Low melting point deep eutectic solvents (DES), can be formed from quaternary ammonium salts with a variety of neutral species.  Eutectic mixtures of such salts with metal halides are well established, Figure 1a. DES employing an ionic liquid component and hydrogen-bond donor, such as CholineChloride/Urea (ChCl/Urea), Figure 1b, are gaining interest as electrolytes, particularly for clean metal extraction and deposition.  DESs can reduce complex processes, avoid hydrogen evolution and embrittlement, deposit reactive metals and metal-alloys unobtainable from aqueous solutions, provide access to novel coating morphologies and enhanced surface properties (increased hardness, reduced roughness), and may offer advantages for the production of nanoporous and nanostructured metals.

DES have properties that are particularly attractive for  large scale technological applications.  DES  are cheap, non-toxic, potentially biodegradable, they have a good electrochemical window, good thermal stability and reasonable electrical conductivity.  DES can dissolve recalcitrant species such as metal oxides and reduce operating temperatures due to the low melting point.  Additional  advantages of the DES include muti-ton production and a high stability with respect to air and moisture.

For example, direct comparison of DESs with standard aqueous solution processes have shown Ni deposition occurs at comparable rates for the same concentration and temperature conditions.  The increased viscosity and reduced conductivity of ChCl/Urea are not rate limiting factors and it has been suggested that speciation within the electrolyte plays the more significant role.¹

            Typically the melting point depression of DES is linked to the formation of a larger more diffuse covalent complexed anion, which has a reduced interaction with associating cations, Figure 1.  However, when the neutral component is an organic species capable of acting as a H-bond donor there are uncertainties with respect to the speciation and bonding within the mixture.  We have employed electronic structure methods to examine the interactions between constituent species of ChCl/Urea.  Within cluster models a subtle interplay of electrostatics and different types of H-bonding are observed and elucidated.  Links can be made between these molecular level interactions and the physico-chemical properties of ChCl/Urea DES.

¹Abbott, A. P., Ballantyne, A., Harris, R. C., Juma, J. A., Ryder, K. S., & Forrest, G., Electrochimica Acta, 2015, 176, 718–726. http://doi.org/10.1016/j.electacta.2015.07.051