Low-Temperature Calorimetric Characterization of Deep Eutectic Solvents With Added Water

Deep eutectic solvents (DES) are combinations of hydrogen bonds donors and acceptors, characterized by significantly lower freezing temperatures than their components. NADES (with natural, biologically compatible components) are becoming popular for a wide applications variety, for their wide solvent capacities, non-toxicity, biodegradability, and inexpensive components. Applications include new means for bioactive compounds extraction and enzymatic reactions, polymer solubilizing and a variety of physicochemical processes, including those taking place at low temperature. Four DES (β-alanine and DL-malic or citric acid (plus water) and choline chloride and ethylene glycol or glycerol) were investigated by differential scanning calorimetry (DSC) at low temperature. Pure DES behave completely reversibly, proof of physicochemical stability. Alanine-DES present glass transitions between -40 and -80°C. Meanwhile, choline-DES show no thermal events between 0° and -80°C. Increasing water produce glass transitions in all DES at decreasing temperatures, and devitrifications and freezing events. Choline-DES show no effects up to 20% water. However, alanine-DES show strong water effects, possibly phase separation into different water populations. Calcium alginate-sucrose beads were used as structural model for diffusion systems. Beads were incubated in DES (20 min/20 hours) and their low-temperature properties characterized. Short incubation periods were enough to radically alter their thermal behavior.