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

Abstract

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.