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

Abstract

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.

Ab Initio Molecular Dynamics Study of Phospho-Amino Acid-Based Ionic Liquids: Formation of Zwitterionic Anions in the Presence of Acidic Side Chains

“…In certain PILs, proton transfer can be enhanced by a Grotthuss-like mechanism when a neutral molecule, such as imidazole, is added to the liquid. A similar mechanism has been detected in PILs with partially neutralized diamines . Further insights into the complexity of certain PILs has been obtained by us; − we have shown that proton transfer in fully ionized liquids can be mediated by the clustering of like-charge ions (specifically amino acid anions).…”

“…A similar mechanism has been detected in PILs with partially neutralized diamines. 18 Further insights into the complexity of certain PILs has been obtained by us; 19−21 we have shown that proton transfer in fully ionized liquids can be mediated by the clustering of like-charge ions (specifically amino acid anions).…”

Surface of Half-Neutralized Diamine Triflate Ionic Liquids. A Molecular Dynamics Study of Structure, Thermodynamics, and Kinetics of Water Absorption and Evaporation

“…Sample Preparation and Simulation Setup. Slabs consisting of 216 [DAEt][Tf] molecules have been prepared starting from a homogeneous liquid sample equilibrated at T = 440 K (see ref 19), increasing the periodicity L z along z in discontinuous steps, while keeping constant the periodicity along xy. At first the change of periodicity has to be slow, and each increase in L z (by ∼1 Å) is followed by minimization of the energy at constant volume, and by brief (50 ps) equilibration at NVT conditions back to 440 K. Soon a density gap forms, separating two adjacent surfaces, and it becomes possible to extend L z by large (∼10 Å) steps without perturbing much the system.…”

“…To investigate all these issues, a paradigmatic role could be attributed to a family of PIL compounds made of aliphatic diamine cations and the trifluoromethanesulfonic (triflate) anion, whose chemical physics properties, and proton conductivity in particular, have been investigated by experiments 16 and by simulation. 19 More precisely, cations in these compounds consist of a conjugated pair of amino (−NH 3 + )/ amine (−NH 2 ) terminations, joined by an aliphatic segment [−(CH 2 ) n −], and their charge is compensated by an equal number of triflate (CF 3 SO 3 − , [Tf] − ) anions. In our study we consider the case n = 2, ethylenediamine triflate, [DAEt][Tf], and, to a lesser extent, the case n = 4, butyldiamine triflate, [DABu][Tf].…”

“…The experimental measurements of ref 16 show that [DAEt][Tf] melts at T m = 351 K, and it is chemically stable up to T = 500 K. Corresponding data for [DABu][Tf] are not reported in the same experimental paper, but they are expected to be similar. Molecular dynamics simulations 19 show a glass transition on cooling at T g = 310 K in [DAEt][Tf] and document the formation and role of mesophases, due to the interplay of Coulomb forces, packing effects, and hydrogen bonding. NMR measurements of the diffusion coefficients in [DAEt][Tf] reveal that protons diffuse faster than cations, implying that vehicular proton transport is supplemented by proton jumps among cations, corresponding to the well-known Grotthuss proton transport in water.…”