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Nanoscopic structures and molecular interactions leading to a dystectic and two eutectic points in [EMIm][Cl]/urea mixtures

Abstract

Deep eutectic solvents (DES) are a novel class of ionic liquid-based solvents, in which a combination of an organic salt and a hydrogen bond acceptor (HBA) leads to a melting point depression. The resulting DES-mixtures feature properties well-known for ionic liquids, which are currently highly investigated as alternative, sustainable solvents and reaction media. In this study, mixtures of 1-Ethyl-3-methyl-imidazolium chloride ([EMIm][Cl]) and urea were investigated at different molar ratios mainly via electron paramagnetic resonance (EPR) spectroscopy on different nitroxide-based spin probes that probe different chemical environments, aided by differential scanning calorimetry (DSC) to obtain insights into the structure and dynamics on the nanoscale and to understand the molecular processes leading to potential function as non-classical solvent. Molecular dynamics simulations, Raman and pulse-field gradient (PFG) NMR spectroscopy were used to substantiate the insights in particular into the dynamic heterogeneities on the nanoscale. We found that indeed the mixing ratios leading to melting point extrema (two eutectic points, one dystectic point) show unusual EPR spectra concerning the reorientational dynamics of the spin probes and their environmental polarity. By thorough EPR spectral analysis and simulation and in combination with data from the other methods, detailed assumptions on the nanostructure and dynamics in this DES could be made. It is shown that the macroscopic DES properties are governed by the nanoscale interface of IL-based nanoregions and urea-enriched regions. This nanointerface crucially depends on the chloride anion and its ability to form hydrogen bonds with urea which leads to distinctive structural changes.

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Publication details

The article was received on 01 Aug 2018, accepted on 01 Oct 2018 and first published on 01 Oct 2018


Article type: Paper
DOI: 10.1039/C8CP04912B
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Nanoscopic structures and molecular interactions leading to a dystectic and two eutectic points in [EMIm][Cl]/urea mixtures

    U. Cerajewski, J. Träger, S. Henkel, A. Roos, M. Brehm and D. Hinderberger, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP04912B

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