| Abstract
| - Dynamic fluorescence Stokes shift measurements of coumarin 153 (C153) have been carried out to study theinfluence 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 concentrationinduces the steady-state emission spectra of C153 to shift gradually toward lower energy. This is attributedto an increase in polarity (due to enhanced hydration) experienced by the probe as a consequence ofincorporation of ionic head groups in the Corona region. The observed dynamic fluorescence Stokes shiftvalue decreases more in mixed micellar systems than in pure copolymer micelles and the trends are quitesimilar in the presence of SDS and CTAC. The spectral shift correlation functions were observed to benonexponential in nature. Critical analysis of the spectral shift correlation function indicates a fastsolvation component (<0.2 ns) in P123 micelles, which was absent in the presence of ionic surfactants. Dueto increased hydration in the presence of ionic surfactants, the initial fast solvation event was elusive inmixed copolymer−surfactant systems, reflecting the absence of faster solvation component and reducedobserved Stokes shift in mixed systems. It has been argued that in the low surfactant concentrationregion, increase in hydration with the incorporation of ionic head groups in the Corona region is mainly dueto increase in mechanically trapped water content. However, at higher surfactant concentrations, boundwater content dominates and leads to slower solvation dynamics. The present results also indicate that thoughCTAC alters the Corona hydration more efficiently than SDS, the overall influence of ionic surfactants onthe Corona hydration is grossly similar irrespective of the cationic or anionic nature of the surfactants. Interactionof 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. Theoverall trends in dynamic fluorescence Stokes shift and solvation times are similar in both the copolymermicelles.
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