Defying gravity with entropy and electrostatics: sedimentation of charged colloids

We combine hydrostatic equilibrium with Donnan equilibrium to identify three different regimes of sedimentation equilibrium in suspensions of highly charged colloids. In the low-density regime the familiar exponential (barometric) distribution is recovered, in an intermediate density regime the profile is found to be linear with height, and in the high-density regime it is exponential again but with a gravitational length that is increased by a factor Z + 1, where Z is the colloidal charge number. The nonbarometric distributions are explained in terms of macroscopic electric fields, generated by macroscopic charge separation as calculated by Poisson-Boltzmann theory.