Properties of Fatty Acid Soap with Sodium N-Methyltaurine as a Counter-ion

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 concen tration. The surface tension and the interfacial tension between water and oils of sodium N-methyltau rate 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 com pared 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 .