A photophysical study of micellization of cetyltrimethylammonium bromide in urea-water binary mixtures

From the photophysical response of different fluorescent probes solubilized in the micellar phase, a study about the effect of urea on micellar properties of cetyltrimethylammonium bromide has been carried out. The critical micelle concentration (CMC) of the surfactant at different urea concentrations was obtained by both the pyrene 1:3 ratio and the conductance methods. The observed increase in the CMC was attributed to a rise in the solubility of the hydrocarbon tails of the surfactant. Micelle aggregation numbers were determined from the static quenching method, using pyrene as a probe and cetylpyridinium chloride as a quencher. Later it was found that the decrease in the micelle aggregation numbers, caused by the addition of urea, is controlled mainly by the rise in the surface area per headgroup of the surfactant. Studies made with pyrene-3-carboxaldehyde, in order to evaluate changes in polarity at the micellar interface, revealed that urea induces desolvation of the probe of the micellar interface, suggesting a direct interaction of urea with the micellar surface. Fluorescence polarization studies performed by using fluorescein as a surface probe indicated that urea causes an increase in the local viscosity of the micellar interface. This fact was also interpreted on the basis of a direct mechanism of urea action.