A theoretical isotherm to predict sorption from the vapour phase for heterogeneous sorbcnts has been derived from thermodynamic considerations. The predicted isotherm is a composite curve comprised of component isotherms arising from three separate processes : (a) monolayer adsorption by strongly binding sites, (b) monolayer adsorption by weakly binding sites, and (c) formation of a multilayer the extent of which is limited by the properties of the substrate. Components (a) and (b) have the form of Langmuir isotherms for monolayer adsorption at surfaces while component (c> describing the formation of a multilayer yields a modified Langmuir-type equation. In general, component (b) can be approximated by an adsorption which is linear with relative vapour pressure, For some substrates one or more of the three component isotherms may be inoperative. The composite isotherm is represented by the equation, and has general applicability to a wide range of systems for water vapour sorption.

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