Fluorometric studies of the polyelectrolyte shell of block copolymer micelles in aqueous media

The fluorescent surfactant 5-(N-dodecanoyl)aminofluorescein, DAF, binds very strongly at amphiphilic surfaces and interfaces. It binds also in the shell of water-soluble polymeric micelles. Dissociation of DAF depends on the polarity of the microenvironment and the charge distribution in the vicinity of the sorbed probe (the apparent dissociation constant of the OH group in DAF changes after binding), and the fluorescence properties of the water-dissolved and the micelle-sorbed probes differ. The distances between the sorbed probes decrease with increasing number of the shell-sorbed probes and the nonfluorescent H-dimers and higher aggregates are formed. The fluorescence emission from the shell-sorbed DAF is quenched due, in major part, to nonradiative energy transfer to H-aggregates that represent efficient energy traps and to a decrease in the fraction of fluorescent species (i.e., DAF monomers). Analysis of the steady-state and time-resolved fluorescence data, based on an appropriate model that assumes the microphase partition equilibrium of probes between the shell of polyelectrolyte micelles and the bulk aqueous phase, and saturation of the shell and the fluorescence self-quenching at high DAF-to-micelle ratios, allows for reliable evaluation of the partition coefficient, Kp, as ca. 4.0 x 10(5).