Thermodynamics of the transport properties: diffusion and thermodiffusion coefficients
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Abstract
Transport coefficients (like diffusion and thermodiffusion) are the key parameters to determine the rates of mixing in the multicomponent mixtures.
For practical applications, it is important to predict them based on the thermodynamic characteristics of a mixture under study: pressure, temperature, composition, and thermodynamic functions, like enthalpies or chemical potentials.
The previous studies approached this problem mainly empirically (apart from the cases of the mixtures of ideal gases), designing correlations or “mixing rules”.
The current study develops a thermodynamic framework for such a prediction. The theory is based on a system of physically interpretable postulates; in this respect, it is better grounded theoretically than the previously suggested models for the transport coefficients.
The n(n+1) /2 independent Onsager coefficients are reduced to 2n+1 determining parameters: the emission functions and the penetration lengths. The transport coefficients are expressed in terms of these parameters.
These expressions are much simplified based on the Onsager symmetry property for the phenomenological coefficients. Simple model expressions for the emission functions and penetration lengths are proposed.
The model is verified by comparison with the known expressions for the diffusion coefficients that were previously considered in the literature, and with the results of the molecular dynamics simulations.