Phases and Phase Transitions in the First Few Layers of Methane, Argon and Krypton Adsorbed on Graphite

Abstract

The equilibrium phase diagrams of monolayers of many substances adsorbed on graphite have long been studied as examples of 2-dimensional (2D) matter. One typically observes 2D gas and liquid phases, and solid phases that may be commensurate or incommensurate with the substrate lattice. Many experimental techniques have been used, but thermodynamic measurements are generally the most useful for tracing out phase boundaries. Recent advances in technique have made it possible to use thermodynamic measurements to study the phase diagrams of the second and higher layers, up to the fifth or sixth. These advances include ultra high resolution scanning calorimetry, and a detailed understanding of the role of capillary condensation in corners and pores of the graphite foam substrate. We find a rich array of phases and phase transitions in multilayer methane, argon and krypton. The second and third layers typically have distinct 2D gas, liquid and solid phases evidenced by 2D triple points and critical points. We observe phase transitions between solid phases that are commensurate and incommensurate with the layer below. We also observe melting of the first layer at higher temperatures, even when one to five additional layers are adsorbed on top of it. In argon and krypton, but not in methane, a strange new phenomenon is observed at temperatures above the gas-liquid critical point of the nth layer for n> 3. Below that temperature, nthlayer gas coexists with a condensed nth layer. At some temperature above it, a new coexistence region is observed in which a partial nth layer coexists with a partial n + 1st layer. This behavior is thought to be evidence for a theoretically predicted phase transition of the bulk interface, called the preroughening transition.