Dynamics of Transitional Phase Inversion Emulsification: Effect of Addition Time on the Type of Inversion and Drop Size

Abstract

Emulsification processes are very sensitive to the time scale during which the dispersed phase is introduced into the continuous phase. A series of experiments were conducted across a transitional phase inversion path, which often leads to formation of nanoemulsions, with the speed of incorporation of the second phase (water) into the first phase (oil) being altered. The optimum condition could only be achieved if the optimum composition was maintained for a critical time. A slow addition (addition time > 60 s) of water to the oil allows the transitional phase inversion to become operative, leading to formation of sub-micrometer droplets. A very fast addition of the water phase (<5.0 s) caused the catastrophic phase inversion mechanism to become dominant, leading to formation of rather large drops. In the intermediate range of addition time, 20−40 s, both inversion mechanisms have contributed to drop formation, at least locally, and as a result skewed or bimodal drop size distributions were formed. The results indicate that while spontaneous emulsification is fast, it is not instantaneous. At a low surfactant concentration, the droplet size was only slightly affected by the rate of addition.