Equilibrium Theory of the Kaolinite-water System at Low Moisture Contents, with Some Remarks concerning Adsorption Hysteresis

A MICROSCOPIC theory of the kaol ini te-water system is presented, based upon the assumption t ha t the clay-water interaction may be envisioned as par t ly hydrat ion of the exchangeable cations and par t ly adsorption by the oxygen and hydroxyl surfaces. The theory t reats exchangeable cation hydrat ion quantum-mechanically as an ion-dipole phenomenon and considers water adsorption by the mineral surface as a problem in hydrogen bonding. A statistical mechanical model incorporating the quantum-theoretical results is then invoked to find the contr ibut ion of each component interaction to the initial port ion of the adsorption isotherm for homoionic kaolinite. Good agreement between the theoretical calculations and available experimental da ta is achieved for water vapor adsorption by Li-, Na-, K-, and Mg-kaolinito, wi thout the use of ad hoc empirical parameters. The concordance, in turn, is used to suggest t ha t the basis for adsorption hysteresis in kaolini te-water vapor systems is the irreversible t ransi t ion: mineral surface water--> cation hydrat ion water. I N T R O D U C T I O N IT XS well known that water vapor interacts to a significant degree with dry kaolinite. Perhaps the clearest non-calorimetric demonstration of this fact is the convex form of the adsorption isotherm at low relative vapor pressures, which immediately suggests that the initially adsorbed water molecules bind even more tenaciously to the clay than they do among themselves. Though simple, this qualitative hypothesis is not at all easy to elaborate in a rigorous manner, chiefly because experimental data that can be used to advantage in the development of a microscopic physical model are almost non-existent. This is not to say that a dearth of purely thermodynamic evidence on the kaolinite-water vapor interaction exists (see, for example, Martin, 1962), but * Present address: Depar tment of Chemistry-Physics, Sonoma State College, Rohner t Park, California.