Solvation dynamics of coumarin 480 in TritonX-100 (TX-100) and bile salt mixed micelles-1

The time-resolved Stokes' shift of the laser dye Coumarin 480 (C-480) has been investigated in the Stern layer of the pure Triton X-100 (TX-100), bile salt (Sodium Deoxycholate, NaDC) and the mixed micelles of bile salt (NaDC), and TX-100 at different compositions using picosecond time-resolved emission spectroscopy. The wavelength dependent decays at the blue end and growth at the red end of the spectrum are observed for the TX-100, NaDC micelles, and mixed micelles above critical micelles concentrations. The average solvation time in pure NaDC and TX-100 are 3.35 and 1.51 ns, respectively, but in the mixed micelles, it is 2.27-3.26 ns. The solvation dynamics of the water molecule in the Stern layer of the mixed micelles is retarded several times compared to that of pure water. The relatively slow solvation dynamics in the Stern layer of the NaDC micelles is due to the motion of the Na+ ions and the water molecules confined in the micellar cavity. The interaction of the Na+ ions with the headgroup of the micelle is also responsible for the slow dynamics. The slow dynamics in the mixed micelles compared to that of pure water is due to the motion of confined water in the micellar core, motion of the Na+ ions, and interaction of the Na+ ions with the headgroup of the micelles. In the mixed micelles, the solvation dynamics is relatively faster because of the removal of Na+ ion from the Stem layer.