Oriented X-ray spacings for vermiculite and montmorillonite were obtained as the sample temperature was changed at a constant rate. Temperatures at which an abrupt change in (001) spacing took place indicated the relative ease of expansion or contraction for a series of interlayer ions. From such temperatures, the difference between contracting and expanding forces was found to be dependent upon interlayer ion charge, distance, and hydration energy, indicating essentially a strong-force balance (ion-water, ion-clay) for both vermiculite and montmorillonite.
Transition temperatures for hydration were lower than the corresponding dehydration temperatures, where small interlayer ions were involved. After removal of a water layer, the layer silicate contracted. Stronger hydrating forces at lower temperatures were required to reexpand the layer silicate prior to sorption of a corresponding water layer. Where the size of the interlayer ion approached that of the water molecule, preventing contraction during dehydration, the hysteresis was eliminated.
Temperatures at which comparable interlayers of water molecules were removed were higher for vermiculite than for montmorillonite, for Li, Na, Mg, Ca, Sr, and Ba ion saturations. A semi-quantitative consideration of the strong-force model indicates the presence of an expanding force in addition to hydration of interlayer ions, and one that is much stronger in vermiculite than in montmorillonite. Hydration of the negative charge sites meets these requirements, but does not explain ion fixation by vermiculite. Hydration of negative charge sites is indicated by plateau formation in the curves, by ion hydration theory and by swelling theory of ion-exchange resins.