Phase transitions of argon multilayer films on graphite: Evolution from multilayer film to bulk solid

Effects of a Free Adsorbate Boundary on the Description of an Argon Adsorbed Film on Graphite below the Bulk Triple Point

Monte Carlo simulations have been carried out to study argon adsorption on graphite at temperatures below the bulk triple point temperature, T_tr^(bulk) = 83.8 K. Two models for graphite have been used to investigate the effects of an adsorbate patch with a free boundary on the layering temperatures, the two-dimensional (2D)-triple point and the 2D-critical point for the three adsorbate layers on the surface. The first model (S-model) has a planar surface of infinite extent in the two directions parallel to the surface, and the second is a finite (2D-patch model). Although simulations using both models describe the characteristic temperatures, only the 2D-patch model can represent the experimental isotherms accurately, and the condensation pressures at which first-order transitions occur, while simulations with the S-model yield many unphysical substeps that are not observed experimentally in the first layer adsorbate, which leads to a poor description of higher adsorbate layers. These results support the interpretation that boundary growth of an adsorbate patch is the mechanism for argon adsorption at temperatures below the bulk triple point temperature. Combining the results derived from this simulation study for temperatures below the bulk triple point temperature, with results reported in the literature for temperatures above T_tr^(bulk) and experimental data, we have constructed a generic pattern for the adsorption isotherms of simple gases on graphite at temperatures ranging from well below the bulk triple point temperature up to the bulk critical temperature, a comprehensive description not widely recognized in the literature.

Thermodynamic analysis of ordered and disordered monolayer of argon adsorption on graphite

We presented a detailed thermodynamic analysis of argon adsorption on a graphitized carbon black with a kinetic Monte Carlo scheme. In this study, we particularly paid attention to the formation of a hexagonal two-dimensional molecular layer on a graphite surface and discuss conditions of its stability and thermodynamic properties of the adsorbed phase as a function of loading. It is found that the simulation results are substantially affected by the dimensions of the simulation box when the monolayer forms a hexagonal ordered structure. This is due to the fact that the lattice constant is constrained by the dimensions of the surface. To circumvent this, we presented a thermodynamic technique, which allows for the variation of the box size as a function of loading, to determine the "intrinsic" lattice constant (rather than apparent average value because of the fixed dimensions of the simulation box) and the thermodynamic functions for the adsorbed phase: the Helmholtz free energy, the chemical potential, and the surface tension. The tangential and normal pressures as a function of the distance from the surface are also discussed.

Resonant frequency and dissipation factor isotherms of adsorbed molecules on solid-liquid interfaces probed by quartz crystal microbalance

Changes in the resonant frequency Deltaf and the dissipation factor DeltaD of a quartz crystal microbalance due to adsorption of molecules onto the electrode surface in solutions at different concentrations have been numerically analyzed. It has been found that the contribution from the solution due to the variation of the concentration can mask the overall behaviors of the Deltaf and DeltaD isotherms. However, if the solution is sufficiently dilute or the contribution of the solution can be characterized accurately, the corrected Deltaf and DeltaD , as a function of the solution concentration, will reveal characteristic features of the adsorption processes for the adsorbed films. Specifically, steplike behaviors will be displayed in Deltaf isotherms, which correspond to layer formation and layer-by-layer growth of the films, but the onsets of the steps usually do not coincide with the layer completion. Oscillatory behaviors will be revealed in DeltaD isotherms if the adsorbed molecular layer is soft at low concentration and becomes rigid near layer completion.