NRET - A rapid method to investigate the water-oil interface in reverse micellar systems

The water-oil interface of AOT/water/isooctane reverse micelles, with different hydration degrees (w0), is investigated by nonradiative energy transfer (NRET) between naphthalene (Np) probe and pyrene grafted poly(acrylic acid) (PAA150-Py32). The polymer is water-soluble while Np has affinity for the organic solvent. UV-Vis spectroscopy reveals that PAA150-Py32 is constrained in the micellar nanocage. The change of the pH of the micellar core and the addition of electrolyte are considered. FTIR and static fluorescence measurements confirm the presence of the polymer at the micellar interface. The stability and size distribution of the reverse micelles, with or without PAA150-Py32 and electrolyte are characterized by DLS. The photophysical parameters of PAA150-Py32 (the polarity index, I1/I3 and the polymer conformation index, IE/IM) are determined at different pHs and electrolyte concentrations. For w0 = 10 and w0 = 15, the pH does not influence the formation of Py excimers, which is an atypical behavior. NRET is strongly dependent on the pH, and for w0 = 10 and pH = 3.6 it reaches a maximum. The optimal salt concentration for which NRET is maximal is 5 × 10− 4 M NaCl. The acquired results shed more light upon the stability of the micellar interface in various experimental conditions.