EFFECT OF XENON UPON THE DYNAMICAL ANOMALIES OF SUPERCOOLED WATER - ATEST OF SCALING-LAW BEHAVIOR

We have studied the molecular reorientation and self-diffusion of wate r molecules in aqueous solutions of xenon in the approximate temperatu re range from 273 to 333 K using D-2 and H-1 magnetic relaxation and s pin-echo techniques. In addition, we report on Xe-131 relaxation rates in this temperature interval. These data, in conjunction with data ob tained recently by us for the self-diffusion of xenon, are evaluated i n terms of scaling laws which are known to account for the peculiar be havior of transport and relaxation coefficients of pure water in the s upercooled regime. In pure water these anomalies are strong enough to suggest a thermodynamic singularity at T-S = 228 K. The results for D- 2 relaxation suggest that xenon shifts this singularity toward higher temperatures. An extrapolation toward the composition of the Xe x 23H( 2)O clathrate yields T-S congruent to 260 K. Essentially the same figu re is obtained from Xe-131 relaxation, which reflects the local dynami cs of water molecules near xenon and may therefore serve as a measure of T-S in clathrate-like domains. This shift of T-S by added xenon con firms expectations that nonpolar solutes stabilize just those structur es of water which are responsible for the anomalies observed in the su percooled regime. In this sense, xenon is acting like a negative hydro static 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 transl ational motions of water near T-S, which becomes apparent by largely d ifferent values for T-S deduced from relaxation and self-diffusion dat a. While reorientational motions reflect the slowing down of molecular motions associated with the approach to T-S, diffusion remains compar atively fast at the same temperature. This decoupling shows a striking resemblance with similar processes observed for other liquids near gl ass transitions.