Effect of surfactant concentration on dispersing mechanism of aqueous particle/surfactant suspension system prepared by ultrasonication

Understanding the dispersing mechanism of particles in aqueous phase using a surfactant is key to the effective preparation of particle suspensions. In this study, the mechanism was elucidated using dynamic light scattering , pulse field gradient nuclear magnetic resonance spectroscopy , and electrophoretic mobility measurements. In this research, calcium carbonate particle suspensions with a particle concentration of 0.2 mg mL −1 (0.0074 vol%) were created in water through ultrasonication, utilizing polyoxyethylene alkylether (Softanol-70) as the stabilizing surfactant. When dispersing particles in an aqueous phase using a surfactant, the concentration of surfactant that was not adsorbed on the particles was affected by the micellar size. When the surfactant concentration was relatively higher (above 0.2 mg mL −1 ), prolonged ultrasonication enabled the particles to reach and retain their smallest size by preventing re-agglomeration. This was due to the sufficient presence of non-adsorbed surfactant, which stabilized the micelle size. Conversely, at lower surfactant concentrations (less than 0.15 mg mL −1 ), short ultrasonic irradiation reduced particle sizes in the liquid phase; however, extending the ultrasonication duration led to re-agglomeration of the particles. The latter phenomenon was induced by the reduction of the micellar size of the surfactant from approximately 8 nm to 2–1 nm, which was related to the reduction of the amount of non-adsorbed surfactant on the particles. Interestingly, the zeta potential of the dispersing particles remained almost unchanged at approximately −24 mV during the dispersing process. This indicated that electrostatic repulsion between particles was not a significant factor in this particle suspension, whereas the micellar size of the surfactant was an important key. This finding could significantly contribute to the effective preparation of a particle suspension. • Effect of surfactant concentration on dispersing of aqueous particle suspension System was investigated. • Micellar size of the surfactant was related to the amount of free diffusing surfactant molecules. • Close correlation between adsorption of the surfactant on the particles and micellar size of the surfactant was observed. • According to DLVO theory, reducing micellar size induced a reduction in the electrostatic repulsion stabilization.