Transport phenomena and electrode reactions generated by an electric fieldin colloidal silica

The kinetics of transport phenomena generated by an electric field and lead ing to the formation of density gradients in suspensions of charged colloid al silica were studied by using various electrodes. The rate of approach to a steady-state density gradient was found to be much higher when using met allic electrodes (Cu, Fe, and Pt) in comparison with graphite (C) electrode s. Nevertheless, the initial rate with C electrodes was substantially incre ased by the addition of hydroquinone-quinone because the redox reactions, n ecessary for electrode-electrolyte current transfer, occur at lower potenti al compared with the electrolysis of water. On the other hand, the products of oxidation of hydroquinone which accumulate in the system bring about an important decrease of the zeta potential of silica particles and progressi ve deceleration of their electrophoretic mobility. A detailed study was car ried out, by using thin-layer isoperichoric focusing, UV-vis spectrophotome try, and voltamperometry, to explain the observed phenomena which can inter fere in electric polarization or focusing field-flow fractionation. (C) 200 0 Academic Press.