Sedimentation profiles of charged colloids: Entropic lift and charge separation

We present Molecular Dynamics simulations 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 simulations provide the first direct confirmation of the theoretical low-salt predictions 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.