Role of anionic surfactant in the generation of bulk nanobubbles by ultrasonication

The role of anionic surfactant in nanobubble (NB) generation behavior by ultrasonication was investigated in terms of zeta potential and concentration of NBs. NBs were generated by ultrasonication in deionized (DI) water and surfactant solutions with different concentrations of surfactant. In pure water, a low number of NBs was generated because the coalescence of bubbles generated by ultrasonication actively occurred and bubble growth was accelerated. On the contrary, as the surfactant concentrations in the solutions were increased, sporadic nucleation occurred because of the influence of surfactant molecules, which were adsorbed to the gas–liquid interface of the bubbles grown from nucleation. Thus, the solutions with surfactants exhibit a strong negative zeta potential characteristic. Such a strong surface electrical property of the generated bubbles strongly suppresses the coalescence behavior by ultrasonication. As a result, this tendency was dominant in the solutions, and the NB concentration considerably increased with increasing surfactant concentration.