Structural Properties and Hydrogen-Bonding Interactions in Binary Mixtures Containing a Deep-Eutectic Solvent and Acetonitrile

Abstract

Deep-eutectic solvents (DESs) are a new class of green solvents. Here, we report the hydrogen bonding and structural properties of the archetypal DES ethaline, a mixture of choline chloride (ChCl) and ethylene glycol (EG) of a 1:2 molar ratio, and its pseudo-binary mixtures with acetonitrile. The investigations were carried out employing Fourier-transform infrared (FTIR) spectroscopy combined with quantum chemical calculations. Excess and two-dimensional (2D)-correlation spectroscopies were used to identify favorable species in the solutions and to explore the heterogeneity. The results show that the mixing process is the transformation from ethaline and CH3CN dimer to the complexes of ethaline–1CH3CN and ethaline–2CH3CN, together with the increased percentages of the EG dimer, EG trimer, and CH3CN monomer with respect to their total amounts in the mixtures. Theoretical calculations show that, for ChCl, the positive charge is located at the methyl groups and methylenes, rendering their ability to form hydrogen bonds. Adding CH3CN to ethaline can hardly break apart the doubly ionic hydrogen bonds between Ch+ and Cl–. The cosolvent molecules mainly surround the core structure of ethaline, forming noncovalent hydrogen bonds with hydroxyl groups of EG/Ch+ but not Cl–. These in-depth studies on the properties of ethaline and CH3CN/CD3CN mixed solvents may shed light on exploring their applications.

Influence of Hydration on the Structure and Interactions of Ethaline Deep‐Eutectic Solvent: A Spectroscopic and Computational Study

“…For the neat ETH, the broad band centered at ∼3325 cm −1 is assigned to the v (O−H) of Ch + and EGs. The peaks in 3050–2750 cm −1 range are assigned to the v (C−H) of Ch + and EG . The broad band centered at 2495 cm −1 is attributed to v (O−D) of D 2 O …”

“…The band at 2938 cm −1 is assigned to an overlap of v s (C−H) of methyls and v as (C−H) of methylenes. The other band at 3030 cm −1 is attributed to v as (C−H) of methyls …”

“…The changes in peak positions and intensities can only provide limited information about the interactions in the mixtures. Excess spectroscopy has the capability of elucidating veiled changes undetectable in original spectra In general, positive and negative peaks of the excess spectra are indicative of the fine structures of the mixtures. Figure B depicts the excess IR absorption spectra of ETH‐D 2 O system in v (O−H) and v (C−H) regions.…”

“…Despite the application potentials of DESs in variety of fields, finding a perfect DES with the exact favorable features is tough. One way to fine‐tune the properties of DESs to a specific application is through mixing them with other cosolvents . For instance, a limiting factor of DESs is their relatively high viscosities which have led to a great challenge in practical processing .…”

“…Furthermore, the molecular level information obtained in this work would help to explain qualitatively the macroscopic properties such as viscosity of the pseudo‐binary mixtures. The DES chosen in this work is the popular (ChCl+2 ethylene glycol), which is known as ethaline (ETH) . Attenuated total reflection Fourier transform infrared (ATR‐FTIR) spectroscopy, density functional theory (DFT) calculations and natural population analysis (NPA) have been employed to do the investigation in the work.…”