The pressure dependence of self-diffusion and spin–lattice relaxation in cold and supercooled H2O and D2O

Measurements of the self-diffusion coefficients D and the oxygen-17 and deuterium spin–lattice relaxation times T1 in light and heavy water have been extended into the supercooled region to a maximum pressure of 400 MPa and to a minimum temperature of 240 K and partly to even lower temperatures. With these measurements the maxima of the T1 isotherms could be established for the first time. For the deuterium and oxygen-17 T1 of heavy water as well as for the oxygen-17 T1 of light water the maxima occur around 250 MPa, their position being independent of temperature. Previously established maxima in the D-isotherms for both liquids are found at around 150 MPa. The heights of the maxima increase strongly with decreasing temperature. The increase in mobility with pressure is much more pronounced for the rotational mobility as given by T1 than for the translational mobility characterised by the self-diffusion coefficient. This anomaly, which appears to have been observed experimentally in cold water only, is discussed qualitatively.