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 ° 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 ° C , respectively. The density is precisely determined on the basis of the P V T dependence of the proton chemical shift. The density-diffusivity products in the zero-density limit divided by the square root of the temperature, ( ρ D ) 0 ∕ T , are 1.03, 1.28, and 1.44 fg m − 1 s − 1 K − 1 ∕ 2 (f, femto) at 200, 300, and 400 ° C , respectively. The ( ρ D ) 0 ∕ 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 ρ D ∕ T against ρ is almost zero at 400 ° C and slightly negative at 300 ° C .