Aggregation of perfluoropolyether carboxylic salts in aqueous solutions. Fluorescence probe study

Aqueous solutions of anionic surfactants Cl(C3F6O)nCF2COOX, consisting of n = 2 and 3 perfluoroisopropoxy units and the counterion X = Na+ or NH4+, were studied by the method of fluorescence quenching with the use of (1-pyrenylbutyl)trimethylammonium bromide as a luminophore, and 1,1'-dimethyl-4,4'bipyridinium dichloride (methyl viologen) as a quencher. From the kinetics of fluorescence decay (time-resolved experiments) micellar aggregation numbers, N, and rate constants of the intramicellar quenching were determined for a wide range of surfactant concentrations, on the basis of the model developed by Infelta and Tachiya. The results are discussed in terms of the shape of the aggregates and the degree of counterion binding. The most important conclusions include: (i) a significant increase of N with increasing surfactant concentration suggests that spherical micelles formed at critical micellar concentration (CMC) transform into ellipsoidal aggregates, (ii) the degree of counterion binding to micelles is higher for NH4+ than for Na+, leading to higher N values in the case of the ammonium salt (n = 2), and (iii) at concentrations close to CMC the longer chain surfactant (n = 3) forms loose aggregates suggesting significant permeation with water molecules. An additional finding of this study is that the micelle-bound luminophore and quencher can form a ground-state complex, and for this reason the N values cannot be evaluated properly from the steady-state fluorescence intensity data using the equation proposed by Turro and Yekta.