WATER VAPOR-SODIUM MONTMORILLONITE INTERACTION

The interaction of water vapor with sodium montmorillonite was investigated with X-ray diffraction and sorption isotherm (gravimetric method) experiments. Expansion of the montmorillonite occurs in three increments. The data suggest that interlayer water builds up in a laminar fashion. The hysteresis of adsorption isotherms is apparently due to the formation of a thixotropic structure and to attractive interlayer forces. BET parameters from adsorption isotherm data reflect adsorption only on external surfaces. Free energy data, computed from adsorption isotherm data and X-ray data, allow separation of the free energy change on adsorption into two components: one for adsorption on external surfaces, and one for adsorption on, and separation of, internal surfaces. The data also permit the estimation of swelling pressures exerted by sodium montmorillonite due to the uptake of interlayer water from the vapor phase. /Author/