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

Thiswork reports Fick diffusion coefficients D (11) in the saturated liquid phase(s) of binary mixturesof nonpolar n-hexane (n-C6H14) or polar 1-hexanol (1-C6H14O) with dissolved krypton (Kr), 1,1,1-trifluoroethane (R143a), orsulfur hexafluoride (SF6). Investigations were performedat temperatures T of 303, 323, and 348 K and gasmole fractions up to 0.999. The comparison between Kr and R143a, bothhaving nearly the same molar mass, allows identification of the impactof molecular structure and polarity of the dissolved gas on D (11). The investigation of SF6 providesinformation about the influence of a centrosymmetric molecule witha large molar mass on D (11). D (11) is experimentally determined by dynamic light scattering(DLS) and predicted by equilibrium molecular dynamics (EMD) simulationsby the independent calculation of the Maxwell-Stefan diffusivityand the thermodynamic factor Gamma (11) in macroscopic thermodynamic equilibrium at or close to saturationconditions. Thermal diffusivity data obtained simultaneously with D (11) by DLS are reported as well. The behaviorof the experimentally determined D (11) asa function of composition and that obtained from EMD simulations showgenerally good agreement. Distinct structural changes of the saturatedliquid phase are reflected by the change of Gamma (11) which extends for the present study between 0.16 and1.3. While for mixtures of 1-C6H14O + Kr, D (11) as a function of composition is nearly constantat a given T, all other mixtures show a distinctcomposition-dependent behavior. For mixtures of 1-C6H14O + R143a and 1-C6H14O + SF6, a strong slowing down of D (11) was observedand related to approaching a liquid-liquid miscibility gap.For all systems, the behavior of D (11) asa function of composition is interpreted with reference to the fluidstructure accessible by EMD.