Equilibrium Structure, Hydrogen Bonding, and Proton Conductivity in Half-Neutralized Diamine Ionic Liquids.

Recent experiments on proton conducting ionic liquids point to half-neutralized diamine-triflate salts as promising candidates for applications in power generation and energy conversion electrochemical devices. Structural and dynamical properties of the simplest among these compounds are investigated by a combination of density functional theory (DFT) and molecular dynamics (MD) simulations based on an empirical force field. Three different cations have been considered, consisting of a pair of amine-ammonium terminations joined by a short aliphatic segment -(CH2) n- with n = 2, 3, and 4. First, the ground state structure, vibrational eigenstates, and hydrogen-bonding properties of single ions, neutral ion pairs, small neutral aggregates of up to eight ions, and molecularly thin hydrogen bonded wires have been investigated by DFT computations. Second, structural and dynamical properties of homogeneous liquid and amorphous phases are investigated by MD simulations over the temperature range of 200 ≤ T ≤ 440 K. Structure factors, radial distribution functions, diffusion coefficient, and electrical conductivity are computed and discussed, highlighting the inherent structural heterogeneity of these compounds. The core investigation, however, is the characterization of connected paths consisting of cation chains that could support proton transport via a Grotthuss-type mechanism. Since simulations are carried out using a force field of fixed bonding topology, this analysis is based on the equilibrium structure only, using geometrical criteria to identify potential paths for proton conduction. Paths of connected cations can reach a length of 80 cations and 30 Å, provided that bridging oxygen atoms from triflate anions are taken into account. The effects of water contamination at 1% weight concentration on the structure, dynamics, and paths for proton transport are discussed.

Linking Structure to Dynamics in Protic Ionic Liquids: A Neutron Scattering Study of Correlated and Single-Particle Motions.

Coupling between dynamical heterogeneity of ionic liquids and their structural periodicity on different length-scales can be directly probed by quasielastic neutron scattering with polarization analysis. The technique provides the tools to investigate single-particle and cooperative ion motions separately and, thus, dynamics of ion associations affecting the net charge transport can be experimentally explored. The focus of this study is the structure-dynamic relationship in the protic ionic liquid, triethylammonium triflate, characterized by strong hydrogen bonds between cations and anions. The site-selective deuterium/hydrogen-isotope substitution was applied to modulate the relative contributions of different atom groups to the total coherent and incoherent scattering signal. This approach in combination with molecular dynamics simulations allowed us to obtain a sophisticated description of cation self-diffusion and confined ion pair dynamics from the incoherent spectral component by using the acidic proton as a tagged particle. The coherent contribution of the neutron spectra demonstrated substantial ion association leading to collective ion migration that preserves charge alteration on picosecond time scale, as well as correlation of the localized dynamics occurring between adjacent ions.

Density Functional Computations and Molecular Dynamics Simulations of the Triethylammonium Triflate Protic Ionic Liquid.

Systematic molecular dynamics simulations based on an empirical force field have been carried out for samples of triethylammonium trifluoromethanesulfonate (triethylammonium triflate, [TEA][Tf]), covering a wide temperature range 200 K ≤ T ≤ 400 K and analyzing a broad set of properties, from self-diffusion and electrical conductivity to rotational relaxation and hydrogen-bond dynamics. The study is motivated by recent quasi-elastic neutron scattering and differential scanning calorimetry measurements on the same system, revealing two successive first order transitions at T ≈ 230 and 310 K (on heating), as well as an intriguing and partly unexplained variety of subdiffusive motions of the acidic proton. Simulations show a weakly discontinuous transition at T = 310 K and highlight an anomaly at T = 260 K in the rotational relaxation of ions that we identify with the simulation analogue of the experimental transition at T = 230 K. Thus, simulations help identifying the nature of the experimental transitions, confirming that the highest temperature one corresponds to melting, while the one taking place at lower T is a transition from the crystal, stable at T ≤ 260 K, to a plastic phase (260 ≤ T ≤ 310 K), in which molecules are able to rotate without diffusing. Rotations, in particular, account for the subdiffusive motion seen at intermediate T both in the experiments and in the simulation. The structure, distribution, and strength of hydrogen bonds are investigated by molecular dynamics and by density functional computations. Clustering of ions of the same sign and the effect of contamination by water at 1% wgt concentration are discussed as well.

Dynamic Heterogeneity and Flexibility of the Alkyl Chain in Pyridinium-Based Ionic Liquids.

Changing the number of carbon atoms in the substituents of ionic liquids (ILs) is a way to shift the balance between Coulomb and van der Waals forces and, thus, to tune physicochemical properties. Here we address this topic on the microscopic level by employing quasielastic neutron scattering (QENS) and provide information about the stochastic ionic motions in the N-alkylpyridinium based ILs in a relatively expanded time range, from short time (subpicosecond) particle rattling to long time diffusive regime (hundreds of picoseconds). We have systematically investigated the effect of the alkyl chain length on the picosecond dynamics by employing partial deuteration of the samples and varying the number of carbon atoms in the alkyl substituent. The localized dynamics of the side groups have appeared to be enhanced for bulkier cations, which is opposite to the trend observed for the translational motion. This result highlights the role of the conformational flexibility of the alkyl group on the dynamical properties of ILs.