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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.
Terrell L. Hill (Mon,) studied this question.
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