This review treated the researches regarding the effect of counter ion on some physicochemical properties of N-acyl amino acid salts, which has the discrimination between its optically active isomer and racemate. Firstly it was shown that the Krafft temperature of vacyl amino acid salt increased with decreasing size of the amino acid residue and counter ion except for the case of acyl phenylalanineate. It was also found from these results that the L-L interaction in solid state of N-acyl amino acid surfactant salt was superior to the D-L interaction for both the alanine and valine systems when the counter ion size increased. Secondly a diastereomer-type surfactant, which was an amino acid surfactant having optically active base as a counter ion, was investigated. The functions of N-acyl amino acids and PEA as an optical resolving agent were examined by drawing the phase diagrams of some diastereomer mixtures. Finally an unique aggregation behavior of potassium N-acyl phenylalaninate in dilute aqueous solution was explained. It was found that relatively large aggregates were appeared in these system at lower concentration. The large aggregates were not observed in these sodium salts systems. It was found, furthermore, that the unique aggregation behavior has never happened in the case which the racemic modification was employed as acyl phenylalanine.

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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.

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