The quasi-two-dimensional regime of fluids absorbed in porous media

We investigate the one-body structure of fluids confined within model pores when the confinement is sufficient to force the fluid into a quasi lower-dimensional regime. The concept of the quasi-two-dimensional regime of atomic fluids confined to symmetric planar slit geometry is introduced and statistical mechanical arguments are shown to predict a universal form for the density profile; or to be more precise, to imply that the many-body contribution to the one-body structure is a particular universal function of position. Computer simulation data of Lennard-Jones fluid confined to planar slit pores are presented, which clearly illustrate the statistical mechanical prediction. Generalizations of this phenomenon to include fluid mixtures, molecular fluids and nonplanar geometries are discussed.