A Phenomenological Theory of the Soret Diffusion

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Abstract
The classical Soret diffusion problem is solved analytically for the case of unrestricted composition in a binary liquid system, taking account of the temperature‐variation of the density of the mixture. Previous treatments have neglected the phenomenon of thermal dilatation entirely and have not developed a unique general solution which applies both to dilute and to nondilute mixtures. The rigorous solution derived in the present work is similar in form to de Groot's well‐known equation for dilute mixtures, but contains additional parameters characterizing the initial composition of the system and its coefficient of thermal expansion. These parameters disappear in the asymptotic approximation for a vanishing temperature gradient, but this approximation differs from that proposed by de Groot, even for dilute solutions. The asymptotic expression has practical importance for estimating the Soret coefficient and the ordinary diffusion coefficient of a system from experimental data taken during the thermodiffusional unmixing period; some examples of its application in this connection are discussed.