Effects of melting and ordering on the isosteric heat and monolayer density of argon adsorption on graphite
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Effects of melting and ordering on the isosteric heat and monolayer density of argon adsorption on graphite Eugene A. Ustinov · Duong D. Do
Received: 13 October 2012 / Accepted: 6 December 2012 / Published online: 18 December 2012 © Springer Science+Business Media New York 2012
Abstract The aim of this paper is to study the effects of temperature on the state of the adsorbed argon on an uniform graphite surface. We applied the kinetic Monte Carlo scheme to simulate adsorption over a very wide range of temperature, which allows us to model the vapor–solid, the vapor–liquid and the order–disorder transition of the monolayer. The main distinction of our methodology is that it accounts for the lattice constant change with loading in the case of formation of an ordered molecular layer by appropriately changing the simulation box size. To do this we enforced the equality of the tangential pressures obtained by the virial and thermodynamic routes, which corresponds to the minimum Helmholtz free energy of a system at a given number of molecules and volume. This criterion is a consequence of the Gibbs–Duhem equation. A significant result obtained by application of the new simulation method was a sharp contraction of the monolayer just after its completion and the onset of the second layer. It manifests itself in an additional heat release. We re-determined the 2D-melting and 2D-critical temperatures of the molecular layer of argon. We also analyzed the order–disorder transition above the 2D-melting and showed that it could occur at some temperatures above the 2D-critical temperature. In this case, a hexagonal lattice appears at a sufficiently large tangential pressure. The effects of loading on the lattice constant, the 2D-critical temperature of the order–disorder transition and the differential heat of adsorption are thoroughly discussed. E.A. Ustinov () Ioffe Physical Technical Institute, 26 Polytechnicheskaya, St. Petersburg 194021, Russia e-mail: [email protected] D.D. Do School of Chemical Engineering, University of Queensland, St. Lucia, QLD 4021, Australia
Keywords Kinetic Monte Carlo · Adsorption on graphite · Monolayer phase transition · Heat of adsorption 1 Introduction Molecular simulation methods have been constant refined and improved, and this motivates a surge of interest to reconsider simple systems, which, in spite of their simplicity, show some discrepancy between the theoretical predictions and the experimental data. A classic example to this is the existence of a heat spike on the heat curve versus loading in the system Ar–graphite at 77 K (Rouquerol et al. 1977; Grillet et al. 1979) at a loading close to the monolayer concentration; the spike corresponds to a two-dimensional (2D) ordering transition. Reproducing of this spike by molecular simulation has been a very challenging task. There were several attempts to describe this phenomenon with grand canonical Monte Carlo method (GCMC) (Nicholson et al. 1977; Nicholson and Parsonage 1986; Wongkoblap and Do 2007; Fan et al. 2010), but they were no
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