Study of the Thermal Diffusion Behavior of Alkane/Benzene Mixtures by Thermal Diffusion Forced Rayleigh Scattering Experiments and Lattice Model Calculations

Abstract

In this work the thermal diffusion behavior of binary mixtures of linear alkanes (heptane, nonane, undecane, tridecane, pentadecane, heptadecane) in benzene has been investigated by thermal diffusion forced Rayleigh scattering (TDFRS) for a range of concentrations and temperatures. The Soret coefficient S T of the alkane was found to be negative for these n-alkane/benzene mixtures indicating that the alkanes are enriched in the warmer regions of the liquid mixtures. For the compositions investigated in this work, the magnitude of the Soret coefficient decreases with increasing chain length and increasing alkane content of the mixtures. The temperature dependence of the Soret coefficient depends on mixture composition and alkane chain length; the slope of S T versus temperature changes from positive to negative with increasing chain length at intermediate compositions. To study the influence of molecular architecture on the Soret effect, mixtures of branched alkanes (2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane and 2,2,4-trimethylpentane) in benzene were also investigated. Our results for the Soret coefficients show that the tendency for the alkanes to move to the warmer regions of the fluid decreases with increasing degree of branching. The branching effect is so strong that for 2,2,4-trimethylpentane/benzene mixtures the Soret coefficient changes sign at high alkane content and that equimolar 2,2,3-trimethylbutane/benzene mixtures have positive Soret coefficients in the investigated temperature range. In order to investigate the effect of molecular interactions on thermal diffusion, we adapted a recently developed two-chamber lattice model to n-alkane/benzene mixtures. The model includes the effects of chain-length, compressibility, and orientation dependence of benzene-benzene interactions and yields good qualitative predictions for the Soret effect in n-alkane/benzene mixtures. For the branched isomers, we find some correlations between the moments of inertia of the molecules and the Soret coefficients.

Temperature Dependence of Soret Coefficient in Aqueous and Nonaqueous Solutions of Pullulan

“…44,52 Systematic experiments for low molecular mixtures show that the Soret coefficient is proportional to the temperature. 33 Table II. The numbers refer to the systems which are mentioned in Table II …”

“…[20][21][22][23][24][25] It has been revealed that a sign of S T of the polymer in solution can occur as function of the solution temperature as well as of the solvent composition. [26][27][28][29][30] For liquid mixtures composed of small molecules thermally induced sign change of S T have been studied by several researchers [31][32][33][34] , sometime those sign changes could be correlated with structural changes in the liquid mixtures. For polymer solutions and colloidal suspensions sometimes sign changes occur und bad solvent conditions.…”

Reverse Nonequilibrium Molecular Dynamics Calculation of the Soret Coefficient in Liquid Heptane/Benzene Mixtures

“…(9) are given by δM = M 1 − M 2 and δI = I 1 − I 2 . In our previous work 14 we have calculated the moments of inertia about the symmetry axis (I zz for the disk-like benzene molecules, I xx for the n-heptane and five heptane isomers, including 3-MH, 2,3-DMP and 2,4-DMP) using an atomistic model for single molecules in vacuum 29 . It was found that the Soret coefficient correlates almost increasing linearly with the moment of inertia I xx in the direction of the main chain.…”

Study of the Soret Effect in Hydrocarbon Chain/Aromatic Compound Mixtures

“…Another large group of substance studied are the hydrocarbons which have been investigated experimentally and theoretically [5][6][7][8][9][10][11] . However, for alkane/benzene mixtures 8 the simple rule of thumb fails.…”