On the use of dyes for the kinetic study of micellar equilibria

Abstract

A number of dyes or fluorescent compounds show an appreciable change of extinction coefficient or quantum yield depending on whether they are in a surfactant solution at concentration above or under the critical micelle concentration ( cmc). This effect was often used to determine cmc values, but it also provides a way of following, by spectrophotometric methods, the chemical relaxation of micellar equilibria upon a fast temperature jump. In this work we have investigated for different types of detergents (anionic, cationic, and nonionic) the effect of addition of dye on the relaxation kinetics of micellar solutions. We have found that: (1) when the ratio r = (detergent)/(dye) is above 150–200, the relaxation time characterizing the micellar solution is the same with and without added dye for the different dye-detergent systems; (2) the same remains true at values of r as low as 20 for the nonionic detergent investigated; and (3) dyes with an electrical charge opposite to that of the micelles are the most suitable for studying the kinetics of micellar equilibria. Acridine orange has been used for such a study of sodium dodecylsulfate that does not contain a chromophoric group, by means of T-jump with spectrophotometric detection. The results indicate a new type of variation of the relaxation time with the detergent concentration.