Abstract
Alkaline and alkaline-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 by using 13C-NMR spectra and phase diagrams. As the result, it is suggested that N-methyltaurine is adsorbed by carboxyl groups in lauric acid as counter-ions. It was also found that quantities of free lauric acid become separated from sodium N-methyltaurate laurate soap solution at higher temperature or lower concentration. The surface tension and the interfacial tension between water and oils of sodium N-methyltaurate laurate solution were low in comparison with those of sodium laurate solution. The C.M.C. of sodium N-methyltaurate laurate exceeded that of sodium laurate. The properties of sodium N-methyltaurate laurate thus differ from those of sodium laurate. Sodium N-methyltaurate laurate as a detergent was compared with other laurate soaps. It turned out that less calcium laurate formed with calcium ions in hard water adsorbed to skin when washing by sodium N-methyltaurate laurate solution compared to other lauric acid soap solutions. This suggests that sodium N-methyltaurate laurate should less likely to lead to skin stiffness so that less calcium laurate adsorbs on to skin.
