NONANALYTICAL CONCENTRATION-DEPENDENCE OF SEDIMENTATION OF CHARGED SILICA SPHERES IN AN ORGANIC-SOLVENT - EXPERIMENTS AND CALCULATIONS

Sedimentation of charged silica spheres in ethanol has been studied as a function of ionic strength and volume fraction phi. It is shown for the first time by experiment that at very low ionic strength the norm alized sedimentation velocity U/U-0 follows a remarkable phi dependenc e of the form 1 - p phi(1/3), which leads to a strong decrease in U/U- 0 at very low concentrations. This dependence agrees with theoretical calculations of the sedimentation velocity. The calculated numerical c oefficient p, however, is smaller than the experimental value. Both ex periments and calculations show that addition of salt gradually change s the phi(1/3) dependence to the linear phi dependence characteristic for hard spheres. The phi(1/3) behavior for salt-free dispersions rend ers it very difficult to measure quantities at ''infinite dilution.'' (C) 1995 Academic Press, Inc.