Macroscopic electric field and osmotic pressure in ultracentrifugal sedimentation-diffusion equilibria of charged colloids

Sedimentation-diffusion (SD) equilibria from analytical ultracentrifugation of well-characterized charged silica spheres in ethanol deviate strongly from a barometric profile and demonstrate the existence and substantial effects of a recently predicted internal macroscopic electric field (van Roij 2003 J. Phys.: Condens. Matter 15 S3569). Experimental SD-profiles yield the gradient of the electrostatic potential energy of the colloids, which clearly manifests an almost homogeneous macroscopic electric field. Electrochemical Donnan potential measurements confirm a difference in electrical potential between the top and bottom of the profiles. A 'non-barometric' limiting law derived from electroneutrality explains the trends in the SD-profiles quite well. Our analysis of osmotic pressures (obtained from integrating SD-profiles) beyond this simple law includes, among other things, colloid-ion attractions and extra volume terms in the free energy.