| Abstract
| - In a previous study (Langmuir 2004, 20, 10324), the electrokinetic properties of diffuse soft layers weretheoretically investigated within the framework of the Debye−Hückel approximation valid in the limitof sufficiently low values for the Donnan potential. In the current paper, the electrokinetics is tackled onthe basis of the rigorous nonlinearized Poisson−Boltzmann equation, the numerical evaluation of theelectroosmotic velocity profile, and the analytically derived hydrodynamic velocity profile. The results areillustrated and discussed for a diffuse soft interface characterized by a linear gradient for the frictioncoefficient and the density of hydrodynamically immobile ionogenic groups in the transition region separatingthe bulk soft layer and the bulk electrolyte solution. In particular, it is shown how the strong asymmetryfor the potential distribution, as met for high values of the bulk fixed charge density and/or low electrolyteconcentrations, is reflected in the electrokinetic features of the diffuse soft layer. The analysis clearlyhighlights the shortcomings of the discontinuous approximation by Ohshima and others for the modelingof the friction and electrostatic properties of soft layers exhibiting high Donnan potentials. This is in linewith reported electrokinetic measurements of various soft particles and permeable gels at low electrolyteconcentrations which fail to match predictions based on Ohshima's theory.
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