Remarks on the Donnan condenser in the sedimentation-diffusion equilibrium of charged colloids

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