Mutual and Thermal Diffusivities in Mixtures of Cyclohexane, n-Hexadecane, n-Octacosane, or n-Hexanoic Acid with Carbon Dioxide Obtained by Dynamic Light Scattering and Molecular Dynamics Simulations

Abstract

The present work reports Fick diffusion coefficients D 11 in the saturated liquid phases of binary mixtures of cyclohexane, n-hexadecane, noctacosane, or n-hexanoic acid and dissolved carbon dioxide (CO 2 ) as a function of composition. D 11 is experimentally determined by dynamic light scattering (DLS) and predicted by equilibrium molecular dynamics (EMD) simulations within or close to saturation conditions at temperatures between 303 and 423 K and CO 2 mole fractions x CO2 up to 0.99. Thermal diffusivity data obtained simultaneously by DLS are reported as well. The mixture composition of the liquid phases is experimentally determined by polarization-difference Raman spectroscopy (PDRS). The combination of PDRS experiments and EMD simulations, where the latter allows a direct insight into or the detection of changes of the fluid structure, is used to investigate the influence of the phase behavior and the resulting fluid structure on D 11 . The behavior of the experimentally determined D 11 and that obtained from EMD simulations by the independent calculation of the Maxwell−Stefan diffusivity and the thermodynamic factor show generally good agreement. Distinct structural changes are reflected by the thermodynamic factors, which are between about 0.35 and 1.4. D 11 as a function of x CO2 at a given temperature is nearly constant in mixtures of n-hexanoic acid and CO 2 due to the opposing kinetic and thermodynamic contributions to D 11 . Pronounced composition-dependent behaviors are observed in the other mixtures due to the proximity to a liquid−liquid miscibility gap or the mixtures' critical line.

Diffusivities in Binary Mixtures of Cyclohexane or Ethyl Butanoate with Dissolved CH4 or R143a Close to Infinite Dilution

“…Furthermore, the AARD of 7.7% of D self,1 from EMD simulations from the T-dependent Arrhenius-type fits of the experimental DLS data for the C 6 H 12 -based systems shows that the FF modification for C 6 H 12 proposed in our previous work is transferable. 38 In agreement with previous experimental investigations of binary mixtures consisting of a liquid and a dissolved gas close to infinite dilution, a generally decreases with increasing T within uncertainties and no distinct effect of p or the amount or type of the dissolved gas on a was found for the systems studied in this work. Until now, for most of the solvent−solute combinations investigated in previous studies, systems composed of molecules exhibiting larger molar mass showed a smaller D 11 value at a given T. In contrast, D 11 The semiempirical prediction model of Wu et al, 9 which was developed in connection with n-alkane-and 1-alcoholbased mixtures with dissolved gases at the beginning of this project, can predict D 11 in mixtures based on C 6 H 12 well even if a refrigerant like R143a is dissolved.…”

“…Except for the eth-C 6 H 12 O 2 + R143a system, which is constantly underpredicted, the EMD simulations are able to reproduce the experimental data in a purely predictive manner. Furthermore, the AARD of 7.7% of D self,1 from EMD simulations from the T -dependent Arrhenius-type fits of the experimental DLS data for the C 6 H 12 -based systems shows that the FF modification for C 6 H 12 proposed in our previous work is transferable …”

“…The inter- and intramolecular interactions within the fluid ensemble are described with all-atom FFs based on the L-optimized potentials for liquid simulations (OPLS)-T FF, which is an extended version of the all-atom OPLS FF , for long-chain hydrocarbons (L-) , with the application of a T -dependent modification proposed by Klein et al (-T). In this work, the FFs for the four substances eth-C 6 H 12 O 2 , C 6 H 12 , CH 4 , and R143a are identical to those used in refs , , . In our last publication examining the composition dependence of D 11 in binary mixtures of an acid or linear and cyclic alkanes with dissolved CO 2 , the FF for C 6 H 12 was slightly modified to improve the predictive quality of the dynamic properties.…”

“…In this work, the FFs for the four substances eth-C 6 H 12 O 2 , C 6 H 12 , CH 4 , and R143a are identical to those used in refs , , . In our last publication examining the composition dependence of D 11 in binary mixtures of an acid or linear and cyclic alkanes with dissolved CO 2 , the FF for C 6 H 12 was slightly modified to improve the predictive quality of the dynamic properties. There, the σ i parameter of the Lennard-Jones (LJ) potential for both the C and H atoms was decreased by 2.0% relative to the original OPLS value.…”

Solubility and Liquid Density of Binary Mixtures of n-Hexane or 1-Hexanol with Krypton, Sulfur Hexafluoride, or R143a

“…The present study is connected with investigations on how the characteristics of the involved molecules affect diffusion in the saturated liquid phase(s) of binary systems consisting of gases dissolved in liquids in vapor–liquid or vapor–liquid–liquid equilibrium (VLE or VLLE). In this context, binary systems consisting of carbon dioxide (CO 2 ) combined with n -hexane (C 6 H 14 ), cyclohexane, n -hexadecane, n -octacosane, n -hexanoic acid, or 1-hexanol (C 6 H 14 O) over a wide concentration range were investigated using dynamic light scattering (DLS) combined with Raman spectroscopy (RS) and equilibrium molecular dynamics simulations. These studies allowed to examine the impact of varying alkyl chain length as well as of different functional groups for hydrocarbons with the same number of carbon atoms on the phase behavior and Fick diffusion coefficient D 11 .…”