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 steady-state spectrum, time-dependent fluorescence Stokes shift of coumarin 153 in aqueous solution of typical neutral surfactant Triton X-100 was measured at high pressures using time-correlated single-photon counting techniques with a time-resolution of 20 ps. It was found that polarity of the microenvironment decreases with increasing pressure. The solvation dynamics exhibits bimodal relaxation behavior with two characteristic time constants of 198 ps and 1.63 ns at atmospheric pressure. The solvation time becomes shorter with increasing pressure. The bimodal relaxation and its pressure dependence is considered as closely connected with the pressure effect on the hydrogen bonding of water molecules. And also the model assuming an equilibrium between "bound water" and "free water" within the Stem layer of micelles is discussed.