(181n) Deep Eutectic Solvents: An Experimental and Molecular Simulation Study

Ionic liquids (ILs) have received prominent interest due to their low melting points, negligible vapor pressure, non-flammability, and unusual solvent properties. Because of these interesting properties, ILs are being used for different applications such as separations, fuel cells, lubricants, novel solvents, and for other materials engineering applications.  Deep eutectic solvents (DES), considered ILs analogues, show promise for many engineering applications, since many of them are readily available and inexpensive compared to some of the typical ILs¹.

In this work, a combined experimental and molecular simulations approach is used to characterize hydrogen bonding and ionic interactions in order to understand the interactions of ILs with mineral surfaces and their role in hydrocarbon/sand separation processes. Specific attention is focused on choline chloride-based eutectic mixtures, i.e ions in choline chloride and a complex agent (i.e. urea, malonic acid, etc) because they are relatively inexpensive and environmentally benign.

Experimental IR spectra are reported for choline chloride-urea mixtures at different molar ratios and for the pure components. Ab-initio and atomistic molecular dynamics simulations has also been performed on these systems.  The geometries of the systems studied were first optimized by using gas phase ab-initio calculations in Gaussian for individual ions and molecules. Atomistic simulations were performed for 1 to 2 choline chloride to urea mixture ratios using the General Amber Force Field, GAFF, functional form. Atomic partial charges obtained from the ab-initio simulations where transferred to the force field. Comparison of the experimental and simulated IR spectra of the choline chloride/urea system in a 1:2 ratio, showed interesting interactions around the NH₂coupling vibrations that are seen by the reduction of three peaks to two peaks when relating solid urea to the complex mixture.  Finally, a hydrogen bond analysis between the moieties shows that a complex structure forms due to the presence of stable hydrogen bonds. The influence of these hydrogen bonds on stability is discussed, as well as their influence on the phase behavior of this deep-eutectic IL system.

1. Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R.K. & Tambyrajah, V. Chem Commun. 2003, 70-71