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

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 (D₁₁). The investigations were performed within or close to saturation conditions at temperatures between (298.15 and 348.15) K and CO₂ mole fractions (xCO₂) 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, D₁₁ 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 D₁₁ from DLS and EMD simulations for CO₂ mole fractions up to 0.5. In general, an increasing D₁₁ with increasing xCO₂ 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 xCO₂ over the entire studied composition range and a decreasing thermodynamic factor at xCO₂ above 0.7. Finally, PDRS and EMD simulations were combined to investigate the influence of the fluid structure on the diffusive process.