Fluid–Solid Phase Transition in Molecular Layers Adsorbed on a Smooth Surface: A New Insight from Molecular Simulations

Abstract

Two-dimensional (2D) phase transitions in molecular layers adsorbed on various surfaces have attracted much research attention over the past decades. Special attention is paid to the solidification/melting transition using methods based on molecular simulation techniques. The main research interest is focused on parameters of the liquid–solid or fluid–solid transitions; that is, the pressure, the chemical potential, and the density change. It is implied that the liquid and solid phases coexist like vapor and liquid with a flat interface, and the transition occurs simply via gradual increase of the solid fraction at a constant pressure. However, visualization of the transition based on molecular simulation in a canonical Monte Carlo method in a relatively large simulation cell has shown that the liquid and solid phases form islands in the liquid or “lakes” in the solid of arbitrary size and shape without a distinct interface. For this reason, the aim of this study was a detailed analysis of the 2D phase transition with the emphasis on the properties of the intermediate state between the pure liquid (fluid) and the crystal. Thermodynamically, the transition phase behaves like a two-phase system: when the density changes over a certain range at a specified temperature, the tangential pressure is nearly constant. However, no energy barrier was observed at the transition from the pure liquid or fluid to the intermediate phase. The absence of the van der Waals loop provides a very efficient way to determine parameters of the 2D liquid/fluid–solid transition and the chemical potential of the crystalline phase by a thermodynamic integration or direct evaluation with the kinetic Monte Carlo simulation in a sufficiently large simulation cell. The advantage of this way is that it makes the liquid–solid transition reversible without resorting to any special procedures.