Rotational diffusion of some polydisperse clay mineral sols

Abstract

Flow birefringence studies at low shear (generally, less than 20 sec. 1) of polydisperse aqueous suspensions of the clay minerals hectorite, montmorillonite, and attapulgite gave gradient and concentration dependent rotational diffusion constants for such systems. The shear dependent constants could be extrapolated linearly to yield concentration dependent zero-gradient constants; these could then be extrapolated (linearly, if the inverse of the constant is plotted as a function of concentration) to infinite dilution. The resulting zero-concentration constants ( θ G=0, c=0 ) were compared with corresponding theoretical values calculated from the particle size distribution (determined by direct counting from electron micrographs) by means of the following equation due to Joly, φ G=0,φ=0 = ∑ i q φ i θ i ∑ i q φ i θ i 2 , where φ i is the volume fraction and θ i the rotational diffusion constant of the ith component of a polydisperse system containing q noninteracting components. There is good agreement between these calculated constants and those obtained by the linear extrapolation. At a very low gradient (dependent on the particular clay and concentration) an unusual effect was noted; i.e., the rotational diffusion constant increased sharply with decreasing shear.