Spontaneous deformation of an oil droplet induced by the cooperative transport of cationic and anionic surfactants through the interface.

Spontaneous deformation of a tetradecane droplet with palmitic acid on an aqueous phase with stearyltrimethylammonium chloride is reported. Palmitic acid is transported from the oil droplet to the aqueous phase by the concentration difference between the organic and the aqueous phases. The transport of palmitic acid causes the oil droplet interface to undergo various spontaneous deformations. When the oil droplet is placed on an aqueous surface, its diameter shrinks. Several tens of seconds later, the oil droplet suddenly expands and then shrinks in a second. After such a dramatic deformation, the oil droplet undergoes blebbing on its oil-water interface for over 1 h. We investigated the physicochemical mechanism of these phenomena. We discuss the cause of these deformations in terms of the spatiotemporal variation of the interfacial tension and elucidate that the blebbing deformation is due to the surfactant aggregate generated by cationic and anionic surfactants.