Surface thermodynamic properties of ionic liquids from new molecular thermodynamic model and ion-contribution equation of state

Abstract

We aim to develop a new molecular thermodynamic model for predicting the surface thermodynamic properties of pure ionic liquids (ILs) based on the statistical mechanical expression developed by Winterfeld et al. (1978. AIChE. J. 24, 1010). In this respect, contributions to surface tension from the hard-sphere repulsion, Lennard–Jones dispersion force, and electrostatic interactions were considered and assumed to be additive in the development of the model. According to Winterfeld et al. approach, the surface tension of ILs is closely related to their pair potential function, pair radial distribution function and liquid densities. The required densities were predicted from our previous ion-contribution equation of state. The reliability of the proposed model was checked by calculating 464 surface tension data points for 13 different ILs over temperature range within 268.6–393K. The overall average absolute deviation of the correlated surface tensions from the experimental ones was found to be 1.55%. This result demonstrates the good performance of the proposed model and the rationality of molecular parameters, i.e., the soft-sphere diameter, σ dispersive energy, ε and inverse shielding length parameter, Γ. The uncertainty of the predicted surface tension was of the order of ±8.14%. Finally, our method has also been employed to estimate the sound velocities in ILs. The uncertainty of the calculated sound velocities was equal to ±9.25%.