Dynamic Viscosity of Tetracyanoborate- and Tricyanomethanide-Based Ionic Liquids by Dynamic Light Scattering

Low-viscosity ionic liquids (ILs) based on the anions [B­(CN)4]− (tetracyanoborate) and [C­(CN)3]− (tricyanomethanide) are currently discussed as alternative working fluids for various applications. The dynamic viscosity of such ILs carrying [1-alkyl-MIM]+ (1-alkyl-3-methylimidazolium) cations was investigated via determination of the translational particle diffusion coefficient by dynamic light scattering (DLS). The long-term stability of suspensions of different particles in the ILs, their particle diameters, and the influence of laser power on the measured particle diffusion coefficient for semitransparent ILs were studied. For temperatures from 283 to 353 K at atmospheric pressure, the dynamic viscosity of the four pure ILs forming stable suspensions was obtained with an uncertainty of less than 5% (k = 2) and agrees with literature. Absorption of CO2 at pressures up to 0.87 MPa induced a distinct decrease in the dynamic viscosity. Differences between the viscosities of the different systems can be explained by varying strength of molecular interactions.