Response of the Surface Phase Transition to an Abrupt Pressure Change: Comparative Study of the 2D Gas–Liquid and 2D Fluid–Solid Phase Transitions

Abstract

The manner in which gaseous CH₄ and N₂ respond to an abrupt pressure change through adsorption and desorption on exfoliated graphite held at a temperature between 77 K and 90 K has been studied with particular emphasis on the role of the 2D (two-dimensional) Gas–Liquid phase transition of the second layer of adsorbed CH₄ and of the 2D Fluid–Solid phase transition of adsorbed N₂. The pressure relaxation was found to consist of two exponential decay components: a fast one and a slow one. For CH₄, the 2D Gas–Liquid phase transition is involved in the fast decay component with a time constant of 2–3 s, while the slow decay component with a time constant of 7–40 s is minor and has been attributed to ripening or coalescence processes in the adsorbed phase. In contrast, the 2D Fluid–Solid phase transition of N₂ involves both the fast decay component with a time constant of 2–3 s and the slow decay component with a time constant of 14–16 s, both having nearly equal magnitudes. The difference in the pressure response between the two phase transitions is discussed.