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

摘要

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 deg 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 deg 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,(rhoD)~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 (pD)~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 rho D T against rho is almost zero at 400 deg C and slightly negative at 300 deg C.