Understanding Structure-Property Relationships in Deep Eutectic Solvents

There is a vital need for sustainable solvents that can effectively replace conventional organic solvents which are toxic and hazardous. Deep eutectic solvents (DESs) are a mixture of a hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) which results in a depressed melting temperature significantly below the parent compounds. They can be made from cheap and renewable resources, and have advantageous properties, such as wide liquidus and electrochemical windows, nonflammibility, and nontoxicity. However, the current state of DESs has some general drawbacks including high viscosity and low ionic conductivity compared to conventional solvents which reduces their appeal for commercial use. However, there are an estimated 1019 possible DESs that have yet to be explored. In order to rationally investigate these options, a fundamental understanding of structure-property relationships should be established. Therefore, this dissertation seeks to understand how composition, local structure and dynamics, and interactions affect the macroscopic properties observed. Here, we study two DESs in unprecedented detail: Ethaline, a 33mol% choline chloride (ChCl) in ethylene glycol, and Glyceline, 33mol% ChCl to glycerol. We observed that for both DESs, ChCl weakens but does not fully disrupt the hydrogen bonded network of the neat HBD. This led to an increase in the rate of dynamics and therefore decrease in melting/glass temperature. The trends in other properties were somewhat different between the two DESs. In Glyceline, the fluidity, ionic conductivity, and dynamics were all enhanced at the eutectic point. In Ethaline, we found that the actual eutectic composition is in the 15-20mol% ChCl in ethylene glycol range. Additionally, the dynamics and ionic conductivity are enhanced in this range, but not the fluidity (previously reported).