Femtosecond OH Bending Dynamics of Water Nanopools Confined In Reverse Micelles

The relaxation of OH bend excitations of H2O confined in reverse micelles is studied by femtosecond broadband pump probe spectroscopy for the two ionic systems dioctyl sodium sulfosuccinate (AOT) and dioleoylphosphatidylcholine (DOPC) and for the nonionic tetraethylene glycol dodecyl ether (Brij-30). In the ionic AOT/DOPC reverse micelles, the OH bending lifetime T-1 decreases from T-1 > 615 fs for a 3:1 ratio of water and AOT/DOPC molecules (w(0) = 3) to T-1 = 345 Is for a 16:1 ratio (w(0) = 16). In contrast, H20 in Brij-30 reverse micelles shows a much shorter = 400 fs at w(0) = 2 which decreases to T, = 250 fs at w(0) = 8. OH bend relaxation proceeds mainly via librational overtones of the bend-excited water molecules with a rate correlating with the energy mismatch between the v = 1 OH bend state and the librational overtone. In the ionic systems, the lower librational frequencies at small wo result in a larger mismatch and longer T, times. In the nonionic case, the w(0)-independent librational frequencies with a small energy mismatch lead to shorter T, times. For w(0) >= 8, the energy flow into the first hydration shell of the bend-excited molecules makes an additional contribution to the relaxation rate in all systems.