The effect of surfactants on the acid-base behaviour of methyl red. Part I. A spectrophotometric comparison of the effect of CPB and SDS

Abstract

The effect of a cationic (CPB) and an anionic (SDS) surfactant on the bands in the absorption spectra of methyl red was studied. The interactions of methyl red with the two surfactants were monitored on the basis of the changes in the absorption curves that were mathematically deconvoluted into Gaussian shape absorption bands. The methyl red dissociation constants were determined in the presence of the two surfactants at sub- and supercritical micelle concentrations. The methyl red conditional dissociation constants changed by ΔpKa = -1.47 for c(CPB) = 5 mmol l-1 and ΔpKa = 0.81 for c(SDS) = 4 mmol l-1. The contributions of the individual forms of methyl red to the interactions with CPB and SDS were discussed.