The association of montmorillonite studied by light scattering in electric fields

Abstract

Studies of the angular dependence of the light scattered from fresh and a 4-week-old dilute suspension of montmorillonite particles have been augmented by measurements of the changes in the scattered intensity (Δ I) when alternating electric fields were applied to the suspensions. Prior to the application of the fields, the molecular weight and radii of gyration of the particles in each suspension were determined. The fresh sols consisted on average of rectangular plates whose length was some five times their breadth and whose thickness was that of a single unit layer. Measurements of Δ I as a function of field strength up to 600 v cm −1 and frequency up to 5 kHz showed these plates to be rigid and predominantly polar along the transverse axis. The particles of the aged sol consisted of aggregates of at least 9 platelets of the fresh suspension. Theoretical radii of gyration were calculated for a number of complicated structures. By considering the experimental radius of gyration, rotary diffusion constant and direction of the dipole moment, 11 original models were reduced to two possible structures. Of these, one was especially favored for reasons given herein. This is an open stacked arrangement in which the rectangular plates encountered in the fresh sols are associated alternately edge to face center, the common edge and face being the long side of the rectangle. This structure provides experimental verification of the hitherto hypothetical suggestion of a previous worker. The reported study involved the determination of both the geometric and electric properties of the particles in both the fresh and aged suspensions and well demonstrates the value and versatility of scattering experiments from solutions and suspensions subjected to electric fields.