Confining deep eutectic solvents in nanopores: insight into thermodynamics and chemical activity

TL;DR

This study investigates how deep eutectic solvents behave when confined in nanopores, revealing altered thermodynamics and chemical activity due to nanoscale effects and confinement, with implications for understanding water anomalies and nanostructure formation.

Contribution

It provides a detailed phase diagram of a prototypical DES in confinement, extending classical thermodynamics to explain observed deviations in chemical activity and phase behavior.

Findings

Confinement leads to deep melting depressions in DES.

Water activity in confined DES deviates from bulk behavior.

Crystallization occurs above a specific hydration threshold in confinement.

Abstract

We have established the detailed phase diagram of the prototypical deep eutectic solvent ethaline (ethylene glycol / choline chloride 2:1) as a function of the hydration level, in the bulk state and confined in the nanochannels of mesostructured porous silica matrices MCM-41 and SBA-15, with pore radii $R_P$ = 1.8 nm and 4.15 nm. For neat and moderately hydrated DESs, freezing was avoided and glassforming solutions were formed in all cases. For mass fraction of water above a threshold value $W_g'\approx30\%$, crystallization occurred and led to the formation of a maximally-freeze concentrated DES solution. In this case, 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 an extended version of the classical Gibbs-Thomson and Raoult thermodynamic approaches. 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.