Theory and Observation of Displacement Phenomena in Coadsorbed Films

Displacement refers to a surprising phenomenon observed in a number of adsorption systems. At low temperatures, some relatively inert gases, such as krypton (Kr) or methane ( CH₄), are found to displace molecules from preadsorbed monolayer films of a more condensable species, such as carbon tetrachloride ( CCl₄), from a graphite substrate. We present a simple thermodynamic model to explain displacement and make a prediction applicable to both first-order and continuous displacement processes. We also present measurements on CH₄/CCl₄ and Kr/CCl₄ coadsorbed films from 77–112 K that verify our prediction and yield the CCl₄ film spreading pressure.