Ultrasonic formation of nanobubbles and their zeta-potentials in aqueous electrolyte and surfactant solutions

The electrokinetic behaviors of nanobubbles internally generated by ultrasonication were investigated in terms of the stability of bubble, size distribution and zeta-potentials. When aqueous solutions were sonicated with a palladium electrode, stable nanobubbles having effective diameters of several hundreds nanometer were formed within a few minutes, and the sizes and size distributions remained stable for up to 1 h. The bubble sizes slightly increased when salts added and significantly reduced when surfactants added. As the chain length of alkyl group of CnTAB increased, the size and size distribution increased with n. A micellar model was proposed to explain the formation of nanobubbles and their size reduction. Zeta-potentials of nanobubbles showed a sharp change at CMC, increasing linearly with surfactant concentrations at low concentrations and invariant at high concentrations. The surface ionization and adsorbed monolayers at a bubble surface would result in a degree of dissociation of a planar air/water interface, but the calculated degree of dissociation at the bubble surface is very low compared with that of micelles. It would be attributed to the counter ion binding at interface or curvature effect.