Cluster growth and coalescence of argon on weakly adsorbing substrates: The origin of the thin-to-thick film transition

Abstract

We have studied argon adsorption on weakly adsorbing substrates using Monte Carlo simulation in both the canonical and grand canonical ensembles in order to determine the microscopic mechanism of the non-wetting/wetting/pre-wetting transitions, and have compared the results with adsorption on graphite (a strong adsorbent). To characterize the affinity of a substrate for a given adsorbate, we used the ratio D*, of the depth of the solid-fluid potential energy of a single molecule to the depth of the pairwise fluid-fluid potential energy. The range of D* investigated for argon is from 1.7 to 9, where 1.7 represents a very weak substrate and 9 represents a very strong substrate (graphite). Argon adsorption on graphite (D* = 9) exhibits a mechanism of wetting (molecular layering) of the first adsorbate layer while non-wetting is the mechanism for the weak substrate with D* = 1.7 at all temperatures. For the intermediate substrate, with D* = 4, the mechanism switches from non-wetting at low temperatures to wetting/pre-wetting at higher temperatures, with a so-called thin-to-thick film transition occurring at temperatures falling between the wetting temperature and the critical pre-wetting temperature. At the wetting temperature the isotherm exhibits a transition from non-wetting to an infinitely thick adsorbed film at P0, and at the critical pre-wetting temperature the “thin to thick” transition disappears. Below the wetting temperature the adsorption is non-wetting and above the critical pre-wetting temperature, adsorption follows a continuous wetting process. Analysis of the microscopic details of the canonical and grand canonical simulation results, reveals that the origin of the “thin-to-thick film” transition, is in fact a sequence of processes of cluster growth, coalescence and molecular layering. It is the coalescence of clusters to form molecular layers that is responsible for the sharp jump in the adsorbed density.