Tailored Ternary Deep Eutectic Solvents Based on Choline Chloride, Urea, and Oxalic Acid: Preparation, Characterization and Application for Polysaccharide Extraction

Abstract

Three new ternary deep eutectic solvents (TDESs) were prepared using choline chloride/urea/oxalic acid with the following molar ratios: TDES-1 (1:2:0.01), TDES-2 (1:2:0.03), and TDES-3 (1:2:0.05). Hydrogen bonding interactions were confirmed by FT-IR and NMR spectroscopy. Computational predicting model using COSMO-RS supported the molecular interactions in these systems. The physicochemical properties of TDESs, such as density, viscosity, and thermal decomposition temperatures, were studied. The addition of oxalic acid increases the density and viscosity of the eutectic solvents. Thermogravimetric analysis (TGA) indicated that all TDESs exhibited good thermal stability up to 200–220 °C. The prepared TDESs are classified as hydrophilic and miscible with polar solvents. Extraction experiments showed that TDES-3 achieved a good yield of crude sulfated polysaccharides (17.55% ± 0.34) from the marine algae Codium vermilara, compared to the other DESs and conventional hot water extraction. These results reveal that choline chloride/urea/oxalic acid TDESs, particularly at a 1:2:0.05 ratio, are promising green solvents and may be useful for extracting polysaccharides from marine macroalgae.