Research on the strengths of electrostatic and van der Waals interactions in ionic liquids

The special properties of ionic liquids are due to the simultaneous presence of Coulombic and van der Waals forces. Evaluation of their interaction strengths is a basic work for explaining the macroscopic physical properties of ionic liquids, understanding the molecular dynamics and microstructure of ionic liquids, and assessing the binding force induced by ions in biological systems. In this paper, a modified Coulomb's law was constructed to calculate the average electrostatic force in ionic liquids through treating an ionic liquid molecule as two charges separated by some organic groups and taking these organic groups as an organic molecule. The modified Coulomb's law was verified with the surface tension data of 117 non-ionic liquids and 153 ionic liquids. According to the modified Coulomb's law, the average strengths of the electrostatic and van der Waals interactions between a pair of cation-anion in the 153 ionic liquids were almost equal, which were about 20 pN.