Fate of a Deep Eutectic Solvent upon Cosolvent Addition: Choline Chloride-Sesamol 1:3 Mixtures with Methanol.

Matteo Busato,Pierpaolo Tomai,Valentina Migliorati,Andrea Martinelli,Paola D’Angelo,Alessandra Del Giudice,Alessandra Gentili,Valerio Di Lisio

doi:10.1021/acssuschemeng.1c03809

Matteo Busato, Pierpaolo Tomai + Show 6 more

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https://doi.org/10.1021/acssuschemeng.1c03809

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Abstract

The changes upon methanol (MeOH) addition in the structural arrangement of the highly eco-friendly deep eutectic solvent (DES) formed by choline chloride (ChCl) and sesamol in 1:3 molar ratio have been studied by means of attenuated total reflection Fourier transform infrared spectroscopy, small- and wide-angle X-ray scattering (SWAXS), and molecular dynamics simulations. The introduction of MeOH into the DES promotes the increase of the number of Cl–MeOH hydrogen bonds (HBs) through the replacement of sesamol and choline molecules from the chloride anion coordination sphere. This effect does not promote the sesamol–sesamol, choline–choline, and sesamol–choline interactions, which remain as negligible as in the pure DES. Differently, the displaced sesamol and choline molecules are solvated by MeOH, which also forms HBs with other MeOH molecules, so that the system arranges itself to keep the overall amount of HBs maximized. SWAXS measurements show that this mechanism is predominant up to MeOH/DES molar ratios of 20–24, while after this ratio value, the scattering profile is progressively diluted in the cosolvent background and decreases toward the signal of pure MeOH. The ability of MeOH to interplay with all of the DES components produces mixtures with neither segregation of the components at nanoscale lengths nor macroscopic phase separation even for high MeOH contents. These findings have important implications for application purposes since the understanding of the pseudophase aggregates formed by a DES with a dispersing cosolvent can help in addressing an efficient extraction procedure.

Highlights

Deep eutectic solvents (DESs) are gaining increasing attention as a more sustainable alternative to traditional organic solvents for several applications.[1,2] deep eutectic solvent (DES) are a compositionally heterogeneous class of solvents formed by both molecular and ionic compounds acting as hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs), very often based on quaternary-ammonium salts such as choline chloride (ChCl) and HBDs such as amines, amides, carboxylic acids, and alcohols.[3,4] The combination of the HBA and HBD in proper proportions gives rise to a eutectic with a melting point in the phase diagram that is lower than those of the individual components
As far as the ChCl/sesamol 1:3 DES (M = 0) is concerned, the broad band centered at 3189 cm−1 (νOH(DES)) is assigned to the O−H stretching of both ChCl and sesamol hydroxyl groups, which absorb in the same spectral range
The additional spectral features at lower wavenumbers are assigned exclusively to the sesamol moiety, in particular the C=C stretching at 1610 cm−1, the C−H bending region (1500−1300 cm−1), the strong C−O stretching absorption between 1300 and 1000 cm−1, and the out-of-plane ring deformation (δring(sesamol)) at 765 cm−1.63 bands related to ChCl only are too weak and unresolved to be employed in the analysis

Summary

Introduction

Deep eutectic solvents (DESs) are gaining increasing attention as a more sustainable alternative to traditional organic solvents for several applications.[1,2] DESs are a compositionally heterogeneous class of solvents formed by both molecular and ionic compounds acting as hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs), very often based on quaternary-ammonium salts such as choline chloride (ChCl) and HBDs such as amines, amides, carboxylic acids, and alcohols.[3,4] The combination of the HBA and HBD in proper proportions gives rise to a eutectic with a melting point in the phase diagram that is lower than those of the individual components The origins of such behavior have been long debated and rely on the extensive hydrogen bond (HB) network that is established among the components upon the melting process so that the system is usually arranged to maximize the molecular interactions.[5−8] Once discovered, DESs suddenly gained much attention owing to some outstanding properties like negligible vapor pressure, nonflammability, high conductivity, high solvation ability, and low toxicity.[9] The term “natural deep eutectic solvents” (NADES). Was coined to describe a subset of DESs obtained by components that are primary metabolites of living cells, like amino acids, organic acids, sugars, and choline derivatives, dramatically increasing the biocompatibility and eco-friendliness of these materials.[10−12] DESs show an intrinsic nature of “designer solvents”, since the chemical nature of the constituents can be tailored to meet desired chemical−physical requirements.[9,13]

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