Ionic equilibria of fluorophores in organized solutions: The influence of micellar microenvironment on protolytic and photophysical properties of rhodamine B

Ionic equilibria and fluorescence decay of rhodamine B were studied in micellar solutions of sodium n-dodecyl sulfate (SDS) with various additives (NaCl, pentanol-1, crown ether, and tetra-n-butyl ammonium salt), and five nonionic surfactants (Brij 35, nonyl phenol 12, Triton X-100, Triton X-305, Tween 80), as well as in beta-cyclodextrin solutions. The apparent dissociation constants, K-a(a), of rhodamine B (HR+ reversible arrow R + H+) were obtained. The distribution of the dye species HR+ and R in the ultramicroheterogeneous systems was studied using absorption and emission spectra, fluorescence lifetimes, tau, and the plots of pK(a)(a) versus surfactant concentration. The pK(a)(a) values under conditions of complete binding, pK(a)(ac), were found to be markedly higher than that in water (pK(a)(w)). The medium effects, DeltapK(a)(ac) = pK(a)(ac) - pK(a)(w), in organized solutions studied are in accord with the charge type of the acid-base couple +/+/-, confirming the zwitterionic nature of rhodamine B neutral species. Both tau and DeltapK(a)(ac) values of the dye were shown to sense the changes in the micellar microenvironments, and the possibility of using rhodamine B as an interfacial acid-base indicator, for example, for monitoring of surface potentials and of bulk ionic strength, were demonstrated.