Influence of Concentration and Temperature on the Dynamics of
Water in the Hydrophobic Hydration Shell of Tetramethylurea,
K.-J. Tielrooij, J. Hunger, G. Hefter, R. Buchner, M. Bonn and H. J. Bakker,
J. Am. Chem. Soc. 132 (2010) 15671-15678.
Abstract
We study the influence of the amphipilic compound tetramethylurea (TMU) on the dynamical properties of water,
using dielectric relaxation spectroscopy in the regime between 0.2 GHz and 2 THz. This technique is capable
of resolving different water species, their relative fractions, and their corresponding reorientation
dynamics. We find that the reorientation dynamics of water molecules in the hydration shell of the
hydrophobic groups of TMU is between 3 (at low concentrations) and 10 (at higher concentrations) times
slower than the dynamics of bulk water. The data indicate that the effect of hydrophobic groups on water
is strong but relatively short-ranged. With increasing temperature, the fraction of water contained in the
hydrophobic hydration shell decreases, which implies that the overall effect of hydrophobic groups on water
becomes smaller.
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