On the similarity of the self-diffusion and shear viscosity coefficients in low-molecular liquids

Abstract

We discuss the similarity of the self-diffusion and kinematic shear viscosity coefficients for low-molecular liquids. The rotational motion of molecules is the main prerequisite of this similarity. Due to it, the interparticle potentials are selves-averaged and they become argon-like. The applicability of the van der Waals equation of state for low-molecular liquids is one of the important consequences of this fact. Taking into account that as rule the period of rotational motion is essentially smaller in comparison with the characteristic time needed for a change of molecular configuration formed by the nearest neighbors, we conclude that the similarity should also take place for mentioned kinetic coefficients. We illustrate the manifestation of the argon-like similarity for atomic liquids of argon type, dumbbell a disk-like liquid of nitrogen and benzene types, water, and other associated liquids. At that, liquid states of associated liquids are divided into two regions: one of which adjoins to the critical point and corresponds to argon-like translational thermal motion, and the second includes the triple point and the rotational motion of molecules has activation character in it. In this region, the similarity relations also take place although they have another character in comparison with that for argon-like liquids. The inapplicability of standard activation representations for low-molecular liquids is in details discussed. The careful comparison with experimental data is carried out.