Quasi elastic neutron scattering study of dimethyl sulfoxide water mixtures probing molecular mobility in a nonideal solution

The translational and rotational motions of water and dimethyl sulfoxide, DMSO, CH3 2SO have been investigated using quasi elastic neutron scattering. Water?DMSO mixtures at five DMSO mole fractions, cDMSO, ranging from 0 to 0.75, were measured. Hydrogen deuterium substitution was used to extract independently the water proton dynamics d DMSO H2O , the DMSO methyl proton dynamics h DMSO D2O and to obtain background corrections d DMSO D2O . The translational diffusion of water slows down significantly compared to bulk water at all cDMSO gt; 0. The rotational time constant for water exhibits a maximum at cDMSO 0.33 that corresponds to the observed maximum of the viscosity of the mixture. Data for DMSO can be analyzed in terms of a relatively slow tumbling of the molecule about its center of mass in conjunction with random translational diffusion. The rotational time constant for this motion exhibits some dependence on cDMSO , while the translational diffusion constant shows no clear variation for cDMSO gt; 0. The results presented reinforce the idea that due to the stronger associative nature of DMSO, DMSO water aggregates are formed over the whole composition range, disturbing the tetrahedral natural arrangement of the water molecules. As a consequence adding DMSO to water causes a drastic slowing down of the dynamics of the water molecule, and vice vers

Ultrafast vibrational dynamics of water confined in phospholipid reverse micelles

We study the ultrafast dynamics of OH stretching and bending vibrations of water inside dioleoylphosphatidylcholine (DOPC) reverse micelles in a wide range of hydration. A strong hydration level dependence for the spectral diffusion rates is found and explained by the distinctly different environment for single water molecules bound to the anionic phosphate group. We show that the energy relaxation pathway of the OH stretching vibration at low hydration level involves the OH bending