Determination of the Critical Micelle Concentrations and Microviscosity with a Fluorescence Probe, Auramine

A new method based on the fluorescence intensity of bis[4-(dimethylamino)phenyl]methanimine (auramine) was proposed in order to determine the critical micelle concentration (cmc). The cmc values of the following surfactants were determined by the present method, and were compared with those by a surface-tension method. The nonionic surfactants used were homogeneous polyethylene glycol monododecyl ether (C12E5, C12E6, C12E7, and C12E8) and N-acyl-N-methylglucamine (MEGA-7, -8, -9, -10, and -12). The cationic surfactants were dodecyltrimethylammonium chloride and bromide (DTAC, DTAB), tetradecyltrimethylammonium bromide (TTAB), hexadecyltrimethylammonium chloride and bromide (CTAC, CTAB). The potassium salt of N-acylalanines, the sodium salt of N-acylvalines, and sodium dodecyl sulfate were used as anionic surfactants. The cmcs measured by both methods were in good agreement with each other in all systems of surfactants used. The effects of added salt, temperature, and size of the acyl group on the microviscosity were also estimated.