Pressure and temperature dependence of the self diffusion coefficient of water and oxygen-18 water

The self diffusion coefficients of water, DS(H216O), and of oxygen-18 water, DS(H218O), have been measured over the temperature range 277–333 K and to pressures of 300 MPa by the n.m.r. spin echo technique. The precision was ± 1%. The isotherms show the now familiar broad maximum below ≈ 308 K. The ratio of DS(H216O) to DS(H218O) is constant over the whole region studied and is equal to m(H218O)/m(H216O)½. Comparison of the self diffusion data with tracer data for HTO in water show the data to be consistent with a slightly greater coupling of translational and rotational motion of HTO relative to H2O in pure water at low pressures, i.e. the pressure dependence of DS is less marked for H2O, though the effect is rather small.