Sedimentation of strongly and weakly charged colloidal particles: Prediction of fractional density dependence

We report on calculations of the reduced sedimentation velocity U/U-0 in ho mogenous suspensions of strongly and weakly charged colloidal spheres as a function of particle volume fraction phi. For dilute suspensions of strongl y charged spheres at low salinity, U/U-0 is well represented by the paramet ric form 1 - p phi(alpha) with a fractional exponent alpha = 1/3 and a para meter p similar or equal to 1.8, which is essentially independent from the macroion charge Z. This nonlinear volume fraction dependence can be quantit atively understood in terms of a model of effective hard spheres with phi-d ependent diameter. For weakly charged spheres in a deionized solvent, we sh ow that the exponent alpha can be equal to 1/2, if an expression for U/U-0 given by Petsev and Denkov (1992, J. Colloid Interface Sci. 149, 329) is em ployed. We further show that the range of validity of this expression is li mited to very small values of phi and Z, which are probably not accessible in sedimentation experiments. The presented results might also hold for oth er systems such as spherical proteins or ionic micelles. (C) 1999 Academic Press.