Experimental Measurement and Theoretical Modeling of the Viscosity of an Ionic Liquid Saturated with CO2

Under ambient or near-ambient conditions, ionic liquids have shown potential in oral and biomedical applications, where their viscosity plays a key role in processability, diffusion, penetration, and transport behavior. Beyond these ambient-condition applications, gas-loaded ionic liquids are also relevant to gas absorption, separation, and other CO2-related processes, in which dissolved gases can significantly alter the viscosity and transport behavior of ionic liquids. Therefore, systematic viscosity data for CO2-containing ionic-liquid mixtures over broad temperature, pressure, and composition ranges are needed to understand their transport behavior and develop predictive models. Accordingly, in this work, the viscosities of CO2-saturated [Hmim][OTf] and [Hmim][Tf2N] were measured from 303.15 to 343.15 K and at pressures up to 7.5 MPa using a high-pressure capillary viscometry system. The standard uncertainties were u(T) = 0.023 K and u(p) = 0.0025 MPa, while the expanded relative uncertainty of viscosity was Ur(η) < 0.05 at an approximately 0.95 coverage probability (k = 2). The experimental results show that the viscosity decreases markedly with increasing CO2 loading, and under low-temperature conditions, the viscosity reduction exceeds 50% when the CO2 mole fraction is around 0.3. The Soft-SAFT equation of state combined with entropy scaling was then employed to correlate the viscosity behavior of the two systems. The results show that the model satisfactorily correlates the effects of CO2 loading and temperature on mixture viscosity.