Small Angle X-Ray Scattering from Montmorillonite Clay Suspensions. II

Abstract

Small angle x-ray scattering has been used to study the interactions between clay platelets in a number of samples. Sodium montmorillonite clays have been studied in concentrations ranging from 5% to 85% by weight. The scattering from the samples with concentrations from 10% to 40% was analyzed by Fourier transformation to obtain the distribution functions for the platelet spacing. For the higher concentrations, the scattering curves were interpreted by fitting the data with theoretical curves for aggregates of platelets. The most probable platelet spacing ranged from 116 Å for the 10% concentration to 13 Å for the 85% concentration. If one plots platelet spacing as a function of grams of H2O per gram of clay for this series of samples, a reasonably linear relation is obtained. The effect of adding NaCl, CaCl2, and AlCl3 to 2% sodium montmorillonite suspensions was studied. Until the salt concentration is raised to a critical value, the scattering curve is identical to the curve obtained for dilute Na montmorillonite suspensions with no salt added. The latter scattering curve has been previously interpreted [R. Hight, Jr., W. T. Higdon, and P. W. Schmidt, J. Chem. Phys. 33, 1656 (1960)] as being due to independent montmorillonite layers 10 Å thick. The critical concentration decreased as the valence of the added salt was increased. Above the critical concentration, the scattering data show that aggregation takes place, with the aggregates consisting of about 6 to 8 platelets and with a platelet spacing of 19 to 21 Å. Calcium montmorillonite suspensions show essentially identical behavior. Fresh hydrogen montmorillonite samples show no aggregation, but in samples aged about four months, the scattering curves indicate the presence of aggregates with a platelet spacing of 22–24 Å and with 2 or 3 platelets per aggregate.