MICROSCOPIC STRUCTURE OF A PURE NEAR-CRITICAL FLUID CONFINED TO A MESOSCOPIC SLIT-PORE

The adsorption behavior of a near-critical fluid (pore fluid) in a mesoscopic slit-pore is investigated in grand canonical ensemble Monte Carlo (GCEMC) simulations. In these simulations the chemical potential mu(T) (T is the temperature) is chosen such that the pore fluid is in thermodynamic equilibrium with a homogeneous bulk fluid reservoir maintained at the critical density rho(c). These conditions mimic recent adsorption experiments [M. Thommes, G. H. Findenegg, and H. Lewandowski, Ber. Bunsenges. Phys. Chem. 98, 477 (1994)] in which, after a maximum, a sharp decrease of the pore average density rho(p) is observed as T approaches T-c from above (i.e., T --> T-c+). The GCEMC simulations offer a microscopic explanation of this effect in terms of the local density rho((1))(z) of the pore fluid. It is found that the mean density in the core region of the pore and the mean density of the first layers next to the walls exhibit an opposite temperature dependence: while the density near the wall increases with decreasing temperature (as is to be expected for sufficiently strong attractive fluid-wall interactions) the density in the core region decreases and falls below rho(c) as T --> T-c+. This latter effect dominates in the near-critical region and causes a net decrease of the pore density. This depletion effect is different from drying which is characterized by a decreasing density in the vicinity of the walls. Depletion depends on the interplay between the strength of the fluid-wall potential and the width of the pore's cross section. By employing a two-fluid van der Waals model for the fluid in the core region and the bulk reservoir it is shown that depletion is closely related to restricted density fluctuations in confined fluids.