Spontaneous interfacial tension changes at the interface of a ZnCl2 nitrobenzene solution and aqueous stearyltrimethylammonium chloride solution

•Spontaneous motions at the interface of STA-Cl aq./ZnCl2 nitrobenzene solution was observed.•High ion-transfer potentials of Zn2+ is important.•Concentration distribution of Cl− at/near the interface is also important. This study examines the roles of chemical compounds that can generate spontaneous interfacial motion, especially in the Nakache and Dupeyrat system, which normally used a stearyltrimethylammonium chloride (STA-Cl) aqueous solution and a KI+I2 nitrobenzene solution. Spontaneous behavior at the interface of a system comprising the STA-Cl aqueous solution and a nitrobenzene solution of ZnCl2 or FeCl3 was observed. To explore the mechanism, measurements of the interfacial tension, critical micelle concentration (CMC), and solubilization rate were performed for the ZnCl2 system. The aggregates produced at the oil/water interface were subjected to infrared spectroscopic analysis, and the time dependence of aggregate coverage at the interface was measured. The analyses suggested that the spontaneous changes were generated because Fe3+ and Zn2+, which have high ion-transfer potentials, could not easily move to the aqueous phase from the oil phase, and the counterion, Cl−, also remained near the interface, resulting in a reduction of the CMC and promotion of solubilization. This information affords an understanding of the kinds of materials that can be used to produce a high level of spontaneous interfacial motion and aids in the development of a spontaneous interfacial motion system not utilizing toxic substances.