Self-Propulsion of a Soap at an Oil/Aqueous Interface Depending on pH

A self-propelled sodium oleate (OleNa) disk was investigated at an oil/aqueous interface prepared in an annular channel to induce characteristic features of self-propulsion. When the pH of the aqueous phase was changed, three types of motion were observed, i.e., unidirectional motion at 3.0 ≤ pH ≤ 6.0, motion with inversion at 8.0 ≤ pH ≤ 9.0, and no motion at 11.0 ≤ pH ≤ 12.0. At pH = 8.0, the interfacial tension and complementary contact angle of the meniscus oscillated simultaneously. The mechanism of the three types of motion is discussed in relation to the acidity constant (pK_a) between protonated and deprotonated oleic acids, their distribution ratios in the oil and aqueous phases, and the driving force of motion. The present study suggests that the mode of self-propulsion is determined by the nature of the energy-source molecule, specifically its pK_a, interfacial tension, and adsorption/desorption of protonated and deprotonated oleic acid molecules at the interface.