Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure−property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.
The 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.
Solvation
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.