A Viscosity Model for Deep Eutectic Solvents Based on the Friction Theory

Abstract

A previously developed six-parameter viscosity model based on the friction theory (FT) in combination with a cubic equation of state (CEoS), and applied to pure ionic liquids, was modified here to represent the dynamic viscosity of deep eutectic solvents (DES). Upon analyzing the different viscous contributions of the aforementioned model to the total viscosity, we found out that, in the case of DES, the second-order repulsive term was the most dominant dragging force contribution within the FT framework; it is actually the main responsible in capturing, at least qualitatively, the correct variation of viscosity with both temperature and pressure exhibited by a DES. This finding allowed us to reasonably reduce the number of adjustable parameters of the model without sacrificing model accuracy. Thus, by preserving the same form of second-order repulsive term, simplifying the first-order attractive term and neglecting the first-order repulsive term, the new model now contains three parameters. As earlier mentioned, the use of a simple CEoS (Soave-Redlich-Kwong or Peng-Robinson) served to calculate the first-order attractive and second-order repulsive pressures. Interestingly, we were also able to confirm the predictive capabilities of the present 3-parameter viscosity model by applying the model to estimate high-pressure DES viscosity data using the same model parameters previously obtained only using viscosity data at atmospheric pressure, thus, developing a model capable of representing the viscosity and density variations of DES with respect to different ?? and ?? range.