Properties of Fatty Acid Soap that Counter-ion is Sodium N-Methyltaurine

Abstract

Alkali and alkali earth metals and triethanolamine are used as counter-ions of fatty acid soaps. Basic amino acids such as lysine and arginine have also come to be used in recent years. It is well known that fatty acid salts each have distinctive properties. However, until the present, compounds used as counter-ions of fatty acid have all been alkaline. In this study, examination was made of the effects of amphoteric compounds such as N-methyltaurine on sodium laurate using ¹³C-NMR spectra and phase equilibrium diagrams. With ¹³C-NMR spectra, chemical shifts of carboxyl groups in lauric acid were measured in the course of their neutralization by sodium hydroxide and sodium N-methyltaurine respectively. These shift values in lauric acid neutralized completely by sodium N-methyltaurine differed from those when using sodium hydroxide. From this fact it is thought that the environments around the carboxyl groups when neutralized by sodium N-methyltaurine and when neutralized by sodium hydroxide differ from each other. It is suggested that N-methyltaurine is adsorbed by carboxyl groups in lauric acid as counter-ions. It was also found that a considerable amount of free lauric acid becomes separated from sodium N-methyltaurate laurate soap solution at higher temperature or lower concentration. Surface tension of sodium N-methyltaurate laurate solution and its interfacial tension with silicone oil are low compared to sodium laurate solution. The c.m.c. of sodium N-methyltaurate laurate is higher than that of sodium laurate. N-methyltaurine would thus appear to function as a fatty acid counter-ion.