Modeling Viscosity of Ionic Liquids with Electrolyte Perturbed-Chain Statistical Associating Fluid Theory and Free Volume Theory

Viscosity is one of the most important physical properties when developing ionic liquids (ILs) for industrial applications such as CO₂ separation. The viscosities of ILs have been measured experimentally, while the modeling work is still limited. In this work, the electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) was combined with the free volume theory (FVT) to model the viscosities of pure ILs and IL mixtures up to high pressures and temperatures, in which the ePC-SAFT was used to calculate the density as inputs for modeling the viscosity of ILs with FVT. The ILs under consideration contain one of the IL cations [C_nmim]⁺, [C_npy]⁺, [C_nmpy]⁺, [C_nmpyr]⁺, or [THTDP]⁺ and one of the IL anions [Tf₂N]^-, [PF₆]^-, [BF₄]^-, [tfo]^-, [DCA]^-, [SCN]^-, [C₁SO₄]^-, [C₂SO₄]^-, [eFAP]^-, Cl^-, [Ac]^-, or Br^-. In total, 89 ILs were considered combined with a thorough literature survey of the available experimental viscosity data and evaluation. The comparison with the available experimental viscosities shows that the model can provide reliable representation and prediction for most of the pure ILs in a wide temperature and pressure range, and it can be further used to predict and describe the viscosity of IL mixtures reliably.