Effect of Arginine as a Counterion on Surfactant Properties of Fatty Acid Salts

Abstract

Arginine (Arg) is considered to be useful as a counterion with its large hydration radius and high bulkiness due to high dissociation constant compared to other organic bases, the presence of a ganido group, and its high molecular weight. The effects of Arg as a counterion on the surfactant properties of fatty acids were studied in comparison to triethanolamine (TEA), ammonia (NH3), and sodium ions (Na). The critical micelle concentrations (CMCs) and the degree of counterion binding to the micelles were determined based on electrical conductivity measurements. The degree of binding of argininium (Arg) counterions to micelles of fatty acids was the same as for triethanolammonium (TEA) counterions, and greater than that of ammonium (NH4) and Na counterions, attributed to the bulkiness of the Arg counterion. The CMCs of aqueous solutions of fatty acid Arg salts and fatty acid TEA salts were smaller than the CMCs of the corresponding aqueous solutions of fatty acid NH4 salts and fatty acid Na salts. The effect of the different counterions on the change in Gibbs hydrophilic energy was not investigated. Surface adsorption was determined through surface tension measurements, and it was found that when the bulk concentration was lower than the CMC, without micelle formation, the surface adsorption of fatty acid Arg salts was the same as for fatty acid Na salts and less than that for TEA and NH4 salts. This adsorption trend is attributed to the large hydration radius of the Arg counterion.