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.