COMPARISON OF TEMPERATURE DEPENDENCES OF ACTIVATION ENERGIES FOR FLOW AND DIFFUSION IN PURE WATER

Abstract

The problems of calculations and the nature of intermolecular interactions forces in liquids, as their activation energies, in a wide range of temperatures of the existence of the liquid phase are considered by example of pure water. The main methodological problem in standard calculations of the activation energy is the lack of conservation of the number of molecules in the system where temperature and pressure changes. The problem is solved by writing down the Arrhenius correlation and calculating the activation energy through the kinematic viscosity, for which the number of molecules in the system is conserved. When calculating the activation energy for the diffusion process, an alternative calculation method is proposed. It’s based on the viscosity of a given liquid, which is determined much more accurately than diffusion. The calculations of the activation energies in pure water are carried out on the saturation line for the processes of flow and diffusion in the region of existence of its liquid phase, from the melting point till the critical temperature. Approximating formulas containing power and exponential contributions are obtained for them. Good correlation of the approximations with the calculations for temperatures from 0 ° С to 230 ° С is observed. It is shown that the difference between the activation energies of the processes is due to the differences in their physical mechanisms. The activation energy for the diffusion process is larger and decreases more slowly with heating than for the flow process, since all the bonds of molecules for diffusion are three-dimensional and isotropic in space. The flow process is caused by external forces that stimulate the rupture of intermolecular bonds between the layers of the fluid flow, and these interactions become quasi-two-dimensional. As a result, the activation energy for the diffusion process decreases to the critical temperature more slowly, like a root of the second degree, while for a flow it decreases linearly. For both activation energies, exponential contributions are observed near the melting point (0 ... 90 ° C). They are caused by the destruction of the remains of the crystal structure of ice in water in the form of water clusters. In the liquid phase of water, stronger and more stable in time intermolecular bonds of molecules, observed in closed hexagonal rings of water structure, which exponentially decay with increasing temperature, are retained for them.