Soret effect in molecular mixtures

A theoretical approach to the description of the Soret effect in binary nonpolar liquids is proposed. The temperature gradient of the partial pressure is determined as the driving force of thermal diffusion. The hard-sphere fluid is chosen as a reference system and an explicit relation for the Soret coefficient is found. Two additive contributions owing to steric repulsions and attractive interactions form the so-called chemical contribution to $S_T$ [C. Debuschewitz and W. Köhler, Phys. Rev. Lett. 87, 055901 (2001)]. The parameters of interparticle interactions are defined with the help of the solvation theory. In particular, the van der Waals constant of cross interactions is expressed via the excess volume of mixture. The proposed theory is applied to the benzene-cyclohexane system. A reasonable agreement of theoretical and experimental results is revealed for the Soret coefficient and its temperature dependence.

Figure 1

(Color online) The hard-sphere diameter $d$ and the van der Waals constant $a$ of cyclohexane as a function of temperature. The values of $d$ are given in angstroms.

Figure 2

Thermal diffusion factor $ST$ as a function of diameter ratio for equimolar mixture of hard spheres for $\eta =0.209$. Diamonds are the data from Ref. [47]. The line is a result of linear fitting of $1/S$ upon extreme points $1/S^{\text{HS}}\left(x=0\right)$ and $1/S^{\text{HS}}\left(x=1\right)$.

Figure 3

(Color online) The chemical contribution to the Soret coefficient of benzene/cyclohexane at $T=25°\text{C}$. Line 1 refers to the data from Ref. [58]. Diamonds are the result of present work. Line 2 is a polynomial fit to the result.

Figure 4

(Color online) The chemical contribution to the Soret coefficient of benzene/cyclohexane mixture at different temperatures relative to its value at $T=25°\text{C}$. Curves are linear fit upon extreme points $x=0$ and $x=1$.