Charge transport and mass transport in imidazolium-based ionic liquids

How hydrogen bonds influence the mobility of imidazolium-based ionic liquids. A combined theoretical and experimental study of 1-n-butyl-3-methylimidazolium bromide

The virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence of hydrogen bonding on imidazolium-based ionic liquids in molecular dynamics simulations. One set provides diffusion coefficients in agreement with the experiment and is therefore a good model for real-world systems. Comparison with the other sets indicates hydrogen bonding to influence structure and dynamics differently. Furthermore, in one case the total charge was increased and in another decreased by 0.1 e. Both the most acidic proton as well as the corresponding carbon atom were artificially set to zero, sequentially and simultaneously. In the final setup a negative charge was placed on the proton in order to introduce a barrier for the anion to contact the cation via this most acidic hydrogen atom. The following observations were made: changing the hydrogen bonding ability strongly influences the structure while the dynamic properties, such as diffusion and viscosity, are only weakly changed. However, the introduction of larger alterations (stronger hydrogen bonding and antihydrogen bonding) also strongly influences the diffusion coefficients. The dynamics of the hydrogen bond, ion pairing, and the ion cage are all affected by the level of hydrogen bonding. A change in total charges predominantly influences transport properties rather than structure. For ion cage dynamics with respect to transport porperties, we find a good correlation and a weak or no correlation for the ion pair or the hydrogen bond dynamics, respectively. Nevertheless, the hydrogen bond does influence ion cage dynamics. Therefore, we confirm that ionic liquids rather consist of loosely interacting counterions than of discrete ion pairs. Hydrogen bonding affects the properties only in a secondary or indirect manner.

Space Charge Polarization Measurements as a Method to Determine the Temperature Dependence of the Number Density of Mobile Cations in Ion Conducting Glasses

A method is proposed using mean-field theories to help solve a long standing problem in the study of ionically solid electrolytes. While it has been long known that the ionic conductivity in solid electrolytes is comprised of two terms, the mobility and number of mobile charge carriers, there has been no accurate method developed to determine these two quantities independently. In this paper, we apply a mean-field method based upon low frequency a.c. impedance measurements of the limiting low frequency space charge polarization capacitance that develops as a result of the mobile carrier population diffusing to blocking electrodes. The space charge capacitance that develops is shown to be a simple function of the number of charge carriers and is found to be independent of the d.c. conductivity, but strongly dependent upon temperature. Measurements on two simple but well studied ion conducting glasses, LiPO3 and NaPO3, suggest that the carrier population is thermally activated where only a small fraction of the cations are mobile in the glass. The activation energy for carrier creation in LiPO3 is larger (49 kJ/mol) than that for NaPO3 glass (44 kJ/mol) and is in agreement with models of activation energies in ion conducting glasses that associate the creation energy with the cation charge density.

Unusual mechanism for the short-range electron transfer within gold-alkanethiol-ionic-liquid films of subnanometer thickness

Exploiting nanoscopically tunable composite gold-alkanethiol-ionic-liquid/ferrocene self-assembled systems with tunable electron transfer distance, we discovered in the case of thinner alkanethiol films a thermally activated electron transfer pattern totally controlled by the viscosity-related slow relaxation mode(s) of the ionic liquid acting as the reactant's fluctuating environment. This pattern manifested through the activation enthalpy and volume parameters that are identical to those for viscous flow was explained in terms of the extreme adiabatic mechanism with a vanishing Marcus barrier (via the exponential Franck-Condon-like term approaching unity).