Self-diffusion in compressed liquid heavy water

Abstract

The self-diffusion coefficients D in liquid heavy water have been measured by the NMR spin-echo method over the temperature range 10–200°C and the pressure range 1 bar to 9 kbar. A good agreement with tracer diffusion studies by Woolf performed over a more limited range of temperatures and pressures has been obtained. As in other studies, we find an anomalous initial increase in the diffusion coefficients with pressure at temperatures below approximately 30°C. At high temperatures compression leads to a monotonic decrease of the self-diffusion. The fact that the activation energies at constant density decrease both with increasing temperature and increasing density indicates that both high temperature and high compression significantly affect the hydrogen bond network and produce a more ’’normal’’ liquid. We find that the Stokes–Einstein equation is accurate to within 10% over the range of experimental conditions studied. Phenomenological interpretation of plots of D/T vs molar volume supports the view that at high compression and high temperature the dynamic behavior of heavy water resembles that of a hard sphere liquid. All experimental results are in agreement with our conclusions obtained in our earlier study of NMR relaxation in compressed liquid water and heavy water.

NMR‐Based Structural Studies of the pNR‐2/pS2 Single Domain Trefoil Peptide

“…This possibility was investigated by estimating the apparent molecular mass of the protein in solution from measurements of its overall rotational correlation times at 278 K, 298 K and 313 K. The correlation times were derived from estimates of 'H-IH cross-relaxation rates obtained from measurements of the build-up rates for intramolecular NOE between the HN-cl and H-Sl/H-l2 protons of Trp43 in pNR-2/pS2 [44]. is in agreement with typical values for protein solutions [45]. It has been reported by others [46] and observed by ourselves (Chadwick, M., May, F. and Westley, B., unpublished results) that native pNR-2/pS2 forms disulphide-linked dimers through Cys58.…”

Room temperature compressibility and diffusivity of liquid water from first principles

“…39,61 Our large 200-molecule simulation box and reasonably long AIMD runs have reduced the uncertainty enough to observe a clear trend in D(ρ), as shown in Fig. 12 We find vdW-DF PBE to be extremely successful, perfectly reproducing NMR spin-echo measurements 62 within the statistical errors. Furthermore, within our density resolution, D(ρ) shows a maximum at 1.05 g/cm 3 , in agreement with experiment.…”

“…Indeed, experimental results show that an increase of 10 K in temperature can cause an increase of 0.5-0.6 g/cm 3 in the self-diffusion coefficient both for light and heavy water. 56,57,62 On the other hand, it also possible that the diffusivity of the liquid is mostly determined by the original temperature (and the corresponding structure) at which the system was equilibrated (300 K for all points), rather than the average temperature reached during the NVE simulation. In favor of this latter hypothesis, we note that the simulations at 1.15 g/cm 3 and 1.20 g/cm 3 also featured a similar increase in temperature; however, applying an approximate correction only to these three points results in a less smooth trend overall, with a minimum at 1.05 g/cm 3 and a maximum at 1.10 g/cm 3 .…”

Are polar liquids less simple?

“…However, Hbonding in water is atypical, being associated with an extended network structure that imposes specific distances and molecular orientations, due to the ability of one water molecule to form multiple Hbonds. These factors cause the concentration of H-bonds in water to decrease with pressure as the network is disturbed [13,14,15]. This reduction in H-bonding enhances molecular motions, countervailing the direct effect of densification on the dynamics.…”