Role of the collective self-diffusion in water and other liquids

This paper is devoted to collective effects in the transport processes in liquids. Special attention is paid to the collective contributions Dc to self-diffusion coefficients Ds of liquids, especially for normal and supercooled water. The theoretical estimates for Dc are obtained on the basis of the Lagrange theory of thermal hydrodynamic fluctuations, the experimental ones by analysis of the wave vector dependence of the half-width of the quasi-elastic incoherent neutron scattering peak. It is shown that theoretical predictions and experimental values of Dc are in good agreement. The ratio Dc/Ds increases from (0.05-0.2) for liquids of different types near their crystallization point up to (0.3-0.5) at T / Tc ∼ 0.8, where Tc is the critical temperature. This shows that collective effects play a very important role in the transport phenomena in liquids. The similarity and the difference of the self-diffusion and shear viscosity in water and simple liquids are carefully discussed. It is shown that the crystal-like representations of thermal motion in water are applicable only up to the temperature TH ∈ (315 ÷ 320)K, which can be interpreted as the temperature of dynamic phase transition.