Phase behavior of a simple fluid confined between chemically corrugated substrates

The phase behavior of a molecularly thin fluid film of Lennard-Jones (LJ)(12,6) fluid confined to a chemically heterogeneous slit-shaped pore was investigated by the grand canonical ensemble Monte Carlo (GCEMC) method. The slit-shaped pore comprises two identical plane-parallel solid substrates, each of which consists of alternating strips of LJ(12,6) solid of two types: strongly (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 isothermal compressibility as functions of substrate separation $s_z$ and for various 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 consist of inhomogeneous gaslike or liquidlike phases filling the entire volume between the substrates. In addition, liquid “bridges” may form as a third phase consisting of stratified liquid stabilized by the “strong” strips and separated from two surrounding gaslike regions by an interface. The phase diagram involving all three phases was determined for a mean-field lattice-gas model similar to the one investigated recently by Röcken and Tarazona [J. Chem. Phys. 105, 2034 (1996)]. The lattice-gas calculations permit a qualitative interpretation of the complex dependence of the GCEMC results on both $s_z$ and $c_r.$