Nanostructure of aqueous ternary amino-acid-based natural deep eutectic solvents by water bridging

Abstract

Natural deep eutectic solvents (NADESs) have drawn significant attention as green solvents for their special properties like biodegradability and biocompatibility. Nevertheless, the high viscosity of NADESs tends to prevent their implementation in industry. Even though dilution with water will lower the viscosity of NADESs effectively, fundamental knowledge of the intermolecular network in NADESs as well as the effect of water on the cooperative hydrogen-bonding network has not been evaluated systematically. Herein, solutions spanning a range of molar ratios of DL-Malic acid, L-Serine and water were synthesized. The experimental results demonstrate that it takes at least three moles of water to achieve a stable NADES. Besides, density functional theory (DFT) calculations indicate that water molecules connect DL-Malic acid and L-Serine by a water bridge framework. Furthermore, the progressive addition of water facilitates the proton transfer, making the hydrogen atom of the carboxyl group gradually ionize and approach the amino group. We hope the findings would provide a better understanding of the structural characteristics of NADESs and shed light on the applications.