Effect of ionic surfactants on the hydration behavior of triblock copolymer micelles: a solvation dynamics study of coumarin 153

Dynamic fluorescence Stokes shift measurements of coumarin 153 (C153) have been carried out to study the influence of ionic surfactants (sodium dodecyl sulfate, SDS and hexadecyltrimethylammonium chloride, CTAC) on the hydration behavior of aqueous poly(ethylene oxide)(20)-poly(propylene oxide)(70)-poly(ethylene oxide)20 (P123) block copolymer micelles. Increase in SDS or CTAC concentration at a fixed P123 concentration induces the steady-state emission spectra of C153 to shift gradually toward lower energy. This is attributed to an increase in polarity (due to enhanced hydration) experienced by the probe as a consequence of incorporation of ionic head groups in the Corona region. The observed dynamic fluorescence Stokes shift value decreases more in mixed micellar systems than in pure copolymer micelles and the trends are quite similar in the presence of SDS and CTAC. The spectral shift correlation functions were observed to be nonexponential in nature. Critical analysis of the spectral shift correlation function indicates a fast solvation component (<0.2 ns) in P123 micelles, which was absent in the presence of ionic surfactants. Due to increased hydration in the presence of ionic surfactants, the initial fast solvation event was elusive in mixed copolymer-surfactant systems, reflecting the absence of faster solvation component and reduced observed Stokes shift in mixed systems. It has been argued that in the low surfactant concentration region, increase in hydration with the incorporation of ionic head groups in the Corona region is mainly due to increase in mechanically trapped water content. However, at higher surfactant concentrations, bound water content dominates and leads to slower solvation dynamics. The present results also indicate that though CTAC alters the Corona hydration more efficiently than SDS, the overall influence of ionic surfactants on the Corona hydration is grossly similar irrespective of the cationic or anionic nature of the surfactants. Interaction of SDS and CTAC with poly(ethylene oxide)(100)-poly(propylene oxide)(70)-poly(ethylene oxide)(100) (F127) block copolymer micelles has also been studied to comprehend the effect of copolymer composition. The overall trends in dynamic fluorescence Stokes shift and solvation times are similar in both the copolymer micelles.