Thermodynamic properties, electrochemical properties, and interaction behaviors of quaternary ammonium salts-based deep eutectic solvents

To gain a deeper understanding of the formation mechanism for quaternary ammonium salts-based deep eutectic solvents (DESs), it is of great significance to investigate the interaction between hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA), as well as their physicochemical properties. In this study, tetrapropylammonium bromide (TPAB) or tetrabutylammonium bromide (TBAB) as HBA and malonic acid (MA) as HBD with a molar ratio of 1:1 or 1:1.5 were to synthesize four DESs of [TPAB]:[MA]-1, [TPAB]:[MA]-1.5, [TBAB]:[MA]-1 and [TBAB]:[MA]-1.5. The thermodynamic properties, electrochemical properties, and interaction behaviors of these DESs were investigated. The thermodynamic parameters of the standard entropy, lattice energy, molar refraction, molar polarization, and molar electrical conductivity for all DESs were calculated in terms of the Glasser's classical theory, the molar surface Gibbs energy definition, the molar electrical conductivity definition, and Lorentz-Lorenz equation. Through linear scanning curve tests based on the three-electrode method, electrochemical potential windows (EPW) of all DESs were significantly higher than 2 V. Furthermore, according to DFT calculation and COSMO-RS theoretical model, the sequence of interaction strength for quaternary ammonium salts-based DESs was arranged as follows: [TBAB]:[MA]-1 > [TPAB]:[MA]-1 > [TBAB]:[MA]-1.5 > [TPAB]: [MA]-1.5. These fundamental theoretical studies help to provide significant theoretical reference value for the design of DESs with special properties, and promote the application range of DESs.