Sedimentation-diffusion profiles and layered sedimentation of charged colloids at low ionic strength

We report the formation of strongly inflated sedimentation-diffusion concentration profiles for charged monodisperse colloidal spheres in absolute ethanol. Various additional experiments, such as light scattering, confirm that the very dilute supernatants, left behind by the majority of settling colloids, contain spheres that repel each other at distances of micrometers. We attribute these unusual profiles to a significant counter-ion contribution to the osmotic pressure and to the Debye screening length. An approximate osmotic equation-of-state at the level of the second virial coefficient for dispersions at very low ionic strength indeed implies an algebraic long-distance decay of sedimentation-diffusion profiles, together with significant lowering of the effective colloid mass by an entropic lift due to counter-ions. We have also observed that sedimenting dispersions sometimes demix into two layers, which are both disordered fluids. Since the colloids are clearly repulsive on the DLVO pair level, this layering possibly manifests a phase transition driven by many-body attractions. (C) 2002 Elsevier Science B.V. All rights reserved.