The crystalline swelling of sodium and potassium montmorillonites immersed respectively in solutions of sodium and potassium chloride of different concentration exhibits hysteresis in the swelling-contraction cycles. The position and extent of the hysteresis can be varied by altering the method of preparation of the clays; in all cases the hysteresis loops for potassium montmorillonite were more extensive than those for sodium montmorillonite.
The results have been explained in terms of a structural rigidity of montmorillonite crystals, where the crystals consist of five to ten aluminosilicate sheets (lamellae). The extent of hysteresis depends on this rigidity, which varies in a co-operative manner with the spacing between the lamellae in the crystal.
Electrical interactions between pairs of lamellae are also considered in analysing the swelling behaviour.