Interfacial tension and hydrogen-ion concentration

Abstract

Interfacial tension was measured by a drop weight method at the interface between water containing a buffer mixture and benzene containing a long-chain carbon compound. With fatty acids the interfacial tension decreased regularly as the pH of the aqueous phase was increased from 5.5 to 9.5, but the curves did not have the shape of dissociation curves. With hexadecyl amine the direction of the changes was reversed, and the curve resembled the dissociation residue curve of a weak base. The Ka and Kb of[long dash]COOH and[long dash]NH2 in the interface were calculated from these results to be about 32 X 10-8 and 1.0 X 10-7. The curve for ethyl palmitate resembled those for the fatty acids, but the fall began only above pH 7. Hexadecyl alcohol showed practically no change in interfacial tension with variations in pH. Some possible biological applications of changes of interfacial tension with pH are briefly mentioned.