Structural dynamics and transport in deep eutectic solvents

Non-aqueous solvents that are scalable, easy to prepare and functionalize are needed in many applications, especially energy storage. Deep eutectic solvents (DESs) present a large design space and are therefore tunable, in principle, for such targeted applications. To understand the correlation between local structure and macroscopic dynamics, detailed studies of model DESs were carried out utilizing a wide range of techniques, including classical molecular dynamics, broadband dielectric, nuclear magnetic resonance, dynamic mechanical and femtosecond transient absorption spectroscopy, and differential scanning calorimetry. The evolution of the local structure in varying compositions of hydrogen bond donor (HBD) to hydrogen bond acceptor (HBA) is studied from molecular to macroscopic length-scales. We find that the microscopic heterogeneities induced in the HBD by the addition of the HBA lead to new slow, dynamic modes that are absent in the HBD. These results provide a unified framework for rationalizing the key features of DESs including a decrease in the glass transition temperature, corresponding to an increase in dc ionic conductivity, fluidity, diffusivity, and the mean rates of orientational dynamics in the vicinity of the eutectic composition.