Pressure dependence of NMR proton spin--lattice relaxation times and shear viscosity in liquid water in the temperature range -15--10 degreeC

The NMR proton spin--lattice relaxation times T1 and shear viscosities have been measured as functions of pressure in the temperature interval -15--10 degreeC. At low temperatures the low pressure boundary of the experiments is ice I, whereas ice V represents the high pressure extreme of our measurements. The initial compression at all temperatures covered in our study results in higher motional freedom of water molecules so that the pressure dependence exhibits a minimum in viscosity and a maximum in T1. This is a consequence of significant distortion of the hydrogen bond network due to compression which also seems to weaken the hydrogen bonds. Further compression leads to restricted motional freedom due to increased packing of the molecules. This anomalous behavior of spin--lattice relaxation and shear viscosity with compression is more pronounced at lower temperatures since the hydrogen bond network is better developed at lower temperatures. In agreement with our earlier data covering the 10--90 degreeC temperature range, we find that compression under isothermal conditions distorts the random hydrogen bond network, leading to diminished coupling between the rotational and translational motions of water molecules. The data indicate that the Debye equation describes the relationship between the reorientational correlation time and shear viscosity at constant volume but is not applicable to describe the density effects on water reorientation. In general, pressure and temperature have parallel effects on many dynamic properties at temperatures below 40 degreeC and pressures below 2 kbar, whereas at higher temperatures and pressures their effects are just the opposite. Hard core repulsive interactions become more important than the directional interactions of hydrogen bonding at high compression