THE CASE FOR LANGMUIR CASE-VI

Attention is drawn to the equation (case VI) derived by Langmuir in 1918 for multilayer adsorption, which has been largely neglected. When derived by a statistical mechanical route as an alternative to Langmuir's kinetic derivation, the physical significance of the parameters is clarified. With the assumption that all adsorbed layers after the first have similar properties, but differ from the bulk liquid, the resulting equation, in the form of a modified BET equation, extends the applicability of the BET equation to high relative pressures. This has been tested by using recent precise measurements on a range of adsorption systems, and illustrative examples are presented. The superiority of the case VI equation over the conventional BET equation is clearly demonstrated. The soundness of the theoretical basis of this equation requires tabulated data over a range of temperature, but no published data are available.