Tyler Cosby

Dynamic-Mechanical and Dielectric Evidence of Long-Lived Mesoscale Organization in Ionic Liquids

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.

Proton Transport in Imidazoles: Unraveling the Role of Supramolecular Structure

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.

Dynamic and structural evidence of mesoscopic aggregation in phosphonium ionic liquids

Mesoscopic aggregation in aprotic ionic liquids due to the microphase separation of polar and non-polar components is expected to correlate strongly with the physicochemical properties of ionic liquids and therefore their potential applications. The most commonly cited experimental evidence of such aggregation is the observation of a low-q pre-peak in the x-ray and neutron scattering profiles, attributed to the polarity alternation of polar and apolar phases. In this work, a homologous series of phosphonium ionic liquids with the bis(trifluoromethylsulfonyl)imide anion and systematically varying alkyl chain lengths on the phosphonium cation are investigated by small and wide-angle x-ray scattering, dynamic-mechanical spectroscopy, and broadband dielectric spectroscopy. A comparison of the real space correlation distance corresponding to the pre-peak and the presence or absence of the slow sub-α dielectric relaxation previously associated with the motion of mesoscale aggregates reveals a disruption of mesoscale aggregates with increasing symmetry of the quaternary phosphonium cation. These findings contribute to the broader understanding of the interplay of molecular structures, mesoscale aggregation, and physicochemical properties in aprotic ionic liquids.

Rotational and Translational Diffusion in Ionic Liquids

Dynamic glass transition and charge transport in a variety of glass-forming aprotic ionic liquids (ILs) are investigated in wide frequency and temperature ranges by means of broadband dielectric spectroscopy (BDS) , pulsed-field gradient nuclear magnetic resonance (PFG NMR), differential scanning calorimetry and dynamic mechanical spectroscopy. On the low-frequency side, the dielectric spectra exhibit electrode polarization effects, while hopping conduction in a disordered matrix dominates the spectra of ionic liquids at higher frequencies. Upon systematic variation of the molecular structure of the ionic liquids, it is observed that the absolute values of dc conductivity and viscosity span more than 11 orders of magnitude with temperature. However, quantitative agreement is found between the characteristic charge transport and the structural α-relaxation rates. These results are discussed in the context of dynamic glass transition-assisted hopping as the underlying mechanism of charge transport in the ionic liquids investigated. In addition, a novel approach to determine diffusion coefficients from dielectric spectra in quantitative agreement with PFG NMR is proposed. This makes it possible to separately determine the effective number densities and mobilities of the charge carriers and the type of their temperature dependence. The observed Vogel–Fulcher–Tammann (VFT) dependence of the dc conductivity is shown to be due to a similar temperature dependence of the mobility while Arrhenius type of thermal activation is found for the number density.