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 φ. It is shown for the first time by experiment that at very low ionic strength the normalized sedimentation velocityU/U₀follows a remarkable φ dependence of the form 1 -pφ^1/3, which leads to a strong decrease inU/U₀at very low concentrations. This dependence agrees with theoretical calculations of the sedimentation velocity. The calculated numerical coefficientp, however, is smaller than the experimental value. Both experiments and calculations show that addition of salt gradually changes the φ^1/3dependence to the linear φ dependence characteristic for hard spheres. The φ^1/3behavior for salt-free dispersions renders it very difficult to measure quantities at "infinite dilution.".