Deep eutectic solvent (DES), formed by the intermolecular hydrogen bonding between its components of an H–bond acceptor (HBA) and an H–bond donor (HBD), has been extensively studied in the area of material synthesis, catalysis, electrochemistry, etc. The properties of DES including melting point, color, density, viscosity, conductivity, etc. are closely related to the H-bond network. Establishing an understanding of the H-bond structure offers an opportunity to fine-tune these properties.

Hence, in this study, we systematically investigated the effects of halide anion type and alkyl chain length on the hydrogen bond network of a series of eutectic solvent systems and attempted to establish an unambiguous H-bond correlation with the conductivity. We varied the halide anion from Cl⁻ to I⁻ (Cl⁻, Br⁻, I⁻) and the alkyl chain length from methyl to butyl (n_c = 1, 2, 4), and performed FTIR and conductivity measurements. Results show that the OH stretching peak exhibits a blue shift with the halide anion varies in the order of Cl⁻, Br⁻, I⁻ when compared with the neat ethylene glycol (Figure 1a), indicating an increase in H-bond strength. A similar trend can be found with the increasing alkyl chain length as well. A stronger H-bond network inhibits the mobility of these halide anions in the eutectic system, as a result, the conductivity will decrease as shown in Figure 1b. Further probing the H-bond network with ultrafast spectroscopy is currently underway.

Figure 1. (a) FTIR spectrum of neat EG and TBAX:EG(1:10) sample, X= Cl⁻, Br⁻,I⁻. (b) Trend between the OH stretching peak shift (vs. neat EG) and conductivity of these investigated samples.

Acknowledgememts

This work was supported as part of the Breakthrough Electrolytes for Energy Storage (BEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019409