Liquid structure and dynamics in the choline acetate:urea 1:2 deep eutectic solvent

Abstract

We report on the thermodynamic, structural and dynamic properties of a recently proposed Deep Eutectic Solvent (DES), formed by choline acetate and urea at the stoichiometric ratio 1:2, hereinafter indicated as acetaline. Acetaline, although melting at ca. 40 °C, can be easily supercooled in its liquid state at sub-ambient conditions. The existence of a crystalline phase has been detected, together with the glass-liquid transition at -50 °C. Synchrotron high energy X-ray scattering experiments provide the experimental data for supporting a Reverse Monte Carlo analysis to extract structural information at atomistic level. This exploration of acetaline's liquid structure reveals the major role played by hydrogen bonding in determining interspecies correlations: both acetate and urea are strong hydrogen bond acceptor sites, while both the choline hydroxyl and urea act as HB donors. All acetaline moieties are involved in mutual interactions, with acetate and urea strongly interacting through hydrogen bonding, while choline being involved mostly in van der Waals mediated interactions. Such a structural situation is mirrored by the dynamic evidences obtained by means of 1 H NMR techniques, that show how urea and acetate species experience higher translational activation energy than choline, fingerprinting their stronger commitments into the extended hydrogen bonding network established in acetaline.

In Competition for Water: Hydrated Choline Chloride:Urea vs Choline Acetate:Urea Deep Eutectic Solvents

“…29 Another similarity between the two systems is that there is almost no change in the signals of choline (the only exception is the CH 2 OH group, as discussed in the following), meaning they are not particularly affected by the addition of water. This is not surprising given the minor role of choline cations in the intermolecular network of nonhydrated ChCl:U and ChOAc:U, 49 and the fact that water is not expected to form specific interactions with its (hydrophobic) alkyl portion.…”

“…In particular, the carbonyl carbon of acetate is the site showing the most intense downfield shift, suggesting a strong competition with urea CO in the hydrated DES. Indeed, the oxygen acceptor of acetate was found to be involved in the shortest and strongest H-bond interactions in neat ChOAc:U, 49 and its major role is even strengthened upon the addition of water. Dynamic Scenario: Changes in Local and Long-Range Mobilities upon Dilution.…”

“…Although ChCl-based DESs, and in particular ChCl:U, are relatively well characterized in terms of structure and H-bond interactions, the network existing between ionic and neutral species in ChOAc-based DESs has scarcely been investigated in detail at the molecular level yet . Moreover, unlike reline, no systematic study has been reported on the effect of progressive dilution on ChOAc-based DESs, to the best of our knowledge.…”

Vapor pressures and vapor phase compositions of choline chloride urea and choline chloride ethylene glycol deep eutectic solvents from molecular simulation

“…9 Recently, a special issue of the Journal of Chemical Physics titled "Chemical Physics of Deep Eutectic Solvents" was launched, to which the reader is referred for more information on DESs. [10][11][12][13][14][15][16][17][18][19] DESs and ionic liquids (ILs) are often regarded as superior solvents compared to volatile organic compounds (VOCs) due to a negligible vapor pressure at room temperature. 2,5,6,20 Such a low vapor pressure would limit the emission and loss of the DESs into the atmosphere and may facilitate the removal of solutes from DESs by distillation.…”

Choline chloride and ethylene glycol based deep eutectic solvent (DES) versus hydroxyl functionalized room temperature ionic liquids (RTILs): assessing the differences in microscopic behaviour between the DES and RTILs

“…27,45,51,69 Previously, it has been demonstrated that the distributions of relaxation rates for molecules which reside in different local environments in organized media like ILs are responsible for dynamic heterogeneity in that medium. 44,[69][70][71] We note here that Biswas and co-workers have demonstrated that decoupling of the rotational diffusion of a solute from the viscosity of the medium is caused by the dynamic heterogeneity of the medium. 44,50,51 Thus, the more the deviation of p from unity, the more the viscosity-diffusion decoupling and the more the dynamic heterogeneity of the medium.…”