HYSTERESIS IN CRYSTALLINE SWELLING OF SMECTITES

Hysteresis in crystalline swelling of smectites was investigated with respect to the surface properties of five reference smectites, First, the samples were Na saturated and equilibrated with either 3 M NaCl or distilled water to establish initial basal spacings of 15.5 and >40 A ngstrom, respectively, Then the samples were equilibrated with NaCl so lutions ranging from 0.35 to 3 M, to establish osmotic control of crys talline swelling. Basal spacings of the equilibrated clays were measur ed by X ray diffraction, and hysteresis was quantified as the differen ce in weighted-average H2O activities in the equilibration solutions f or the adsorption (15.5 to 19 Angstrom) and desorption (l9 to 15.5 Ang strom) transitions, The magnitude of hysteresis was similar for all fi ve smectites and was not significantly correlated with surface charge density, percentage of tetrahedral charge, or surface area, The cause of hysteresis in crystalline swelling is attributed to both intrinsic and extrinsic processes. Intrinsic hysteresis is an inherent consequen ce of changes in the expansion and attraction energy levels during swe lling and is believed to be caused by rigidity of the clay-water syste m, Extrinsic hysteresis is caused by factors that contribute additiona l rigidity to smectite quasicrystals, primarily thick quasicrystals. S ample history influences the magnitude of the extrinsic hysteresis. (C ) 1995 Academic Press, Inc.