Abstract

Deep eutectic solvents (DES) are emerging sustainable designer solvents viewed as greener and better alternatives to ionic liquids. Nonionic DESs possess unique properties such as viscosity and hydrophobicity that make them desirable in microextraction applications such as oil-spill remediation. This work builds upon a nonionic DES, NMA–LA DES, previously designed by our group. The NMA–LA DES presents a rich nanoscopic morphology that could be used to allocate solutes of different polarities. In this work, the possibility of solvating different solutes within the nanoscopically heterogeneous molecular structure of the NMA–LA DES is investigated using ionic and molecular solutes. In particular, the localized vibrational transitions in these solutes are used as reporters of the DES molecular structure via vibrational spectroscopy. The FTIR and 2DIR data suggest that the ionic solute is confined in a polar and continuous domain formed by NMA, clearly sensing the direct effect of the change in NMA concentration. In the case of the molecular nonionic and polar solute, the data indicates that the solute resides in the interface between the polar and nonpolar domains. Finally, the results for the nonpolar and nonionic solute (W(CO)6) are unexpected and less conclusive. Contrary to its polarity, the data suggest that the W(CO)6 resides within the NMA polar domain of the DES, probably by inducing a domain restructuring in the solvent. However, the data are not conclusive enough to discard the possibility that the restructuring comprises not only the polar domain but also the interface. Overall, our results demonstrate that the NMA–LA DES has nanoscopic domains with affinity to particular molecular properties, such as polarity. Thus, the presented results have a direct implication to separation science.

Highlights

  • Deep eutectic solvents (DESs) are a new class of sustainable designer solvents with characteristics and physical properties similar to those of ionic liquids (IL), but DESs are inherently different in their formation mechanism, composition, and applications

  • Our results demonstrate that the NMA−LA DES has nanoscopic domains with affinity to particular molecular properties, such as polarity

  • The broadening of the CN stretch band is accompanied by a change in the frequency of the maximum as seen in DES2 where the maximum is located at 2056 cm−1, but in DES4 and neat NMA, the maxima are positioned at 2053 cm−1

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Summary

Introduction

Deep eutectic solvents (DESs) are a new class of sustainable designer solvents with characteristics and physical properties similar to those of ionic liquids (IL), but DESs are inherently different in their formation mechanism, composition, and applications. Unlike ILs, which are entirely formed by a fixed and discrete ratio between the anion and cation, DESs are eutectic mixtures composed of ionic and/or nonionic species.[1] It is accepted that the hydrogen bond interaction between the different DES components reduces the enthalpic component of the energetics, which results in the depression of the melting point of the mixture and gives a DES its liquid state at ambient temperature.[1−3] DESs are simple and low-cost alternatives to ILs because they can be readily made by mixing two components This last characteristic makes them more appealing than ILs because the latter usually require complex synthetic procedures for their preparation. DESs can be designed according to a tailored application.[4−6] DESs have received a remarkable interest since they were first introduced by Abbott and coworkers in 2002.7 DESs have been extensively studied for their applications in green chemistry,[6,8−11] organic synthesis,[12−17] material preparation,[18−23] electrochemistry,[24−27] and analytical and separation sciences.[28−34]

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