Multiple Hydrogen-Bonding Interactions Enhance the Solubility of Starch in Natural Deep Eutectic Solvents: Molecule and Macroscopic Scale Insights

Recently, green chemistry and green processes have attracted a lot of academic and industrial interests. As one of the most consumed products, starch expressed extreme advantages in fields of food, pharmacology, and sustainable chemistry. However, as a polyhydroxy polymer, the strong hydrogen-bond network made them almost have no solubility in most solvents. In this study, the solubility of starch in a series of novel, green, sustainable, and low-cost solvents, namely, natural deep eutectic solvents (NADESs), was explored. A total of 12 kinds of NADESs with high fluidity at 363 K were selected, and the solubility of gelose starch (G50) in them was measured. Although a relative high solubility of 36.68 ± 0.86 and 21.03 ± 3.27 g/100 g of G50 was obtained in betaine–urea (BU) and malic acid–glucose (MGlu), respectively, in most NADESs, G50 was almost insoluble. The results of X-ray diffraction (XRD) analysis suggested that the crystalline structures of G50 were destroyed, and the results of attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR) indicated that hydrogen-bonding interactions were formed between G50 and NADESs. To further study it, a molecular dynamic (MD) study was applied and found that the hydrogen bonding between betaine and G50 plays a key role in the solubility of G50. To study the hydrogen bonds between NADESs, the geometries of NADESs, with the highest (BU) and lowest solubility (CU) of G50, were optimized by quantum chemical calculations, and the results showed that chloride ions of CU were occupied by urea, while the carboxylic ion of BU was free from it, which made a greatly different capacity to form a hydrogen-bonding interaction with G50 and, thus, a greatly different solubility of G50.