Photophysical Changes of Pyranine Induced by Surfactants: Evidence of Premicellar Aggregates

We studied photophysical changes of pyranine in different surfactant environments using spectrophotometry, steady-state fluorescence, and time-resolved fluorescence lifetime. The effect of surfactants on such properties varied as a function of the surfactant charge. Whereas anionic surfactants did not show any kind of interaction, the nonionic surfactant Triton X-100 produced spectral changes in the dye, as a consequence of the shift of equilibrium between its excited species. In the case of fluorescence, these changes allowed the critical micellar concentration of the surfactant to be determined. However, the most important features were obtained from the interaction with cationic surfactants of the n-alkyl trimethylammonium bromides. Such interactions enabled the formation of premicellar aggregates to be determined. In addition, three or four critical concentrations could be defined, which were dependent on the length of the hydrocarbon chain. One of these was the critical micellar concentration of the surfactant. The remaining two or three were in the very dilute concentration domain from which several types of premicellar aggregates are formed. However, the interaction with dodecyltrimethylammonium bromide at submicellar concentrations evidenced a quenching effect of a different nature (either static or combined), depending on the surfactant concentration. By deconvoluting the overall pyranine fluorescence emission spectrum into a sum of overlapping Lorentzian-Gaussian functions, the principal microenvironments of the pyranine molecules can be ascertained. Fluorescence data are consistent with the location of pyranine in a variety of sites, including partitioning that may be influenced by electrostatic and pi-cation interactions in aqueous micelles.