Electrophoretic behaviour of bubbles in aqueous electrolytes

Abstract

A double laser Doppler electrophoresis apparatus has been used to study the effects of bubble size, pH, type and concentration of electrolytes and electric field strength, on the electrophoretic mobilities of electrolytically generated oxygen and hydrogen bubbles in aqueous electrolyte solutions. The pH dependence of the (very large) bubbles' electrophoretic mobilities implied the presence of an isoelectric point between pH 2 and 3 and their (negative) mobilities increased in magnitude with increasing bubble diameter, in the experimentally accessible range of about 60–100 µm. The apparent negative charge of the bubbles in the surfactant-free electrolyte solutions was ascribed to preferential adsorption of OH^– ions. The pH-dependent mean charge density was estimated as ca.–18 µC m^–2 at pH 6.9 by addition of sufficient cationic surfactant (DoTAB) of known adsorption isotherm, to bring the bubble electrophoretic mobility to zero. Although very small, this was similar to the value of ca.–10 µC m^–2 derived from the diameter dependence of the electrophoretic mobilities in surfactant-free NaClO₄ solutions assuming charge polarisation to the extent of greatly diminishing the electrophoretic drag. The very high mobilities enabled complementary experiments involving holding the bubbles stationary against gravity with an applied field, which is seldom possible for solid particles with the same density difference to the suspending liquid; the resulting mobilities were comparable to those obtained with laser Doppler anemometry. The high mobilities were ascribed to polarisation of adsorbed charge at the mobile gas/liquid interface decreasing the electrophoretic drag. Extrapolation to zero applied field gave a finite, though very small mobility, which may be ascribable to hydrodynamic charge polarisation.