Sedimentation of Strongly and Weakly Charged Colloidal Particles: Prediction of Fractional Density Dependence

Abstract

We report on calculations of the reduced sedimentation velocity U/U0 in homogenous suspensions of strongly and weakly charged colloidal spheres as a function of particle volume fraction φ. For dilute suspensions of strongly charged spheres at low salinity, U/U0 is well represented by the parametric form 1 − pφα with a fractional exponent α = 13 and a parameter p ≃ 1.8, which is essentially independent from the macroion charge Z. This nonlinear volume fraction dependence can be quantitatively understood in terms of a model of effective hard spheres with φ-dependent diameter. For weakly charged spheres in a deionized solvent, we show that the exponent α can be equal to 12, if an expression for U/U0 given by Petsev and Denkov (1992, J. Colloid Interface Sci. 149, 329) is employed. We further show that the range of validity of this expression is limited to very small values of φ and Z, which are probably not accessible in sedimentation experiments. The presented results might also hold for other systems such as spherical proteins or ionic micelles.