Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES formed.

Experimental evidence of the dynamics of mesoscopic structure in room-temperature ionic liquids-a feature expected to correlate with many physicochemical properties of these materials-remains limited. Here, we report the observation of slow, sub-α relaxations corresponding to dynamics of nanoscale hydrophobic aggregates in a systematic series of 1-alkyl-3-methylimidazolium-based ionic liquids from detailed analysis of dynamic-mechanical and broad-band dielectric spectra. The emergence of the sub-α relaxations correlates with increases in the zero-shear viscosity and static dielectric permittivity, constituting direct evidence of the influence of mesoscale aggregation on the physicochemical properties of ionic liquids.

Charge transport and structural dynamics in the 1:2 mol ratio mixture of lidocaine and decanoic acid (LID-DA), a model deep eutectic mixture (DEM), have been characterized over a wide temperature range using broad-band dielectric spectroscopy and depolarized dynamic light scattering. Additionally, Fourier transform infrared spectroscopy measurements were performed to assess the degree of proton transfer between the neutral parent molecules. From our detailed analysis of the dielectric spectra, we have determined that this carboxylic-acid-based DEM is approximately 25% ionic at room temperature. Furthermore, we have found that the characteristic diffusion rate of mobile charge carriers is practically identical to the rate of structural relaxation at all measured temperatures, indicating that fast proton transport does not occur in LID-DA. Our results demonstrate that while LID-DA exhibits the thermal characteristics of a DEM, its charge transport properties resemble those of a protic ionic liquid.

Continuous progress in energy storage and conversion technologies necessitates novel experimental approaches that can provide fundamental insights regarding the impact of reduced dimensions on the functional properties of materials. Here, we demonstrate a nondestructive experimental approach to probe nanoscale ion dynamics in ultrathin films of polymerized 1-vinyl-3-ethylimidazolium bis­(trifluoromethylsulfonyl)­imide over a broad frequency range spanning over 6 orders of magnitude by broadband dielectric spectroscopy. The approach involves using an electrode configuration with lithographically patterned silica nanostructures, which allow for an air gap between the confined ion conductor and one of the electrodes. We observe that the characteristic rate of ion dynamics significantly slows down with decreasing film thicknesses above the calorimetric glass transition of the bulk polymer. However, the mean rates remain bulk-like at lower temperatures. These results highlight the increasing influence of the polymer/substrate interactions with decreasing film thickness on ion dynamics.

Polymerized ionic liquids are a promising class of versatile solid-state electrolytes for applications ranging from electrochemical energy storage to flexible smart materials that remain limited by their relatively low ionic conductivities compared to conventional electrolytes. Here, we show that the in situ polymerization of the vinyl cationic monomer, 1-ethyl-3vinylimidazolium with the bis(trifluoromethanesulfonyl)imide counteranion, under nanoconfinement within 7.5 ± 1.0 nm diameter nanopores results in a nearly 1000-fold enhancement in the ionic conductivity compared to the material polymerized in bulk. Using insights from broadband dielectric and Raman spectroscopic techniques, we attribute these results to the role of confinement on molecular conformations, ion coordination, and subsequently the ionic conductivity in the polymerized ionic liquid. These results contribute to the understanding of the dynamics of nanoconfined molecules and show that in situ polymerization under nanoscale geometric confinement is a promising path toward enhancing ion conductivity in polymer electrolytes.

The impact of mesoscale organization on dynamics and ion transport in binary ionic liquid mixtures is investigated by broadband dielectric spectroscopy, dynamic-mechanical spectroscopy,x-ray scattering, and molecular dynamics simulations. The mixtures are found to form distinct liquids with macroscopic properties that significantly deviate from weighted contributions of the neat components. For instance, it is shown that the mesoscale morphologies in ionic liquids can be tuned by mixing to enhance the static dielectric permittivity of the resulting liquid by as high as 100% relative to the neat ionic liquid components. This enhancement is attributed to the intricate role of interfacial dynamics associated with the changes in the mesoscopic aggregate morphologies in these systems. These results demonstrate the potential to design the physicochemical properties of ionic liquids through control of solvophobic aggregation.Elucidating the influence of mesoscale organization on the dynamics and transport properties of ionic liquids is critical to developing design criteria for their applications in chemical synthesis, nanoparticle growth, biomass processing, batteries, solar cells, and supercapacitors.[1-10] In the past decade, the formation of mesoscale polar and non-polar domains in ionic liquids with substantial non-polar, alkyl side groups was recognized in detailed x-ray scattering, neutron scattering, and molecular dynamics (MD) simulation studies. [11][12][13][14][15][16] Mesoscale organization has been used to qualitatively explain numerous experimental findings which imply spatially and temporally distinct regions within bulk ionic liquids. [11,[17][18][19][20][21][22][23][24][25][26] Recent studies suggest that the existence and dynamics of the aggregates in neat ionic liquids, associated with fluctuations of the polar and non-polar regions, correlate strongly with many of the physicochemical properties of ionic liquids including transport properties such as zero-shear viscosity, dc ionic conductivity, and static dielectric permittivity. [27][28][29][30][31][32] However, no efforts to exploit the mesoscale organization to design novel ionic liquids with unique physical and chemical properties have been reported. In this work, it is demonstrated that by mixing ionic liquids with varying degrees of solvophobic aggregation, it is feasible to design distinct liquids with macroscopic properties that significantly deviate from weighted contributions of the neat components.The local organization, or morphology, of the mesoscale aggregates is in part determined by the relative volume fractions of the polar and non-polar groups of the component ions.[11] In amphiphilic imidazolium, pyrrolidinium, piperidinium, quaternary phos-

The impact of molecular structure on ion dynamics and morphology in ammonium-and imidazolium-based glassy polymerized ionic liquids (polyILs) is investigated using broadband dielectric spectroscopy (BDS), wide-angle X-ray scattering (WAXS), and classical molecular dynamics (MD) simulations. It is shown that ammonium-based polyILs exhibit higher dc ionic conductivity at their respective glass transition temperatures (T g ) compared to imidazolium systems. In addition, the length of the alkyl spacer has a more drastic impact on ionic conductivity at comparable time scales of segmental dynamics for ammonium than imidazolium polyILs. Agreement between the characteristic ion diffusion lengths estimated from the dielectric data and the ion-to-ion correlation lengths from the WAXS and all-atom MD simulations is observed. A recently proposed approach is employed to determine ionic mobility in a broad frequency range spanning 5 orders of magnitude below the T g of polyILs studied, providing access to a regime of diffusivities that is inaccessible to many current experimental techniques. The ion mobility is found to be more sensitive to variation of the molecular structure than to the effective number density of the mobile ions. These results showcase the subtle interplay between molecular structure, morphology, and ion dynamics in polymerized ionic liquids.

The impact of supramolecular hydrogen bonded networks on dynamics and charge transport in 2-ethyl-4-methylimidazole (2E4MIm), a model proton-conducting system, is investigated by broadband dielectric spectroscopy, depolarized dynamic light scattering, viscometry, and calorimetry. It is observed that the slow, Debye-like relaxation reflecting the supramolecular structure in neat 2E4MIm is eliminated upon the addition of minute amounts of levulinic acid. This is attributed to the dissociation of imidazole molecules and the breaking down of hydrogen-bonded chains, which leads to a 10-fold enhancement of ionic conductivity.