DOI: https://doi.org/10.55373/mjchem.v27i3.149

Keywords: Deep eutectic solvents; choline chloride; choline dihydrogen citrate; malonic acid; viscosity; density

Abstract

The majority of deep eutectic solvents (DESs) are commonly synthesized utilising choline chloride (ChCl) as the hydrogen bond acceptor (HBA) due to its compatibility with a wide range of hydrogen bond donors (HBDs). However, depending solely on ChCl imposes constraints on the attainable properties suitable for specific industrial and research purposes. Therefore, choline dihydrogen citrate (ChDHCit) was chosen as new HBA to rival ChCl in the formulation of malonic acid (MA) based DESs. Note that the DESs were established at a mole ratio of 1:2 for further investigation of their physicochemical properties. FTIR-ATR analysis confirmed the formation of hydrogen bonds within both DES, with some significant C-O shifts observed in ChDHCit:MA. The complex anion C₆H₅O₇- in ChDHCit:MA caused more spectral shifts in FTIR compared to the anion Cl- in ChCl:MA. Thermal analysis by TGA proved that ChDHCit:MA is less chemically stable against decomposition with a T_d value (227.85°C) lower than that of ChCl:MA (305.26°C) owing to their large anions. The high melting temperature (T_m) of ChDHCit:MA compared to that of ChCl:MA is due to factors such as strong intermolecular forces or less-ordered crystal structures. The hydrogen bonds formation in the DES was confirmed by FTIR-ATR, DSC, and TGA analyses. ChDHCit:MA was found to be less acidic, more viscous, and less dense, with higher surface tension and lower ionic conductivity than ChCl:MA because of the less stable malonate anion, non-symmetrical size of the C₆H₅O₇- anion, and low ion mobility. In conclusion, the complex anions in HBA changed the physicochemical properties of DES.