Preparation of menthol-based hydrophobic deep eutectic solvents for the extraction of triphenylmethane dyes: quantitative properties and extraction mechanism

Abstract

A series of natural, environmentally friendly and low-cost menthol-based hydrophobic deep eutectic solvents (DES) were synthesized to extract and concentrate solutes from dilute aqueous solutions, especially triphenylmethane (TPM) dye micropollutants. The system has excellent extraction performance for TPM. Density functional theory (DFT) and molecular dynamics (MD) simulation were used to quantitatively analyze the effect of the DES composition and TPM structure on the distribution of target molecules in two phases. The solvation free energy of ethyl violet (EV) in DES (−17.128 to −21.681 kcal mol⁻¹) is much larger than that in water (−0.411 kcal mol⁻¹), and increases with the increase of the HBD chain length, which is proportional to the extraction rate, indicating that the TPM molecules are more inclined to the DES environment, especially long-chain DES, than aqueous solution. For the same C₁₂DES, the extraction efficiency of the TPM dyes follows the order: ethyl violet (EV) (99.9%) > crystal violet (CV) (99.6%) > methyl violet (MV) (98.8%). EV has the smallest positive charge and the smallest dipole moment (9.109 D), and the Flory–Huggins parameters of EV (χ_EV–C12DES 0.053) relative to MV and CV are the smallest in C₁₂DES, and are also the largest in water (χ_EV–H2O 0.053), indicating that EV has the largest polarity difference with H₂O and is more easily detached from water and compatible with the long-chain DES phase. The motion of EV and MV on the phase interface of DES and water was calculated to further analyze from the molecular level. At the same time, EV tends to move into the DES phase. In summary, the excellent extraction ability of DES for TPM is verified through experiments and simulations, providing solid theoretical support in terms of separation in other fields.