A fluorimetric method for the estimation of the critical micelle concentration of surfactants

Abstract

The present work investigates the possibility of a rapid estimation of critical micelle concentration (cmc) of surfactants by means of soluble fluorescent probes. The effect of nonionic or differently charged surfactants on the fluorescent properties of the anionic 8-anilino-1-naphtalenesulfonic acid magnesium salt (ANS) or cationic rhodamine 6G has been investigated. The possibility of cmc evaluation depends on the appropriate selection of the dye-detergent couple. ANS has to be used with anionic surfactants; on the other hand, rhodamine 6G has to be used with cationic detergents. Both ANS and rhodamine 6G have been proved to be effective with either zwitterionic or nonionic surfactants. Plots of ANS fluorescence increase or rhodamine 6G decrease vs surfactant concentration give two straight lines whose intersection indicates the cmc of the detergent. Under all these conditions the fluorescent probe does not interfere with the micellization process. Excitation of the fluorescent probes at the isosbestic point does not affect the evaluation of the cmc of the detergent. The method applies for linear or steroid surfactants and is independent of the cmc value within a wide range of concentrations.