Second critical micelle concentration of dodecyldimethylbenzylammonium chloride in aqueous solution at 25 °C

Two breaks have been found on the conductivity, refractive index, density and sound velocity against molality plots for aqueous solutions of dodecyldimethylbenzylammonium chloride at 25 °C, in the absence of any additive. The first break corresponds to the critical micelle concentration, cmc. The second, less distinct break, occurring in the molality range of 0.082 to 0.104 mol kg^-1, in dependence on the technique applied, has been ascribed to the second critical micelle concentration, 2nd cmc, responsible for structural transitions of spherical micelles. Values of cmc and 2nd cmc have been also estimated conductometrically for tetradecyltrimethylammonium bromide and dodecylpyridinium chloride. On the basis of available conductometric data it has been shown that the 2nd cmc/cmc ratio varies in the range of 2 to 10 in dependence on the type of 1:1 ionic surfactant. It has been also shown that for a given class of surfactants, the logarithm of 2nd cmc varies linearly with the number of carbon atoms in the alkyl chain ( n= 12, 14 and 16). Both empirical regression coefficients depend upon the class of surfactants considered.