Pressure effect on solvation dynamics in micellar environment

Solvation dynamics of an excited solvatochromic probe molecule in micellar environment has been studied as a function of pressure. In addition to stea dy-state spectrum, time-dependent fluorescence Stokes shift of coumarin 153 in aqueous solution of typical neutral surfactant Triton X-100 was measure d at high pressures using time-correlated single-photon counting techniques with a time-resolution of 20 ps. It was found that polarity of the microen vironment decreases with increasing pressure. The solvation dynamics exhibi ts bimodal relaxation behavior with two characteristic time constants of 19 8 ps and 1.63 ns at atmospheric pressure. The solvation time becomes shorte r with increasing pressure. The bimodal relaxation and its pressure depende nce is considered as closely connected with the pressure effect on the hydr ogen bonding of water molecules. And also the model assuming an equilibrium between "bound water" and "free water" within the Stem layer of micelles i s discussed.