In-depth theoretical study on the structures of betaine-1,2-propanediol based deep eutectic solvents

Deep eutectic solvents (DESs) have been arousing more and more attention because they meet the important standard of new green solvents, and have been widely used in chemistry. However, the further commercialization is limited by the inherent structural instability and high viscosity. An in-depth understanding of the atomic-level structure and the interactions between hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD) is conducive to better exploring the stability and properties of DESs. In this work, density functional theory (DFT) and molecular dynamics (MD) simulation were used to study the structure and properties of DESs synthesized by betaine and 1,2-propanediol with different molar ratios (1:3 to 1:6). It is confirmed that the relationship between betaine and 1,2-propanediol is essential for the formation of DESs with a reasonable number of hydrogen bonds. In-depth analysis of these hydrogen bonds based on the quantum theory of atoms in molecules shows that they belong to "weak to medium" category. It is mainly the electrostatic interaction between the oxygen atoms of betaine and hydrogen atoms from hydroxyl groups of 1,2-propanediol. In addition, the simulation results showed that the increase of 1,2-propanediol content weakens the hydrogen bond between betaine and 1,2-propanediol, and strengthens the hydrogen bond between 1,2-propanediol molecules. The viscosity of DESs is mainly dependent on the interaction between betaine and 1,2-propanediol. The changes in intramolecular interactions of DESs illustrate that HBD molecules such as 1,2-propanediol play a critical role in the development of application-specific DESs.