The self-diffusion coefficient, D, for pure liquid water has been measured at temperatures between 275.2 and 498.2 K and at pressures up to 1.75 kbart by the proton spin echo method. Our values of D agree, where they overlap, with recently published data which, however, were measured mostly at low temperature and over rather narrow ranges of temperature.The results are discussed in several ways. The Stokes-Einstein relation is found to be obeyed in the slipping boundary limit. The cubic cell model of Houghton accounts satisfactorily for the measured D values, particularly at higher temperatures. A simple test of a hard-sphere model i s found to give poor agreement at lower temperatures but a modified hard-sphere theory seems to be more satisfactory. The activation analysis at constant density shows that water behaves very differently from non-associated liquids. It also suggests that an increase in both temperature and pressure leads to an increase in the fraction of free unbonded water molecules.A free-volume analysis has led to a modified Arrhenius equation which involves pressure-dependent terms. This semi-empirical equation describes the results within experimental error and predicts a glass temperature at 115 K which is in reasonable agreement with the values obtained by other methods.There has recently been an increasing interest in the experimental determination of the self-diffusion coefficient, D, for compressed and heavy4 water at various temperatures. Measurements of D over a range of temperature and density will lead to a better understanding of both the diffusional behaviour of water imbedded in deep rocks and of some molecular transport properties of compressed heavy water such as the proton spin relaxation. However, all measurements of D for H 2 0 under pressure have so far been made1-3*5-9 at moderate temperatures only up to 332.2 K. A detailed discussion of previous self-diffusion measurements for compressed water was recently presented by Woolf2 and will not be repeated here. There also exist numerous measurements of D for liquid water under its saturated vapour pressure (s.v.P.)but only a few covered wide temperatureThe large spread of D values at 298.2 K in the earlier measurements was caused by systematic errors as discussed by Millsl3" and the best value13' is 2.299 xThe present study of self-diffusion in compressed pure water was undertaken to compare our D values measured by the proton spin echo method with those of tracer experiments by Woolf' at lower temperatures and to extend the experimental temperature range up to = 500 K. It seemed interesting to test the applicability of the StokesEinstein relations, the cubic cell rnodelI4 and the hard-sphere model over the widest temperature range. Finally, since the shear viscosity of water at S.V.P. could be represented by a modified Arrhenius equation,15 it was of interest to test the analogous equation for the self-diffusion of compressed water. m2 s-l. E X P E R I M E N T A LThe self-diffusion measurements were made by the n.m.r. spin echo m...

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