Ion Correlation in Choline Chloride-Urea Deep Eutectic Solvent (Reline) from Polarizable Molecular Dynamics Simulations.

Abstract

In recent years, deep eutectic solvents (DESs) emerged as highly tunable and environmentally friendly alternatives to common ionic liquids and organic solvents. In this work, a polarizable model based on the CHARMM Drude polarizable force field is developed for a 1:2 ratio mixture of choline chloride/urea (reline) DES. To successfully reproduce the structure of the liquid as compared to first-principles molecular dynamics simulations, a damping factor was introduced to correct the observed over-binding between the chloride and the hydrogen bonding site of choline. Investigated radial distributions reveal the formation of hydrogen bonds between all the constituents of reline and similar interactions for chloride and urea's oxygen atoms, which could contribute to the melting point depression of the mixture. Predicted dynamic properties from our polarizable force field are in good agreement with experiments, showing significant improvements over nonpolarizable models. Similar to some ionic liquids, an oscillatory behavior in the velocity autocorrelation function of the anion is visible, which can be interpreted as a rattling motion of the lighter anion surrounded by the heavier cations. The obtained results for ionic conductivity of reline show some degree of correlated ion motion in this DES. However, a joint diffusion of ion pairs cannot be observed during the simulations.