Mutual and Thermal Diffusivities in Binary Mixtures of n‑Hexane or 1‑Hexanol with Krypton, R143a, or Sulfur Hexafluoride by Using Dynamic Light Scattering and Molecular Dynamics Simulations

This work reports Fick diffusion coefficients D 11 in the saturated liquid phase(s) of binary mixtures of nonpolar n-hexane (n-C6H14) or polar 1-hexanol (1-C6H14O) with dissolved krypton (Kr), 1,1,1-trifluoroethane (R143a), or sulfur hexafluoride (SF6). Investigations were performed at temperatures T of 303, 323, and 348 K and gas mole fractions up to 0.999. The comparison between Kr and R143a, both having nearly the same molar mass, allows identification of the impact of molecular structure and polarity of the dissolved gas on D 11. The investigation of SF6 provides information about the influence of a centrosymmetric molecule with a large molar mass on D 11. D 11 is experimentally determined by dynamic light scattering (DLS) and predicted by equilibrium molecular dynamics (EMD) simulations by the independent calculation of the Maxwell–Stefan diffusivity and the thermodynamic factor Γ 11 in macroscopic thermodynamic equilibrium at or close to saturation conditions. Thermal diffusivity data obtained simultaneously with D 11 by DLS are reported as well. The behavior of the experimentally determined D 11 as a function of composition and that obtained from EMD simulations show generally good agreement. Distinct structural changes of the saturated liquid phase are reflected by the change of Γ 11 which extends for the present study between 0.16 and 1.3. While for mixtures of 1-C6H14O + Kr, D 11 as a function of composition is nearly constant at a given T, all other mixtures show a distinct composition-dependent behavior. For mixtures of 1-C6H14O + R143a and 1-C6H14O + SF6, a strong slowing down of D 11 was observed and related to approaching a liquid–liquid miscibility gap. For all systems, the behavior of D 11 as a function of composition is interpreted with reference to the fluid structure accessible by EMD.