The zeta-potentials of microbubbles in the 40- to 80-μm size range have been determined by means of a simple microelectrophoresis technique. In general, the sign of the bubble charge is determined by the polar head of the surfactant when an ionic surfactant is used to produce the bubbles. When using hydrolyzable ionic surfactants, however, the bubble charge is significantly affected by the hydrolysis products. In the presence of nonionic surfactants, bubbles can be charged either positively or negatively depending on the pH, and the isoelectric points appear to be related to the oxygen-to-carbon ratio of the surfactant molecule. With ionic surfactants, an increase in concentration results in an increase in bubble charge, while with nonionic surfactants the zeta-potentials change little in the concentration range studied. The negative charges observed with air bubbles and oil droplets in the absence of surfactants can be explained by the differences in the hydration energies of H+and OH-ions. © 1986.
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