Effect of Xenon upon the Dynamical Anomalies of Supercooled Water. A Test of Scaling-Law Behavior

Abstract

We have studied the molecular reorientation and self-diffusion of water molecules in aqueous solutions of xenon in the approximate temperature range from 273 to 333 K using 2D and 1H magnetic relaxation and spin-echo techniques. In addition, we report on 131Xe relaxation rates in this temperature interval. These data, in conjunction with data obtained recently by us for the self-diffusion of xenon, are evaluated in terms of scaling laws which are known to account for the peculiar behavior of transport and relaxation coefficients of pure water in the supercooled regime. In pure water these anomalies are strong enough to suggest a thermodynamic singularity at TS = 228 K. The results for 2D relaxation suggest that xenon shifts this singularity toward higher temperatures. An extrapolation toward the composition of the Xe × 23H2O clathrate yields TS ≅ 260 K. Essentially the same figure is obtained from 131Xe relaxation, which reflects the local dynamics of water molecules near xenon and may therefore serve as a measure of TS in clathrate-like domains. This shift of TS by added xenon confirms expectations that nonpolar solutes stabilize just those structures of water which are responsible for the anomalies observed in the supercooled regime. In this sense, xenon is acting like a negative hydrostatic pressure. It is however difficult to rationalize the data by a universal exponent, as is required by true scaling law behavior. As a further new feature we report on a decoupling of rotational and translational motions of water near TS, which becomes apparent by largely different values for TS deduced from relaxation and self-diffusion data. While reorientational motions reflect the slowing down of molecular motions associated with the approach to TS, diffusion remains comparatively fast at the same temperature. This decoupling shows a striking resemblance with similar processes observed for other liquids near glass transitions.