Interfacial tension and hydrogen-ion concentration

Abstract

The predominance of hydrogen-ion concentration (1) in the colloidal chemistry of the proteins and other substances (Loeb (2)) makes imperative an investigation of those changes at the interface between two phases which are brought about by changes in hydrogen-ion concentration. Previous investigators have directed their attentions to the purely chemical aspects of the interfacial tension between soap solutions and oils, or to non-reactive chemical substances, such as benzene. Using a drop-weight method of measuring interfacial tension, Donnan (3) showed that crude rape oil and crude olive oil showed a much smaller interfacial tension against alkaline solutions (NaOH and Na carbonate) than against distilled water. The same olive oil showed no such difference against N/1000 NaOH after a process of purification, which was intended to remove the fatty acid. He also found that a paraffin showed no difference for interfacial tension when N/100 NaOH was substituted for distilled water, whereas the same paraffin containing 0·6 per cent, stearic acid, showed a large decrease when in contact with the alkaline solution. The decrease in interfacial tension, so produced, was found to be a function both of the amount of fatty acid dissolved in the paraffin, and also of the amount of alkali. It was further shown that of the series of acids, formic, acetic, butyric, caprylic, lauric, and stearic, tried respectively in solution in the paraffin, stearic and lauric acids alone showed any appreciable lowering of surface tension when an alkaline aqueous fluid was substituted for distilled water. Harkins and Humphrey (4) extended Donnan’s work by showing that alkali (NH4OH) lowered the interfacial tension of benzene solutions of butyric and acetic acid against water. The depression of interfacial tension so produced was a function of the concentration of fatty acid, but tended to become maximal for butyric acid at a concentration of 0·5 mols. per litre. Later, Clowes (5) showed that the decrease in interfacial tension produced by alkalis, such as Na, K, Li, at the interface crude olive oilwater, was lessened by the presence of divalent salts, such as Ca and Ba.