Structural characterization of an ionic liquid in bulk and in nano-confined environment using data from MD simulations.

This article contains data on structural characterization of the [C2Mim][NTf2] in bulk and in nano-confined environment obtained using MD simulations. These data supplement those presented in the paper "Insights from Molecular Dynamics Simulations on Structural Organization and Diffusive Dynamics of an Ionic Liquid at Solid and Vacuum Interfaces" [1], where force fields with three different charge methods and three charge scaling factors were used for the analysis of the IL in the bulk, at the interface with the vacuum and the IL film in the contact with a hydroxylated alumina surface. Here, we present details on the construction of the model systems in an extended detailed methods section. Furthermore, for best parametrization, structural and dynamic properties of IL in different environment are studied with certain features presented herein.

Insights from molecular dynamics simulations on structural organization and diffusive dynamics of an ionic liquid at solid and vacuum interfaces.

HYPOTHESIS: A reliable modelling approach is required for simultaneous characterisation of static and dynamic properties of bulk and interfacial ionic liquids (ILs). This is a prerequisite for a successful investigation of experimentally inaccessible, yet important properties, including those that change significantly with the distance from both vacuum and solid interfaces. SIMULATIONS: We perform molecular dynamics simulations of bulk [C2Mim][NTf2], and thick IL films in contact with vacuum and hydroxylated sapphire surface, using the charge methods CHelpG, RESP-HF and RESP-B3LYP with charge scaling factors 1.0, 0.9 and 0.85. FINDINGS: By determining and employing appropriate system sizes and simulations lengths, and by benchmarking against self-diffusion coefficients, surface tension, X-ray reflectivity, and structural data, we identify RESP-HF/0.9 as the best non-polarizable force field for this IL. We use this optimal parametrisation to predict novel physical properties of confined IL films. First we fully characterise the internal configurations and orientations of IL molecules relative to, and as a function of the distance from the solid and vacuum interfaces. Second, we evaluate densities together with mobilities in-plane and normal to the interfaces and find that strong correlations between the IL's stratification and diffusive transport in the interfacial layers persist for several nanometres deep into IL films.

Polaronic transport in iron phosphate glasses containing HfO2 and CeO2.

The electrical and dielectric properties of three series of glasses, xHfO2-(40 - x)Fe2O3-60P2O5, 0 ≤ x ≤ 8 mol%, xCeO2-(40 - x)Fe2O3-60P2O5, 0 ≤ x ≤ 8 mol%, and xHfO2-(38 - x)Fe2O3-2B2O3-60P2O5 2 ≤ x ≤ 6 mol%, have been investigated by impedance spectroscopy over a wide frequency and temperature range. As expected, these glasses exhibit polaronic conductivity which strongly depends on the fraction of ferrous ions, Fe2+/Fetot. Following a detailed discussion on the DC conductivity, we use the MIGRATION concept to model their conductivity spectra. It is found that in each series of glasses, the shape of the conductivity isotherms remains the same indicating that the time-temperature superposition principle is satisfied and that the mechanism of conductivity is the same. Returning to a model-free scaling procedure, namely Summerfield scaling, it is found that while conductivity isotherms for each composition yield a master curve, we need to suitably shift individual master curves on the frequency axis to generate a super-master curve. We examine the dependence of the DC conductivity and the shift factors on the number density of charge carriers. Next, using the fact that the dielectric strength of relaxation for each isotherm is well-defined in these systems, we scale the conductivity isotherms using the Sidebottom scaling procedure. This procedure yields a super-master curve, implying that length scales for polaronic transport also change with composition. Further, using the scaling features of permittivity spectra, we extract in a straightforward way the characteristic spatial extent of localized hopping of polarons and find that it decreases with increasing number density of charge carriers. The magnitude of these values obtained from permittivity spectra lies in the same range as those for the polaron radius calculated using the equation proposed by Bogomolov and Mirilin.

New nearly constant loss feature detected in glass at low temperatures.

