Oscillation of Interfacial Tension and Spontaneous Interfacial Flow at a Water/Oil Interface Composed of Di(2-ethylhexyl)phosphoric Acid

Oscillation of dynamic interfacial tension and spontaneous interfacial flow due to the Marangoni instability were investigated for the water/oil interface containing di(2-ethylhexyl) phosphoric acid (DEHPA) and calcium chloride. Characteristics of interfacial flow were extracted from experimental results with tracer particles. The interfacial flow appeared randomly at the interface. Movement of a small particle put on the interface was essentially a random walk. By application of the theory of random walk, the kinetic energy of the interfacial flow was evaluated. The energy evaluated decayed exponentially with time after the contact of aqueous and organic phases. Equilibrium interfacial tension was measured as a function of surfactant concentration. There was a region in which equilibrium interfacial tension increased with an increase in surfactant concentration. This concentration region agreed with the concentration range where dynamic interfacial tension oscillated. The difference in equilibrium interfacial tensions between CaCl 2 -containing and CaCl 2 -free systems was almost equal to the amplitude of the tension oscillation at the corresponding surfactant concentration. The oscillation of interfacial tension was concluded to reflect alternate changes in interfacial states between DEHPA-rich and its calcium salt rich compositions. The tension oscillation continued even when the kinetic energy of interfacial flow became extremely small. Spontaneous interfacial flow appeared only when the amplitude of the tension oscillation was large enough. Oscillation of interfacial tension did not need appreciable interfacial flow due to the Marangoni instability.