The isotope Soret effect in molecular liquids: a quantum effect at room temperatures

Abstract

Diffusion in multicomponent mixtures is not only driven by concentration but also by temperature gradients. Although this so-called Soret effect plays an important role in such diverse phenomena as the composition variation in oil reservoirs or prebiotic evolution, the underlying molecular mechanisms are still not clear. Here we show that the thermodynamic part of the isotopic contribution to the Soret effect of binary liquids is a quantum effect originating at room temperature from librational and vibrational motions of the molecules. Remarkably, the ‘isotopic’ contribution is not limited to true isotopic substitution but can be the dominating effect in mixtures of chemically similar molecules. We have tested the theory for the binary mixtures from the homologous series of halobenzenes and for halobenzenes in toluene and cyclohexane.