Proton and deuteron NMR spin–lattice relaxation times in liquid water and heavy water were measured as a function of pressure and temperature in the range 10–90°C and 1 bar–9 kbar. D2O was also studied at 150 and 200°C. Availability of density and viscosity data under these experimental conditions enabled us to separate the effects of temperature and density on the spin–lattice relaxation times, T1, and viscosities. Under the assumption that the intermolecular dipolar contribution to the proton T1 follows the changes in shear viscosity with temperature and density, we separated the intramolecular and intermolecular dipolar contributions to the proton T1. We found that at a temperature of 10°C the initial increase in density leads to faster reorientation of the water molecules. The effect was much smaller at 30°C. Analysis of the experimental data on H2O and D2O leads to the conclusion that compression diminishes the coupling between the rotational and translational motions of water molecules. The change in the nature of the rotation–translation coupling with increasing density is mainly responsible for the failure of the Debye equation to describe the density effects on the reorientation of water molecules. In the case of D2O we find a relatively small variation in the deuteron quadrupole coupling constant with increasing density. Its average value is approximately 230 kHz over the range of our experimental conditions. Another experimental finding of this study is the decrease in the activation energies for relaxation and shear viscosity with increasing density. All the experimental evidence indicates that compression of water leads to significant distortion and/or disruption of the hydrogen bond network with the important consequence that the dynamic behavior of water under high compression resembles more that of a ’’normal’’ molecular liquid of comparable molecular size. At high densities the hard core repulsive interactions begin to dominate over the directional interactions which are mainly responsible for the open structure of liquid water at low temperatures and pressures.
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.
Cetane number is a measure of ignition quality, specifically ignition delay, of diesel fuel. It is an engine measure of a kinetic phenomena. While it is typically inappropriate to use a thermodynamic measure, such as molecular structure, to predict kinetic behavior, molecular structure does correlate
The NMR proton spin-lattice relaxation times T t and shear viscosities have been measured as functions of pressure in the temperature interval -IS-IO·C. 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 T t • 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·C 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 4O·C 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.
The NMR deuteron spin–lattice relaxation times T1, self-diffusion coefficients D, and shear viscosities η have been measured as a function of pressure in the temperature interval −15 to 10°C. The low-pressure extreme of the measurements is Ice I, whereas Ice V represents the high-pressure boundary of the experiments. In analogy with anomalous motional behavior in compressed liquid water, the initial compression of liquid D2O in the temperature interval studied results in higher motional freedom of D2O molecules so that T1 and D dependences with pressure exhibit a maximum and shear viscosity shows a minimum. This is a result of distortion and weakening of the hydrogen bond network owing to compression. Further compression hinders molecular motions as a result of increased repulsive interactions due to higher packing. This study also enables us to test the applicability of hydrodynamic equations at the molecular level for liquid heavy water. Analysis of the relaxation and shear viscosity data show that the Debye equation fails to describe the density effects on reorientation of D2O molecules. It appears that the success of the Debye equation to describe temperature effects on reorientation of H2O and D2O molecules at 1 bar is accidental. However, the data show that the deuteron relaxation rate (1/T1)D is proportional to η/T under isochoric conditions. The fact that the slope of the (1/T1)D vs η/T plot diminishes with increasing density indicates that compression leads to diminished coupling between rotational and translational motions of water molecules. The shear viscosity and self-diffusion data show that the Stokes–Einstein equation does not represent the relationship between D and η in liquid heavy water. A brief discussion of the isotope effects on shear viscosity in liquid D2O and H2O is presented.
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