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Show moreThe influence of ionic associations and potential-dependent interactions on the electrode–electrolyte interfacial structure of ionic liquids (ILs) is studied by electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS) for a variety of asymmetric quaternary ammonium ILs. Specifically, the impact of cation alkyl chain length (C = 4, 8 and 16) and ether functionality on the interfacial structuring of ILs at the glassy carbon electrode surface is examined. Ammonium cations with alkyl chain length of 8 and 16 carbons are found to stabilize the formation of the bis(trifluorosulfonyl)imide, [TFSI], anion dense Stern layer at positive electrode potentials leading to larger capacitances. The longer alkyl chain of the cation is believed to screen the ion–ion repulsion among the anions by intruding into the interfacial anion layer. SERS suggests the presence of carbon-containing rings at the interface at both positive and negative electrode potentials, which can be explained by the buckling of the long alkyl chains. Inclusion of an ether functionality allowed for more symmetry in the camel-shaped potential-dependent differential capacitance curves, suggesting similar excess ion density at both positive and negative potentials. This work contributes to understanding and predicting the interfacial electrode capacitance in ILs by understanding the balance of ionic interactions and the associated repulsions at electrode–electrolyte interfaces that are pertinent to electrochemical energy storage, electrocatalysis, and electrochemical sensors.
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Show moreSolvation and transport properties of methly viologen dichloride (MVCl2) in 1:2, 1:4, and 1:6 molar mixtures of choline chloride (ChCl) and ethylene glycol (EG), including the deep eutectic solvent (DES) ethaline (1:2 mixture), were studied through the application of the hole theory to measured physical properties, cyclic voltammetry, and Raman spectroscopy. The ChCl:EG mixtures were compared to the ionic liquid (IL) 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([PYR13][TFSI]) and choline bis(trifluoromethylsulfonyl)imide (ChTFSI) EG mixtures with the same molar ratios in order to understand the impact of the anion and hydrogen bond donor on solvation. Exchanging the chloride anion with TFSI is found to increase the fluidity of the solvent and promote stronger solute–solvent interactions. Raman spectroscopy suggests MVCl2 is strongly solvated by EG in ChTFSI:EG solutions and interstitially accommodated in holes in ChCl:EG mixtures and [PYR13][TFSI]. Complex solvents such as ILs and DESs are regarded as “designer solvents”, and it is demonstrated here that the physical properties and solvation characteristics of these fluids strongly depend on the choice of the anion.
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Show moreA range of techniques including physical property measurements, neutron scattering experiments, ab initio molecular dynamics, and classical molecular dynamics simulations are used to probe the structural, thermodynamic, and transport properties of a deep eutectic solvent comprised of a 1:2 molar ratio of choline chloride and ethylene glycol. This mixture, known as Ethaline, has many desirable properties for use in a range of applications, and therefore, understanding its liquid structure and transport properties is of interest. Simulation results are able to capture experimental densities, diffusivities, viscosities, and structure factors extremely well. The solvation environment is dynamic and dominated by different hydrogen bonding interactions. Dynamic heterogeneities resulting from hydrogen bonding interactions are quantified. Rotational dynamics of molecular dipole moments of choline and ethylene glycol are computed and found to exhibit a fast and slow mode.
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Show moreIn the past decade, reports detailing the preparation, characterization, and application of deep eutectic solvents (DESs) have grown in number significantly, showing signs of increased interest and attention. Indeed, these systems provide tunable polar environments attractive for their ease of synthesis and lack of need for purification. DESs are homogeneous systems composed of two or more components having a significantly depressed melting point compared to either constituent material. As interest and application of DESs grow, the need for a common understanding of their preparation and characterization is required. In this Perspective, we discuss metal-free DESs, focusing on their preparation, characterization of physical properties, and considerations for their application. We highlight inconsistencies or omissions in literature reports as well as factors for researchers to consider when investigating these systems.
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