On the properties of water in betaine - based Deep Eutectic Solvents

Deep eutectic solvents have gained attention as environmentally friendly alternatives in various technological applications. Likewise, it is known the relevant effect of water on the physicochemical properties of Deep Eutectics, especially for hydrophilic ones. In this study, Betaine - based type III Deep Eutectic solvents (with ethylene glycol, fructose, malic acid or citric acid as Hydrogen Bond Donors) are considered and the properties and structuring of water in Deep Eutectic solutions (content lower than 20 %) were investigated considering volumetric properties, optical properties (refraction index), dynamic properties (viscosity, thermal conductivity, and electrical conductivity) as well as relevant characteristics for liquid phase nature such as polarizability and molecular packing. Viscosity measurements demonstrated a wide variation in viscosities, with water content playing a significant role, thus probing how water content largely affect dynamic properties of Deep Eutectics. Additionally, high water content led to improved thermal and electrical conductivity, particularly in mixtures containing citric acid and malic acid as Hydrogen Bond Donors. The polarizability decreased with increasing water content, and packing calculations indicated that the most voluminous molecules occupied interstitial voids. Our findings contribute to the understanding of water in Deep Eutectics solution, aiding their optimization for sustainable applications.