| Abstract
| - Both exact and approximate analytical solutions of the Poisson−Boltzmann equation for two planar, parallelsurfaces are derived for the case when a dispersion medium contains counterions only, and the results obtainedare used to evaluate the critical coagulation concentration of a spherical dispersion. A correction factor, whichis a function of the valence of counterions, the surface potential of a particle, and the potential on the midplanebetween two particles at the onset of coagulation, is derived to modify the classic Schulze−Hardy rule forthe dependence of the critical coagulation concentration on the valence of counterions. The correction factoris found to increase with the increase in the valence of counterions and/or with the increase in the surfacepotential. However, it approaches a constant value of 0.8390 if the surface potential is sufficiently high.
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