Mechanism of Adsorption of Water Vapor by Muscovite: A Model Based on Adsorption Calorimetry
Аннотация
Abstract Adsorption isotherms and differential heats of adsorption of water vapor by muscovite were measured at 303 K. The heats of adsorption are stepwise and each step corresponds to the stoichiometric formation of adsorption complexes of H 2 O molecules with K + ions, (H 2 O) n /K + , ( n = 1–6), which are located on the basal and lateral (edge) faces. At saturation, the ditrigonal cavities of the basal faces are fully occupied by hexameric clusters. It is suggested that half of the K + ions on the basal faces come from neighboring layers by migration under the influence of the adsorbed H 2 O at the initial stage of adsorption. Similar migration of K + to the edges was also hypothesized, suggesting that only every second site can be occupied by a cluster. At the final stage of adsorption, H 2 O molecules are believed to form H-bonded bridges between the hexameric water/cation clusters on the basal faces, whereas on the edge faces no such effect is believed to occur. The mean molar integral adsorption entropy of water is ∼−7 J/mol K less than the molar entropy of the bulk liquid. The mobility of H 2 O on muscovite is slightly less than in bulk water. Migration of K + cations under the influence of adsorbed H 2 O both on the basal and lateral faces of muscovite is reversible.