Effects of alkyl chain length and halide anion on hydrogen bonding, electrochemical transport properties and double layer capacitance in eutectic solvents

Eutectic solvents (ESs) have been widely studied due to their tunable solvation and physical properties. The properties of eutectic solvents that are formed from mixtures of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) are governed by a complex hydrogen (H) bonded network among their constituents. However, an understanding of the key factors effecting the H-bonding structure and dynamics, and the resulting physical and electrochemical properties at interfaces is still in its infancy. In this study, the influence of the cation's alkyl chain length and the anion chemical identity on the characteristics of the H-bonding network in eutectic solvents were examined. In particular, ESs formed from mixtures of tetraalkylammonium salts as HBAs and ethylene glycol as the HBD were studied. Complementary spectroscopic studies revealed the changes in the solvation environment as a function of the ammonium chain length (C1, C2, C3, and C4) and the anion type (Cl−, Br−, I−). With an increase in the alkyl chain length or a decrease in the electronegativity of halide anions (X−), it was found that the O–H bond length shortens and the H···X− bond elongates in the H-bond (O–H···X−) structure, thus resulting in an increase of the solvated ion size and a decrease in the bulk ionic conductivity. Further, double layer capacitance measurements showed that increased capacitance was correlated with shorter H-bonds in anion solvates.