The microscopic dynamics in matrine–fatty acids mixtures: A key to understanding the formation and properties of deep eutectic solvents

Deep eutectic solvents (DESs) have extensive applications in metal processing, organic reaction and extraction, and other fields due to their advantages of facile preparation, environmental compatibility, and non-toxicity. Matrine (Mat) and fatty acids (FAs) are widely used in pharmaceuticals and cosmetics because of their rich biological activities. In this study, Mat-FA DESs (DESs formed by Mat and FAs) are prepared and the corresponding microscopic mechanism for the formation of DESs is elucidated at molecular level systematically by experiments and molecular dynamics (MD) simulations. The results reveal that the Mat-FA interaction is stronger than that between the components themselves, which makes the melting point of the mixture lower than the theoretical eutectic point, and the hydrogen bonding in the system is positively correlated with the deviation of the melting point from the theoretical eutectic point, and the systems with stronger hydrogen bonding are more prone to form DESs with a melting point below 25 °C. Notably, when the interaction of FA-FA is weaker in the system, the DESs also can be observed at higher FA concentrations. In addition, the DES aqueous solutions exhibit different phase behaviors, which can be adjusted on purpose by changing the DES concentration. The phase behavior of DES aqueous solutions is closely related to molecular interactions. The stronger molecular interactions in DES aqueous solutions favor the formation of gels with better mechanical performance. The present work not only provides a theoretical basis for understanding the formation of Mat-FA DESs but also extends the potential applications of DES systems.