Thermodynamics of interlayer adsorption of water in clays. I.—Sodium vermiculite

Adsorption-desorption isotherms for water vapor and a high charge density sodium vermiculite were determined at 25°C. and at 50°C. X-ray patterns were obtained at various stages of hydration of the clay. Heats of immersion were determined calorimetrically for the dry clay and for partially hydrated samples. The adsorption of water appears to take place in two distinct steps corresponding with the intercalation of the lattice with one and with two monomolecular layers of water. The shape of the isotherms can be seen as the superposition of two Langmuir-type isotherms. The observed hysteresis is attributed to a retardation of the adsorption process owing to the development of elastic stresses in the crystallites during the initial peripheral penetration of water between the unit layers. On the assumption that the desorption isotherm represents equilibrium values, the thermodynamic constants for the sorption process were determined.

The calorimetric data suggest that the integral heats of sorption per mole of water are constant during the formation of each layer of water, the heat of sorption for the first layer per mole of water being about twice that for the second layer. Agreement between the calorimetric data and the heats determined from the isotherms at two temperatures is reasonable for the first stage, and good for the second.

The integral entropy of sorption is negative with respect to the entropy of liquid water. Since the system was treated as a one-component system (the adsorbate), the computed integral entropy includes entropy changes in the solid due to the parting of the unit layers and changes in the positions of the interlayer cations.

From the isotherms the net work of unit layer interaction can be computed as a function of unit layer distance, from which the pressures required to squeeze out the water layers are derived. The net work consists of the hydration energy proper minus the potential energy of electrostatic and van der Waals unit layer attraction. From an estimate of the attraction energy, the hydration energy is estimated to be considerably smaller than ion hydration energy in bulk solution.