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

Abstract

The self diffusion coefficients of water, D_S(H₂¹⁶O), and of oxygen-18 water, D_S(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 D_S(H₂¹⁶O) to D_S(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 D_S is less marked for H₂O, though the effect is rather small.