Thermodynamics investigation of phase behavior of deep eutectic solvents-polymer aqueous biphasic systems

Phase behavior including vapor-liquid and liquid-liquid equilibria of aqueous solutions of three deep eutectic solvents (DES), prepared by mixing of choline chloride as HBA, and urea, ethylene glycol and glycerol as HBD at a molar ratio of 1:2 (ChCl:2HBD), were investigated in the absence and presence of water soluble polymers polypropylene glycol400 (PPG400), polyethylene glycol400 (PEG400) and polyethylene glycol10000 (PEG10000) at different temperatures. Among all the DES/Polymer/H2O systems investigated in this work, only in the case of DES/PPG/H2O systems the aqueous biphasic system (ABS) is formed. Evidently, the ABS formation appeared to be controlled by the competition between the preferential hydration and hydrogen bonding interaction depending on the polymer and DES structures. Furthermore the LLE results for DES/PPG/H2O systems showed that the capability of DES for soluting-out of PPG in the aqueous solutions followed the order glyceline > ethaline > reline. In order to gain further insight regarding the DES properties which govern the preferential hydration in DES/PPG/H2O solutions, the ability of each DES component (hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD)) to form ABS with PPG was also evaluated. The results revealed that choline chloride as HBA and only glycerol as HBD could solute-out of PPG and undergo phase separation in the aqueous solutions. The equal water activity lines obtained from the isopiestic measurements for different DES/Polymer/H2O systems indicated a relationship between the vapor-liquid equiliria and liquid-liquid equiliria behavior of these systems. Finally, the effect of temperature, the polymer and DES structures on the equal water activity lines and also the binodal curve of the systems were also investigated and the results were discussed on the basis of the hydrogen bond interactions in these systems.