Hydration is not detrimental for Glycerol:Lactic acid deep eutectic mixtures

Abstract

Natural deep eutectic solvents (NADES) have been proposed as eco-friendly alternatives to organic solvents. A trace amount of water is frequently unavoidable as a contaminant and may be also purposely added to the mixtures to reduce their problematically high viscosity and costly price. It is critical to understand the different effects of water on the characteristics of NADES, particularly for their use in procedures such as bioactives extraction, and enzymatic synthesis, among others. In this work, the effect of water on the physicochemical properties and structural features of glycerol:lactic acid (GLY:LAC) deep eutectic mixture was investigated. The structural changes after water addition were evaluated using FTIR-ATR and 1H NMR spectroscopy. The pH, conductivity, and polarity of the mixtures were also evaluated after the addition of different percentages of water. Molecular dynamics simulations (MD) were applied to assess the role of water on NADES components' distribution and properties, including the number of hydrogen bonds (HB), the radial distribution functions (RDF), and the spatial distribution functions (SDF). The results demonstrated that when water enters the system, the hydrogen bonds between the eutectic components break but new ones are formed creating strong interactions between them and the water. A ternary eutectic combination arises, leading to a melting point lower than the original counterparts, even with 90% water, caused by the formation of stronger intermolecular interactions. The MD simulations indicate no phase separation regardless of the ratio of the components or the water content in the mixture, showing preservation of the eutectic behavior. Overall, these unprecedented results, in which the rupture of the HB between eutectic mixtures is never observed up to 90% water, unravel the potential of these novel mixtures for multiple applications.