Electroviscous effects of concentrated polystyrene latexes

Abstract

Abstract Monodisperse polystyrene latexes with immobilized surface charge groups were used in the study of electroviscous effects of colloidal suspensions. The flow curves of the latexes at solids concentrations 15%–40% by weight and at salt concentrations 10−8-10−1 M NaCl have been determined over shear rates 102-104 sec−1. The latexes at these solids and salt levels were all shear thinning. The viscosity-concentration relationship at any given shear rate was shown to obey Mooney's equation: ln η η 0 = αθ 1−Kθ where the intrinsic viscosity α and the crowding factor k depend on the ionic strength and shear rate. The intrinsic viscosity approaches 2.65 ± 0.02 at high shear rates regardless of the ionic strength. This value compares favorably with the intrinsic viscosity 2.5 theoretically calculated by Einstein for noninteracting spheres, indicating that the primary electroviscous effect vanishes at high shear rate. The slightly higher value than the Einstein constant is attributed to the presence of an emulsifier layer of approximately 16 A thickness. The crowding factor k varies from 1.03 to 1.88 depending on the shear rate and ionic strength. The extrapolated value 1.03 for k at high shear rate, however, is much smaller than the value of 1.35–1.91 estimated by Mooney for noninteracting spheres. The electroviscous effects were shown to be well represented by those of an increase in hydrodynamic volume of the particles. The apparent increase in particle radius was experimentally determined as a function of shear rate and ionic strength, and found larger than the Debye-Huckel double layer thickness 1/κ at shear rates less than ∼1000 sec−1. At shear rates above ∼1000 sec−1 the apparent increase in particle radius is smaller than, and linearly related to, the double-layer thickness 1/κ.