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Using photon correlation spectroscopy, diffusion coefficients of charged colloidal particles surrounded by electrical double layers have been determined. The diffusion constant equals the value of a neutral sphere at high and low electrolyte concentrations, but is reduced by several per cent when the electrical double layer is comparable to the radius of the particle. The reduction in diffusion constant depends on the zeta potential of the particle and the sizes of the ions in the double layer. The diffusion of charged particles can be explained by the theory of Ohshima ef al. (J. Chem. Soc., Furaday Trans. 2, 1984, 80, 1299) for the friction coefficient of charged spheres, assuming that the friction coefficient of a charged sphere in Brownian motion equals the equilibrium friction coefficient of a sedimenting sphere.

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