WATER DYNAMICS NEAR A DISSOLVED NOBLE-GAS - FIRST DIRECT EXPERIMENTAL-EVIDENCE FOR A RETARDATION EFFECT

Deuteron magnetic relaxation rates and self-diffusion coefficients mea sured by the H-1 spin-echo technique are reported for aqueous solution s of xenon in the temperature range from 273 to 333 K at pressures up to 6 MPa. The data enable the determination of the reorientational cor relation time and self-diffusion coefficient of water in the first coo rdination sphere of xenon and provide the first direct experimental pr oof of a perceptible slowing down of these motions. At 298 K we find a retardation by a factor of 1.3 of rotational and translational motion s in the first hydration shell of xenon above those in the bulk. The d ata are compared with the results of computer simulations and with est imates extracted from experimental data for alkyl groups in larger mol ecules. A comparison of results for rotational and translational motio ns shows that at high temperatures both are affected to the same degre e. Below 298 K this strong coupling is lost. This phenomenon appears t o be related to known anomalies of water in the supercooled regime, po ssibly associated with a thermodynamic singularity near T-s = 228 K. X enon may shift T-s to higher temperatures.