Investigation of cavitation bulk nanobubbles in various pH and surfactant concentrations*

Nowadays, nanobubbles (NBs) are attracting interest from many researchers because of their various applications and high stability. However, since it is very challenging to investigate the NBs quantitatively or qualitatively, few studies have been conducted to observe the characteristics of the NBs in various media. In this study, we generated the cavitation bulk NBs by varying the pH and surfactant concentration of the solutions in which NBs were generated. The generated NBs were analyzed in terms of zeta potential, average diameter, and concentration. The zeta potential of the NBs was analyzed by the electrophoretic light scattering (ELS) method and the average diameter and concentration of the NBs were observed by the nanoparticle tracking analysis (NTA) method. The zeta potential of the NBs was directly affected by the properties of the additive which are added to the solution such as HCl, NaOH, a cationic surfactant, and an anionic surfactant. The average diameter of the NBs was highly related to the zeta potential and surfactant concentration. The concentration of the NBs had the highest value nearby the CMC (critical micelle concentration). Moreover, by applying the DLVO theory, the theoretical investigation of the stability of NB was conducted.