Temperature dependence of pyrene fluorescence spectra in aqueous solutions of CnEm (C14E7, C16E7, and C16E6) nonionic surfactant micelles

Apparent hydrodynamic radii (Rhapp) of nonionic surfactant micelles in aqueous solutions of hepta- and hexa-ethyleneoxide monoalkylether (C14E7, C16E7, and C16E6) were measured at 20–40 °C. Fluorescence spectra of pyrene in aqueous solutions of the nonionic surfactants were measured as a function of surfactant concentration and temperature. The cloud point of C16E6 was about 35 °C at concentration range of 10−3 to 10−2 M. However, even at a higher temperature than the cloud point, the fluorescence intensity ratio of pyrene monomer, Im1/Im3, in C16E6 solution behaves similarly to other surfactants (C16E7 and C14E7), indicating that the pyrene molecules reside in a similar location of surfactants. The slope for the temperature dependence of the Im1/Im3 ratio also changed with the surfactant concentration, and the micelle size differed among the surfactants (C16E6, C16E7, and C14E7). The activation enthalpy of pyrene diffusion was evaluated using Stevens–Ban plot, and the value of 20–30 kJ mol−1 was obtained for surfactant concentrations of 5 × 10−4 to 10−2 M. The results were compared with those reported for ionic and nonionic surfactant micelles.