Remarks on the Donnan condenser in the sedimentation–diffusion equilibrium of chargedcolloids

Abstract

The Donnan membrane equilibrium, applied to the sedimentation–diffusion (SD)equilibrium for charged colloids in gravity or an ultracentrifuge, yields limiting lawsfor the SD density profile and the recently predicted macroscopic electric field(van Roij 2003 J. Phys.: Condens. Matter 15 S3569) which necessarily accompaniesthe colloid density gradient as is also illustrated by the Donnan model. For SDequilibrium in an ultracentrifuge the radial ‘Donnan’ electric field accounts for thesignificant apparent mass reduction in thelow-salt limit already discussed by Tiselius(1926 Z. Phys. Chem. 124 449). Several other aspects are discussed, such as the strikinganalogy between the macroscopic Donnan condenser and the usual colloidal diffuse doublelayer.