ULTRATHIN FLUID FILMS CONFINED TO A CHEMICALLY HETEROGENEOUS SLIT-SHAPED NANOPORE

The properties of a molecularly thin film of Lennard-Jones (LJ) (12,6) fluid confined to a chemically heterogeneous slit-shaped pore were in vestigated by the grand canonical ensemble Monte Carlo (GCEMC) method. The slit-shaped pore comprises two identical plane-parallel solid sub strates, each of which consists of alternating strips of LJ(12,6) soli d of two types: strongly adsorbing and weakly adsorbing. With substrat es aligned so that strips of the same type oppose each other, GCEMC wa s used to compute equilibrium properties of the film as functions of t he distance s(z) between the substrates. Results are compared for two well depths epsilon(fs) of the LJ(12,6) potential between molecules in the film and those in the strongly adsorbing strip. Variations in ten sions, mean film density, and isothermal compressibility as functions of s(z) are correlated with structural changes in the film and with it s phase behavior. In both cases, when the substrates are sufficiently close together, liquid bridges exist between the opposing strong strip s, surrounded by dilute gas over the weak ones. The stronger substrate (i.e., the one with the greater value of epsilon(fs)) is capable of s tabilizing a liquidlike phase that fills the whole pore over a certain range of s(z), then abruptly evaporates beyond a critical value of s( z), to leave nanodroplets clinging to the strong strips. For the weake r substrate, however, the liquid bridges collapse to form nanodroplets directly, without the intermediate appearance of a liquid pore-fillin g phase.