Confining deep eutectic solvents in nanopores: insight into thermodynamics and chemical activity
Denis MORINEAU1, Benjamin MALFAIT1, Aicha JANI1
1CNRS - Institute of Physics of Rennes, Rennes, France
For many applications related to catalysis, electrochemistry, gas capture, and energy storage, solvents are not considered as bulk liquid phases but handled at solid interfaces or in mesoporous hosts. However, when they are spatially confined at the nanometer scale, several fundamental properties of liquids considerably differ from those of their bulk counterparts. While the effects of confinement on the physical properties of conventional liquids have been the subject of intense activity over the past two decades, the topic is still in its infancy regarding emerging new classes of alternative solvents.
We have performed, a comprehensive thermodynamic study of the phase behavior of aqueous solutions of the prototypical ethaline (ethylene glycol / choline chloride 2:1) deep eutectic solvent (DES) when confined in well-defined nanochannels of mesotructured porous silica matrices MCM-41 and SBA-15, with pore radii RP = 1.8 nm and 4.15 nm .
Extremely deep melting depressions were attained in the confined states, due to the combination of confinement and cryoscopic effects. These phenomena were analyzed quantitatively, based on a recently proposed extended version of the classical Gibbs-Thomson and Raoult thermodynamic approaches [1, 2]. In this framework, the predicted values of the water chemical activity in the confined systems were shown to systematically deviate from those of the bulk counterparts . The origin of this striking observation is discussed with respect to thermodynamic anomalies of water in the ‘no-man’s land’ and to the probable existence of specific nanostructures in DES solutions when manipulated in nanochannels or at interfaces with solids.
 “Confining deep eutectic solvents in nanopores: insight into thermodynamics and chemical activity”, B. Malfait, A. Jani, D. Morineau, J. Mol. Liquids, 349, 118488 (2022)
 “Extension and Limits of Cryoscopy for Nanoconfined Solutions”, B. Malfait, A. Pouessel, A. Jani, D. Morineau, J. Phys. Chem. Lett. 11 (14), 5763-5769 (2020).
 “Phase behavior of aqueous solutions of ethaline deep eutectic solvent”, A. Jani, T. Sohier, D. Morineau, J. Mol. Liquids, 304, 112701 (2020).