Mutual and thermal diffusivity of binary mixtures of the ionic liquids [BMIM][C(CN)3] and [BMIM][B(CN)4] with dissolved CO2 by dynamic light scattering.

Ionic liquids (ILs) are promising solvents for gas separation processes such as carbon dioxide (CO2) capture from flue gases. For the design of corresponding processes and apparatus, thermophysical properties of ILs containing dissolved gases are required. In the present study, it is demonstrated that with a single optical setup, mutual and thermal diffusivities as well as refractive indices can be measured quasi-simultaneously for such mixtures. Dynamic light scattering (DLS) from bulk fluids was applied to determine mutual and thermal diffusivities for mixtures of 1-butyl-3-methylimidazolium tricyanomethanide ([BMIM][C(CN)3]) or 1-butyl-3-methylimidazolium tetracyanoborate ([BMIM][B(CN)4]) with dissolved CO2 at temperatures from 303.15 to 333.15 K and pressures between 2 and 26 bar in macroscopic thermodynamic equilibrium. Good agreement with literature data and only slight differences between the diffusivities measured for the two systems at the same temperature and comparable mole fractions of CO2 were found. Increasing mutual diffusivities with increasing mole fractions of CO2 are consistent with decreasing viscosities reported for other IL-CO2 mixtures in the literature and can be attributed to weakening of molecular interactions by the dissolved gas. For the conditions studied, no dependence of the thermal diffusivity on the temperature or the mole fraction of CO2 could be found.