk near-criti cal 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), respective ly, where <(rho)over bar>(mid) is the average midpoint density of the core fluid. At densities sufficiently above and below the bulk critica l density thermodynamic states of con and bulk are the same, i.e. mu(b ulk) = mu(core). However, if depletion is observed the density depende nce of mu(bulk) and mu(core) is generally different. By applying a mea n-field theory to mu(bulk) and mu(core) depletion is related to a redu ced degree of criticality of the core fluid compared with the bulk pha se in a self-contained way. (C) 1997 American Institute of Physics.