Investigation on the Microstructures in Mixtures of Water with the Nonionic Surfactants C8E5, C10E6, and C10E8 in the Whole Range of Composition

Abstract

The microstructure in mixtures of water with the three nonionic surfactants C8E5, C10E6, and C10E8 has been investigated in the whole range of compositions, at temperatures where the mixtures remain monophasic. The techniques used were density, viscosity, spectrofluorometry (probes = pyrene and 1-pyrenesulfonate), self-diffusion (open-ended capillary tube method), and time-resolved fluorescence quenching (TRFQ). The spectrofluorometry revealed that the micellar structure is retained up to 50% w/w for the three surfactants. At low surfactant concentration, C, the micelles are close to spherical and their aggregation number N increases little with C when the temperature T is well below the critical temperature TC of the water-surfactant mixture. At higher T ,N increases with C and the micelle polydispersity becomes important. The T-dependence of N appears to be determined by the values of TC - T and of C. At a given C , N first increases slowly with T then much more rapidly at above a temperature TI, which has been shown to decrease upon increasing C. At C > 50% w/w the self-diffusion, spectrofluotometry, and TRFQ results reveal that the micelles progressively lose their identity. The concentrated mixtures tend to resemble microemulsions at above the percolation threshold, with extensive connectivity between micelles in clusters of micelles.