Aggregation behavior of mixed fluorocarbon and hydrocarbon surfactants in aqueous solutions

Abstract

The aggregation behavior of lithium perfluorooctanesulfonate (LiFOS), of mixtures of LiFOS with lithium dodecyl sulfate (LiDS) and with octa- and hexaoxyethylene glycol dodecyl ether (C 12E 8 and C 12E 6), and of C 12E 6LiDS in aqueous solutions has been investigated by means of pyrene fluorescence probing. For the pure LiFOS, the micelle aggregation number N as obtained from the analysis of the fluorescence decay of micelle-solubilized pyrene in the presence of a micelle-solubilized quencher was found to increase considerably with the surfactant concentration. In these micelles, pyrene is located very close to, or at, the micelle surface. The measurements of the pyrene fluorescence lifetime τ 0 and of the ratio I 1 I 3 of the intensities of the first and third bands of the pyrene monomer fluorescence spectrum appear to indicate that only one type of mixed micelle exists in the mixed systems LiDSLiFOS, C 12E 6LiFOS, and C 12E 8LiFOS. The micelle aggregation numbers for these three systems have been determined as has that for the C 12E 6LiDS system. They decrease as the mole fraction of LiFOS in the first three systems is increased. The evidence for probe intermicellar migration in the C 12E 6-containing mixed systems has been obtained. This migration is inhibited by the incorporation of LiFOS or LiDS in C 12E 6 micelles.