abstract
The solids choline chloride and urea, mixed in a 1 : 2 molar proportion, form the iconic deep eutectic solvent "Reline''. A combination of computational and vibrational spectroscopy tools, including inelastic neutron scattering (INS), have been used to probe intermolecular interactions in the eutectic mixture. Reline's experimental spectra were estimated using discrete and periodic ab initio calculations of a molecular aggregate with two choline chloride and four urea units. This is the minimum size required to achieve satisfactory agreement with experiment, as smaller clusters cannot represent all of reline's significant intermolecular interactions. The INS spectrum of reline, compared with that of pure choline chloride, reveals a displacement of chloride anions away from their preferred positions on top of choline's methyl groups, whose torsional movement becomes less hindered in the mixture. Urea, which adopts a planar (sp(2)) shape in the crystal, becomes non-planar (sp(3)) in reline, a feature herein discussed for the first time. In reline, urea molecules form a wide range of hydrogen bonds, from soft contacts to stronger associations, the latter being responsible for the deviation from ideality. The chloride's interactions with choline are largely conserved at the hydroxyl end while becoming weaker at the cationic headgroup. The interplay of soft and strong interactions confers flexibility to the newly formed hydrogen-bond network and allows the ensemble to remain liquid at room temperature.
keywords
IONIC LIQUID ANALOG; CHOLINE CHLORIDE; VIBRATIONAL ANALYSIS; INFRARED-SPECTRA; MATRIX-ISOLATION; UREA; TEMPERATURE; SPECTROSCOPY; HALIDES; PHASE
subject category
Chemistry; Physics
authors
Araujo, CF; Coutinho, JAP; Nolasco, MM; Parker, SF; Ribeiro-Claro, PJA; Rudic, S; Soares, BIG; Vaz, PD
our authors
Belinda Soares
PhD Student
Catarina Araújo
PhD Student
Mariela Martins Nolasco
Assistant Researcher
Paulo Ribeiro-Claro
Associate Professor with Aggregation
Pedro Miguel Duarte Vaz
CollaboratorGroups
G4 - Renewable Materials and Circular Economy
G6 - Virtual Materials and Artificial Intelligence
Other
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Mediaacknowledgements
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013) and project DEEPBIOREFINERY (PTDC/AGR-TEC/1191/2014), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. The STFC Rutherford Appleton Laboratory is thanked for access to neutron beam facilities. CASTEP calculations were made possible due to the computing resources provided by STFC Scientific Computing Department's SCARF cluster.