Phase behavior of a simple fluid confined between chemically corrugated substrates

The phase behavior of a molecularly thin fluid film of Lennard-Jones (LJ)(1 2,6) fluid confined to a chemically heterogeneous slit-shaped pore was inve stigated by the grand canonical ensemble Monte Carlo (GCEMC) method. The sl it-shaped pore comprises two identical plane-parallel solid substrates, eac h of which consists of alternating strips of LJ(12,6) solid of two types: s trongly (width d(s)) and weakly adsorbing (width d(w)). With the substrates aligned so that strips of the same type oppose each other, GCEMC was used to compute the local and mean densities of the fluid as well as its isother mal compressibility as functions of substrate separation s(z) and for vario us degrees of chemical corrugation measured quantitatively in terms of c(r) :=d(s)/(d(s) + d(w)). Depending on s(z) and c(r), the confined fluid may co nsist of inhomogeneous gaslike or liquidlike phases filling the entire volu me between the substrates. In addition, liquid "bridges" may form as a thir d phase consisting of stratified liquid stabilized by the "strong" strips a nd separated from two surrounding gaslike regions by an interface. The phas e diagram involving all three phases was determined for a mean-field lattic e-gas model similar to the one investigated recently by Rocken and Tarazona [J. Chem. Phys. 105, 2034 (1996)]. The lattice-gas calculations permit a q ualitative interpretation of the complex dependence of the GCEMC results on both s(z) and c(r).