Use of alizarin red S for determining critical micelle concentration of cationic surfactants

Abstract The critical micelle concentrations of cetyl trimethyl ammonium bromide and cetyl pyridinium bromide have been calculated by a spectral‐dye method. Alizarin red S at pH 9.12 was found to be suitable for these two cationics. Two shifts in the dye maximum, one due to complex formation (500 mॖ or 525 mॖ) and the other due to the dye‐surfactant complex solubilized in the micelles of the cationics were realized. The critical micelle concentration was represented by the point of intersection of two curves at wavelength 500 mॖ and 550 mॖ for cetyl pyridinium bromide and 525 mॖ and 550 mॖ for cetyl trimethyl ammonium bromide.