Fick Diffusion Coefficient in Binary Mixtures of [HMIM][NTf₂] and Carbon Dioxide by Dynamic Light Scattering and Molecular Dynamics Simulations

Abstract

Dynamic light scattering (DLS) experiments and equilibrium molecular dynamics (EMD) simulations were performed in the saturated liquid phase of the binary mixture of 1-hexyl-3-methylimidazolium bis­(trifluormethylsulfonyl)­imide ([HMIM]­[NTf2]) and carbon dioxide (CO2) to access the Fick diffusion coefficient (D 11). The investigations were performed within or close to saturation conditions at temperatures between (298.15 and 348.15) K and CO2 mole fractions (x CO2) up to 0.81. The DLS experiments were combined with polarization-difference Raman spectroscopy (PDRS) to simultaneously access the composition of the liquid phase. For the first time in an electrolyte-based system, D 11 was directly calculated from EMD simulations by accessing the Maxwell–Stefan (MS) diffusion coefficient and the thermodynamic factor. Agreement within combined uncertainties was found between D 11 from DLS and EMD simulations for CO2 mole fractions up to 0.5. In general, an increasing D 11 with increasing x CO2 could be observed, with a local maximum present at a CO2 mole fraction of about 0.75. The local maximum could be explained by an increasing MS diffusion coefficient with increasing x CO2 over the entire studied composition range and a decreasing thermodynamic factor at x CO2 above 0.7. Finally, PDRS and EMD simulations were combined to investigate the influence of the fluid structure on the diffusive process.

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

“…To limit the extent of the present manuscript, only details necessary for the discussion are given. For more information on the interpretation of the fluid structure by EMD simulations and PDRS, the reader is referred to refs , , .…”

“…In this work, all-atom FFs are used for the solute and solvent molecules. The solute CO 2 is described by the flexible version of the transferable potential for phase equilibria (TraPPE FLEX) FF, 39 which was also employed in our previous work for the determination of D 11 15,25,26 as well as the viscosity and interfacial tension. 40,41 The solvents n-C 16 H 34 , n-C 28 H 58 , and n-C 6 H 12 O 2 are modeled with the L-OPLS-T FF, which is an extended version of the all-atom optimized potentials for liquid simulations (OPLS) 42,43 FF for long-chain hydrocarbons (L-) 44,45 with a T-dependent modification proposed by Klein et al 46 (-T) applied.…”

Diffusivities in Binary Mixtures of n-Decane, n-Hexadecane, n-Octacosane, 2-Methylpentane, 2,2-Dimethylbutane, Cyclohexane, Benzene, Ethanol, 1-Decanol, Ethyl Butanoate, or n-Hexanoic Acid with Dissolved He or Kr Close to Infinite Dilution

“…The fundamentals and the application of the DLS method within thermophysical property research ,, and the use of the DLS method including the experimental realization for the accurate and simultaneous determination of a and D 11 in mixtures consisting of liquids with dissolved gases − ,− have been described in detail in the literature. Thus, only pertinent information about the DLS method and the experimental realization related to the analysis of Rayleigh scattered light is given.…”

“…All further simulation parameters, the simulation procedure, and postprocessing techniques for the calculation of D 1 and its uncertainties are identical to those provided in ref . The only exceptions and the FFs used for the solvents that were not investigated in our previous investigations − are described below.…”

“…Both methods are performed in macroscopic thermodynamic equilibrium under or near saturation conditions. Thus far, this series of investigations has included linear alkanes, primary alcohols, and ionic liquids with dissolved hydrogen, helium (He), nitrogen, carbon monoxide, or carbon dioxide (CO 2 ) close to infinite dilution and studies of n -hexane (C 6 H 14 ), 1-hexanol (C 6 H 14 O), or the ionic liquid [HMIM]­[NTf 2 ] with dissolved CO 2 over a wide composition range. The most recent work focused on investigating how solute properties may influence D 11 by expanding the solute class to methane, neon, krypton (Kr), 1,1,1-trifluoroethane (R143a), sulfur hexafluoride, or 1,1,1,3,3,3-hexafluoropropane (R236fa) dissolved in C 6 H 14 or C 6 H 14 O close to infinite dilution …”