Sedimentation profiles of charged colloids: Entropic lift and charge separation

We present Molecular Dynamics simulation and Poisson-Boltzmann theory of sedimentation equilibrium of suspensions of charged colloids, treated at the level of the primitive model, including the co- and counterions (microions) explicitly. The simulation provide the first, direct confirmation of the theoretical low-salt prediction; of i) a macroscopic separation of colloidal and microionic charge; ii) an almost homogeneous electric field in the suspension. iii) a highly non-barometric colloid density distribution. These effects. which cannot be explained within the usual effective one-component picture; should be measurable experimentally.