Chemically Driven Tension Fluctuation and Motion of Interface with Divalent Cation and Di(2-ethylhexyl)phosphoric Acid

Abstract

Abstract Fluctuation in the interfacial tension in an oil/water interface composed of di(2-ethylhexyl)phosphoric acid (DEHPA) and divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) were investigated. Only a combination of Ca2+ and DEHPA caused noticeable tension fluctuation and spontaneous interfacial flow. Light-scattering experiments and the extraction of cations with DEHPA indicated that both a high reactivity between cation and DEHPA, of which only Ca2+ had a high reactivity, and strong attractive interactions between M(DEHP)2 molecules were required for fluctuation. These may induce a collective desorption of Ca(DEHP)2. This was supported by the observation of alternating pH-change with m-Cresol Purple. An N-shape dependency of interfacial pressure on the DEHPA concentration was observed only for Ca-containing interface. Due to this dependency, there was a DEHPA concentration range in the interfacial pressure vs the concentration curve, which satisfies two different conditions. First, the dependency of the interfacial pressure on the concentration is negative, and second, there are other two concentrations with the same interfacial pressure value. The DEHPA concentration range involving tension fluctuation almost agreed with the concentration range, which is discussed. The interface may be a chemical generator predicted by Vedove and Sanfeld.