At sufficiently low temperatures, disordered ionic materials display the well-known Nearly Constant Loss (NCL) effect, with ionic conductivities becoming approximately proportional to frequency and virtually independent of temperature. There is a broad consensus that the effect is a collective phenomenon, with many interacting ions participating, each of them performing some non-vibrational motion that remains strictly localised. The underlying many-particle dynamics have been analysed in Monte Carlo simulations and also by straightforward modelling. Both kinds of treatment predict that, with decreasing frequency, a frequency squared behaviour should become visible. Here, we report on the experimental detection of the squared to linear crossover in an NCL component of conductivity spectra of sodium borate glasses, xNa(2)O·(1 -x) B(2)O(3) with x = 0.05 and x = 0.1, at temperatures below 100 K. From the composition dependence of the effect it is obvious that it is caused by the sodium ions. We demonstrate that this behaviour corresponds to an almost trivial property of the mean square displacement of the confined, but locally mobile ions, which approaches a temperature-independent long-time value, reflecting the finite size of the accessible volume. In the log-log plot of measured conductivity versus frequency, the transition from slope two to slope one is rather gradual, reflecting the existence of different local neighbourhoods of the sodium ions.

The cationic energy landscape in alkali silicate glasses: Properties and relevance.

Individual cationic site energies are explicitly determined from molecular dynamics simulations of alkali silicate glasses, and the properties and relevance of this local energetics to ion transport are studied. The absence of relaxations on the time scale of ion transport proves the validity of a static description of the energy landscape, as it is generally used in hopping models. The Coulomb interaction among the cations turns out to be essential to obtain an average energy landscape in agreement with typical simplified hopping models. Strong correlations exist both between neighboring sites and between different energetic contributions at one site, and they shape essential characteristics of the energy landscape. A model energy landscape with a single vacancy is used to demonstrate why average site energies, including the full Coulomb interaction, are still insufficient to describe the site population of ions, or their dynamics. This model explains how the relationship between energetics and ion dynamics is weakened, and thus establishes conclusively that a hopping picture with static energies fails to capture all the relevant information. It is therefore suggested that alternative simplified models of ion conduction are needed.

Synthesis and modeling of polysiloxane-based salt-in-polymer electrolytes with various additives.

The effect of both nanoparticles and low molecular weight borate esters on the ionic conductivity of cross-linked polysiloxanes was systematically investigated by means of measuring conductivity spectra in the impedance regime at temperatures between -30 and 90 degrees C. Salt-in-polymer electrolytes were prepared by dissolving lithium triflate (LiSO(3)CF(3)) in comblike polysiloxanes bearing one methyl and one oligoether side group per silicon. An amount of 10 mol % of the oligoether side groups exhibited a terminal allytrimethoxysilane serving as a cross-linker moiety (T(0.1)OPS). Thus prepared polymer electrolyte membranes were completely amorphous and mechanically stable with an optimum conductivity value of 5.7 x 10(-5) S x cm(-1) at 15 wt % of lithium triflate (LiSO(3)CF(3)) at room temperature (T(0.1)OPS + 15 wt % LiSO(3)CF(3)). Further investigations concerned the influence of additives, i.e., nanosized ceramic fillers (alpha-Al(2)O(3) and SiO(2), up to 10 wt %) as well as two low molecular weight borate esters (tris(2-(2-methoxyethoxy)ethyl) borate (B2) and tris(2-(2-(2-methoxyethoxy)ethoxy)ethyl) borate (B3)) with maximum concentrations of 40 wt % as referred to polysiloxane T(0.1)OPS. The addition of borate esters resulted in a considerable increase of the conductivity, while still maintaining the mechanical stability. Optimum conductivities of 3.7 x 10(-5) and 1.6 x 10(-4) S x cm(-1) were measured for B2 and B3, respectively, at room temperature. A fit of the temperature-dependent DC conductivity by the empirical Vogel-Tammann-Fulcher (VTF) equation showed that there was an increased number density of mobile charge carriers in the case of borate esters as additives. However, the shape of the conductivity spectra in the dispersive regime changed considerably in going from nanoparticles as additives to borate esters. A careful and consistent modeling of the conductivity spectra and of the temperature dependence of the DC conductivity was done within the framework of the MIGRATION concept. The result was that the addition of borate esters to the polymer host most probably increased both number density of mobile charge carriers as well as their mobility.

Frequency-dependent fluidity and conductivity of an ionic liquid.

