Physicochemical properties of deep eutectic solvents formed by choline chloride and phenolic compounds at TÂ =Â (293.15 to 333.15)Â K: The influence of electronic effect of substitution group

Three deep eutectic solvents (DESs) were synthesized using choline chloride (ChCl) and phenolic compounds (phenol, p-cresol, and p-chlorophenol) as hydrogen bond donors (HBD) with a molar ratio of 1:2 (ChCl:HBD). Densities (ρ), viscosities (η), and refractive indices (nD) of DESs were measured at T = (293.15 to 333.15) K under atmospheric pressure. Temperature dependence of ρ and η were fitted by empirical equations, and thermal expansion coefficient (αp) and activation energy for viscous flow (Eη) were obtained accordingly. FT-IR spectra of DESs were determined and compared with the corresponding HBDs. Based on physicochemical properties and FT-IR spectra, the influence of electronic effect of substitution group of HBD on hydrogen bond interaction between ChCl and HBD was explored. The results show that the electronic effect of HBD plays an important role on the strength of hydrogen bond interaction of OH ⋯ Cl− of DES. The strong electron-withdrawing property of Cl group of p-chlorophenol can strengthen the hydrogen bond interaction of OH ⋯ Cl−, which leads to higher ρ, η, and nD of DES formed by ChCl and p-chlorophenol. Conversely, CH3 group of p-cresol exhibits electron-donating effect, resulting in weaker hydrogen bond interaction and lower ρ and nD of DES formed by ChCl and p-cresol. Consequently, the strength of hydrogen bond interaction of OH ⋯ Cl− between ChCl and HBD of DES is as follows: p-chlorophenol > phenol > p-cresol.