Role of micellar size and hydration on solvation dynamics: A temperature dependent study in Triton-X-100 and Brij-35 micelles

The temperature effect on solvation dynamics has been investigated in two neutral micelles, namely, Triton X-100 (TX100) and Brij-35 (BJ-35), using dynamic fluorescence Stokes' shift method, to explore the role of micellar size and hydration on the solvation process. In TX-100, the temperature effect on C(t) is not only very strong but shows an unusual inversion around 298 K. On the contrary, for the BJ-35 micelle, the temperature effect is not that significant. Present results on solvation dynamics in the two micelles have been rationalized on the basis of the temperature dependent changes in micellar size and hydration, which are reported to be very large for TX-100 but very marginal for BJ-35. Observed inversion in the solvation rate in TX-100 around 298 K is inferred to be arising due to the interplay of two factors, one is the largely reduced micellar size at lower temperature, which causes the bulk water to come reasonably closer to the probe and thus contribute to the solvation process, and the other is the largely increased hydration at higher temperature that makes the micellar structure loose and consequent enhancement in the solvation rates. With intermediate micellar size-and hydration, the solvation dynamics is slowest at around 298 K. For the BJ-35 micelle, as the micellar size and hydration does not change to any significant extent, there is almost no temperature effect on the solvation dynamics for this micelle.