Self-diffusion of supercritical water in extremely low-density region

The self-diffusion coefficient D for super- and subcritical water is determined by using the proton pulsed-field-gradient spin echo method at high temperatures and low densities. The density of water is ranged in the steamlike region from 0.0046 to 0.0650 g cm(-3) at a supercritical temperature of 400 degrees C, also at 0.0046-0.0079 and 0.0046-0.0462 g cm(-3) (the steam-branch densities on the coexistence curve and lower) at 200 and 300 degrees C, respectively. The density is precisely determined on the basis of the PVT dependence of the proton chemical shift. The density-diffusivity products in the zero-density limit divided by the square root of the temperature, (rho D)(0)/root T, are 1.03, 1.28, and 1.44 fg m(-1) s(-1) K-1/2 (f, femto) at 200, 300, and 400 degrees C, respectively. The (rho D)(0)/root T obtained decreases with decreasing temperature and is significantly smaller than the temperature-independent value from the hard sphere model, 1.95 fg m(-1) s(-1) K-1/2. The marked temperature dependence reflects the presence of the strong attractive interaction between a pair of water molecules. The magnitude of the experimental D values and the temperature dependence are well reproduced by the molecular dynamics simulation using TIP4P-FQ model. The initial slope of the product rho D/root T against rho is almost zero at 400 degrees C and slightly negative at 300 degrees C. (c) 2006 American Institute of Physics.