Determination of critical micelle concentrations of some surfactants by keto‐enol tautomerism of benzoylacetone

Abstract

Abstract Benzoylacetone, keto‐enol tautomer, was dissolved in aqueous solutions of sodium dodecyl sulfate, dodecyl trimethyl ammonium chloride, hexaethyleneglycol dodecyl ether, polyoxyethylene tert‐octyl phenyl ether, and polyoxyethylene nonyl phenyl ether, respectively. The effect of surfactants on this keto‐enol equilibrium was examined by measuring the intensity of two spectral bands at ca. 250 nm and 315 nm due to ketonic and enolic form of this compound. The existence of keto‐enol equilibrium of the benzoylacetone was indicated by an isosbestic point among the two bands. The absorbances of the two bands showed hardly any change in the range of surfactant concentrations below the critical micelle concentration (cmc) but, in the concentration range above the cmc, the absorbance of the enolic form increased and that of the ketonic form decreased abruptly. These changes at cmc were very noticeable in the case of ionic surfactant solution. The concentrations corresponding to abrupt change of the spectra used for the determination of cmc were in fair agreement with data obtained by other methods. However, in the case of nonionic surfactant solutions, the absorbance due to the enolic form increased gradually with increase of the surfactant concentration, and the distinct break point was not observed at cmc.