The frequency- and temperature-dependent shear fluidity, f(nu,T), of the ionic liquid [BMIm]BF(4) is presented and compared with its ionic conductivity, sigma(nu,T). [BMIm]BF(4) is short for 1-butyl-3-methyl-imidazolium tetrafluoroborate. Its DC fluidity, f(DC)(T), and DC conductivity, sigma(DC)(T), are non-Arrhenius and superimpose in an Arrhenius-type representation if the respective inverse temperature axes are made to differ by a small amount, Delta = (1/T(multiply sign in circle)- 1/T) > 0. The observed superposition suggests that f(nu,T) should display a frequency dependence similar to sigma(nu,T(multiply sign in circle)). We have therefore measured f(nu,T) of [BMIm]BF(4) over five decades of frequency at different temperatures. The spectra thus obtained corroborate our expectations. We model our experimental results in terms of the MIGRATION concept and arrive at the conclusion that f(nu,T) and sigma(nu,T(multiply sign in circle)) are Fourier transforms of quite similar time correlation functions.

Nearly constant loss effects in borate glasses.

Different nearly constant loss phenomena are investigated in borate glasses with compositions xNa(2)O.(1-x)B(2)O(3), for 0 < or =x< or = 0.3. The ionic conductivities caused by these effects are studied in wide ranges of temperature and frequency, spanning 4.3 K to 573 K and 100 mHz to 1 MHz, respectively. In a first step, we show how to identify the nearly constant loss (NCL) in 0.3Na(2)O.0.7B(2)O(3) glass. In the procedure, the scaling property of the conductivity caused by ordinary hopping is used to remove this component from the total conductivity as measured as a function of temperature at fixed frequency. The resulting NCL component is seen to be proportional to frequency and to display no temperature dependence. In a second step, a broad-band relaxation process is shown to exist in amorphous boron oxide and in sodium borate glasses with x< or = 0.1. It is most probably due to the presence of traces of water, with hydrogen ions behaving as reorienting and interacting local dipoles. In a third step, we propose a simple formal treatment of the NCL phenomenon, tracing it back to a large number of interacting ions, each of them moving locally. The key aspect is a "see-saw-type" time dependence of their individual single-particle double-well potentials, which is due to their Coulomb interactions. The individual ion does, therefore, not require thermal activation and is thus kept in motion even at cryogenic temperatures.

Low-temperature phases of rubidium silver iodide: crystal structures and dynamics of the mobile silver ions.

Recently, broad-band conductivity spectra have been taken in the low-temperature gamma-phase of the archetypal fast ion conductor RbAg4I5. Attempts to reproduce the experimental data in a simple model calculation have led to the conclusion that strictly localized displacive movements of interacting ionic charge carriers should play an important role in the low-temperature phase. However, with no detailed structural study of gamma-RbAg4I5 available, the relevant processes could not be identified within the crystal structure. This state of affairs has triggered the present investigation of the structures of all three phases of rubidium silver iodide. Powder diffraction data of RbAg4I5 have been collected at the high-resolution powder diffractometer at ID31 at the European Synchrotron Radiation Facility (ESRF). The structure of the gamma-phase has been solved by successive Rietveld refinements in combination with difference Fourier analyses. The same structural principle is found to prevail in all three phases, interconnected distorted RbI6 octahedra forming a three-dimensional framework, which undergoes only displacive structural changes during the alpha-beta and beta-gamma phase transitions. With decreasing temperature, the disorder in the silver sublattice is found to decrease, and a clustering of the disordered silver ions is found to develop. In the gamma-phase, "pockets" containing partially occupied silver sites have been identified, and it is suggested that the localized displacive motion detected by conductivity spectroscopy is performed by the silver ions located within these pockets.

Non-Arrhenius viscosity related to short-time ion dynamics in a fragile molten salt.

The equation T x sigmaDC(T) = alpha x exp[--(E*/kappa(B)T)--gamma x exp(E*/kappa(B)T)] has been used to understand the non-Arrhenius behaviour of the DC conductivity in supercooled glass-forming melts. Here, alpha, gamma and E* are parameters, E* denoting the activation energy for an elementary displacive step. Unlike the empirical VTF relation, our equation provides a link between the long-time and the short-time ion dynamics as observed in broad-band conductivity spectra. Surprisingly, the same equation with the same value of E* but different gamma successfully describes the fluidity (inverse viscosity) of a fragile glass-forming melt. This opens up the possibility of relating non-Arrhenius viscosities to short-time properties, which is in agreement with recent experimental and computer-simulation results.