Influence of Concentration and Temperature on the Dynamics of Water in the Hydrophobic Hydration Shell of Tetramethylurea

Abstract

We study the influence of the amphipilic compd. tetramethylurea (TMU) on the dynamical properties of water, using dielec. 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 mols. in the hydration shell of the hydrophobic groups of TMU is between 3 (at low concns.) and 10 (at higher concns.) 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 temp., the fraction of water contained in the hydrophobic hydration shell decreases, which implies that the overall effect of hydrophobic groups on water becomes smaller.