The viscous behaviour and structure of aqueous suspensions of goethite

Aqueous suspensions of goethite, prepared at neutral pH, were found to be flocculated, highly viscous and extremely shear thinning at particle volume fractions from 0.01 to 0.08. Then a suspension having a volume fraction of 0.02 was observed by optical microscopy over a range in pH values, and fluid structure correlated with viscous behaviour, that is, a pH that produced a more dispersed structure yielded a lower viscosity and a lower degree of shear thinning. Flocculation and high viscosity were found to persist at high zeta potentials (ζ). The persistence could not be predicted using classical DLVO theory, which indicated the presence of an attractive force in addition to van der Waals forces. It was hypothesized that the different crystal faces of goethite possess different charges and thus provide an additional attractive force, one that is electrostatic. The theory was supported by electron microscopy studies of mixtures of goethite and charged latex spheres.