Controllable emulsification by dissolved gas in water: Formation and stability of surfactant-free oil nanodroplets

Surfactant-free emulsions represent the ideal system to investigate properties like droplet size, surface potentials, and colloidal stability which are mainly driven by oil-water interactions. The dissolved gases in solution, which are often neglected due to low solubility, have been found to pose significant impacts in the dispersion process. So far, the role of dissolved gases in the emulsions system remains unclear. Here, we systematically studied the effect of different concentrations of dissolved gases in water on the formation and stability of surfactant-free oil nanodroplets. By increasing the dissolved gas in water, more concentrated and smaller nanodroplets were pro-duced by ultrasonication but with a lowered growth rate. While the removal of dissolved gas severely restricted the emulsification process and only a few nanodroplets were produced. The growth pattern of nanodroplets in different gas-saturated water is similar, first increasing rapidly and reaching equilibrium eventually. The stability of nanodroplets is found to be largely independent of the dissolved gas in water. The growth mechanisms for surfactant-free nanodroplets in gas-oversaturated water possibly combine Ostwald ripening, coalescence, and creaming. Our study will offer valuable insights into the production of nanoemulsions, oil-water separation, and constraints on ultrasonication.