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

Abstract

This paper is devoted to collective effects in the transport processes in liquids. Special attention is paid to the collective contributions D c to self-diffusion coefficients D s of liquids, especially for normal and supercooled water. The theoretical estimates for D c 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 D c are in good agreement. The ratio D c/ D s increases from (0.05–0.2) for liquids of different types near their crystallization point up to (0.3–0.5) at T / T c ∼ 0.8, where T c 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 T H ∈ (315 ÷ 320) K, which can be interpreted as the temperature of dynamic phase transition.