Jt. Cabral et al., Single-particle dynamics in dimethyl-sulfoxide/water eutectic mixture by neutron scattering, J CHEM PHYS, 113(19), 2000, pp. 8736-8745
We study water and dimethyl-sulfoxide (DMSO) dynamics in the eutectic DMSO/
water mixture, 2H(2)O:1DMSO, by incoherent quasielastic and inelastic neutr
on scattering. A temperature range from room down to -27 degreesC is invest
igated. Both water and DMSO translational dynamics are significantly slowed
in the mixture compared to pure liquids. They exhibit different diffusion
dynamics, pointing to the absence of stable hydrogen bonded complexes. Furt
her, the presence of a solute suppresses the dynamic anomalies of water: se
lf-diffusion coefficients for water in the mixture, as well as residence ti
mes for jump diffusion reveal an Arrhenius temperature dependence, in contr
ast to the strong non-Arrhenius behavior in pure water. The density of vibr
ational states shows a shift of the characteristic librational peak in pure
water towards higher energies in the mixture, reflecting hindrance of larg
e amplitude proton motions. These results are compatible with computer simu
lations which predict longer hydrogen-bond lifetimes in the mixture, compar
ed to pure water. However, rotational relaxation times for water protons in
the mixture are estimated to be similar to1 ps, as for pure water. This ex
perimental finding suggests that unlike in pure, low temperature water, pro
ton librations should no longer be the main mechanism responsible for hydro
gen-bond breaking in the mixture. Thus, the correspondence between the prot
on hindered rotational relaxation time and the "hydrogen-bond lifetime" sho
uld not apply in DMSO-water mixtures. (C) 2000 American Institute of Physic
s. [S0021-9606(00)51242-2].