## Diffusion in condensed media

## General

Thermodynamics of multicomponent mass transfer in liquids is described by nonlinear differential equations. Various modifications of the Fick laws are the basis of macroscopic theory of interdiffusion , also there are many theories that treat diffusion as a linear process. However, the feature of multicomponent solutions is that alongside the usual streams the additional flows associated with the mutual diffusion of different components arise. In the presence of interaction between particles such flows are nonlinear, as transport of particle of some sort is accompanied by transfer of particles of other varieties. It is also believed that the diffusion coefficient is a constant, however, numerous experimental data show that Fick diffusion coefficient strongly depends on the concentration of diffusing substances.

The theory of liquids is still underdeveloped area compared to solids, gases, plasma. There is no model that takes into account the simplest break of bonds, describes method of formation of clusters, and so on. Maybe that's why that there is no generally convenient theory that would explain the concentration dependence of the diffusion coefficient for liquids, and this is a significant gap in modern physics.

## History

From 90s, V.V. Obukhovsky has developed an alternative approach to describe diffusion in liquid solutions in which the diffusion is described by a new non-linear law, and material parameters of liquid are described by interdiffusion coefficients of the components. Methods of determining the numerical values of these coefficients by experimental curves are practically absent. So, there is a task to learn to find a matrix of diffusion for specific solutions. Comparison of the developed theory with experiment may provide new information on the models of liquid solutions.

During our investigation, the diffusion in multicomponent liquid systems is studied subject to course of quasichemical reactions using non-linear diffusion law obtained by V.V. Obukhovsky within the model of "exchange of positions" in liquid systems. For simple molecular mixtures the method of finding both the diffusion matrix and percentage of components of the solution by the experimental data of nonmonotonous dependence of diffusion coefficients is developed. Processing of experimental data for simple molecular mixtures is carried out. Analyzing the chemical reactions in the mixture, both influence of complexation processes on the thermodynamic properties of mixtures and dependence of viscosity and refractive index of solution on concentration are explained.