The formulation and physicochemical properties of betaine-based natural deep eutectic solvent

Eleven different types of natural deep eutectic solvent (NADES) were synthesised using betaine (Bet) as a hydrogen bond acceptor (HBA) with glycerol (Gly), 1,2-propanediol (1,2P), lactic acid (LA), levulinic acid (Lev), anhydrous citric acid (CA), malic acid (MA), tartaric acid (TA), xylitol (Xyl), sorbitol (Sor), glucose (Glu), and sucrose (Suc) as the hydrogen bond donors (HBD). They were synthesised at different molar ratios (3:1, 2:1, 1:1, 1:2, 1:3) to determine their appropriate formulation. Formulations that gave a clear, homogenous liquid were identified as Bet-Gly (1:2), Bet-1,2P (1:4), Bet-LA (1:2), Bet-Lev (1:2), Bet-CA (1:1, 20 % w/w water), Bet-MA (1:1, 20 % w/w water), Bet-TA (1:1, 20 % w/w water), Bet-Xyl (1:2, 20 % w/w water), Bet-Sor (1:2, 20 % w/w water), Bet-Glu (1:1, 20 % w/w water), and Bet-Suc (1:1, 20 % w/w water). The successfully synthesised NADESs remained as stable liquids for more than 30 days. Based on the analysis using Fourier transform infrared (FTIR) spectroscopy, it was confirmed that a hydrogen bonding interaction was established between the HBA and the HBD in the NADES system. In addition, the thermal stabilities of these NADESs were evaluated through a thermogravimetry analysis (TGA). The onset decomposition temperatures of the NADESs were in the range of 106.92 °C–233.66 °C. Furthermore, the NADESs were evaluated for their physicochemical properties, including viscosity, density, polarity, surface tension, electrical conductivity, and pH. The NADESs were often diluted with water to reduce viscosity during their application. Therefore, the influence of the addition of water on the physicochemical properties of NADESs was evaluated. This knowledge provides an in-depth understanding of the characteristics and behaviour of the NADESs during any process.