The z-potential and the surface conductivity of pure anatase and anatase doped with Li , Nb5 and W6 prepared with both the precipitation (continuous crystallization at constant supersaturation) and the sol-gel method have been determined at two pH values (3.3. and 6.0 or 6.5). This was carried out by measuring the streaming potential and electrical conductivity of a porous plug containing the anatase particles. The sol-gel preparations exhibited a higher density than those obtained by precipitation. Plots of the plug conductivity as a function of the conductivity of the solution are essentially linear, allowing for the establishment of the surface conductivity, K. The main contribution to K stemmed from the counter charge behind the slip plane. At pH 3.3, i.e. much lower than the i.e.p., doping with Nb5 and Li caused a considerable increase of the surface conductivity in both preparations, whereas at a pH 6.0 or 6.5, i.e. near the i.e.p., doping led to a slight decrease of the relatively high surface conductivity of the sample prepared by precipitation. At pH 6.5 sol-gel preparations showed negligible surface conductivity, independent of the doping, with the exception of the preparation with a high concentration of W6 . Moreover, doping with W6 did not considerably change the surface conductivity. There is some indirect correlation between the surface and the bulk conductivity of the various samples. Tangential mobilities of Cl- ions in the stagnant layers are of the same order of magnitude as those in the bulk solution.
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