Statistical Mechanics of Multimolecular Adsorption II. Localized and Mobile Adsorption and Absorption

Abstract

It is shown that the transition from localized to mobile adsorption (in the first adsorbed layer) takes place at rather low temperatures for potential barriers of around 1000 cal./mole or less, so that localized physical adsorption should be a very rare phenomenon at the temperatures usually employed in adsorption experiments. Since the BET (and Langmuir) isotherm actually assumes localized adsorption, a new approximate isotherm equation is derived here on the basis of a mobile first layer obeying a two‐dimensional van der Waals' equation. This isotherm is in semiquantitative agreement with the two‐dimensional phase changes observed by Jura and co‐workers, and by earlier workers, at very low pressures. The localized‐mobile transition in the absorption of hydrogen by metals is mentioned briefly. The relationships between (1) the two‐dimensional equation of state of a monolayer, (2) the adsorption isotherm of the monolayer, and (3) the ordinary three‐dimensional equation of state of the gas being adsorbed are discussed in some detail.