Abstract
The physically adsorbed monolayer of water vapor on hydroxylated α-Fe 2O 3 is shown to be immobile through application of adsorption thermodynamics and statistical mechanics. The integral entropy of adsorption obtained from multitemperature isotherms at constant spreading pressure is −33.2 eu for half a monolayer. This value compares favorably with the theoretical value of −35.8 eu calculated from statistical mechanics for immobile adsorption, but not with the value −15.9 eu similarly derived for mobile adsorption. The experimental and theoretical entropies for immobile adsorption agree quite well over the entire monolayer. Spreading pressures required to evaluate the experimental integral entropies were obtained in two ways: (1) graphically; and (2) analytically, with a two-part integration of the Gibbs equation based on Dubinin-Radushkevich behavior in the monolayer and on Frenkel-Halsey-Hill multilayers. Both methods gave the same spreading pressure for a given coverage. The mechanism by which the monolayer is localized is presumably hydrogen bonding of a single water molecule to two underlying hydroxyl groups.
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