Structural origin of proton mobility in a protic ionic liquid/imidazole mixture: insights from computational and experimental results

Abstract

The structure, dynamics, and phase behavior of a binary mixture based on the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide (C2HImTFSI) and imidazole are investigated by (1)H NMR spectroscopy, vibrational spectroscopy, diffusion NMR, calorimetric measurements, and molecular dynamics simulations. Particular attention is given to the nature of the H-bonds established and the consequent occurrence of the Grotthuss mechanism of proton transfer. We find that due to their structural similarity, the imidazolium cation and the imidazole molecule behave as interchangeable and competing sites of interaction for the TFSI anion. All investigated properties, that is the phase behavior, strength of ion-ion and ion-imidazole interactions, number of specific H-bonds, density, and self-diffusivity, are composition dependent and show trend changes at mole fractions of imidazole (χ) approximately equal to 0.2 and 0.5. Beyond χ = 0.8 imidazole is not miscible in C2HImTFSI at room temperature. We find that at the equimolar composition (χ ≈ 0.5) a structural transition occurs from an ionic network mainly stabilized by coulombic forces to a mixed phase held together by site specific H-bonds. The same composition also marks a steeper decrease in density and increase in diffusivity, resulting from the preference of imidazole molecules to H-bond to each other in a chain-like manner. As a result of these structural features the Grotthuss mechanism of proton transfer is less favored at the equimolar composition where H-bonds are too stable. By contrast, the Grotthuss mechanism is more pronounced in the low concentration range where imidazole acts as a base pulling the proton of the imidazolium cation. At high imidazole concentrations the contribution from the vehicular mechanism dominates.

Applying neutron diffraction with isotopic substitution to the structure and proton-transport pathways in protic imidazolium bis{(trifluoromethyl)sulfonyl}imide ionic liquids

“…These demonstrate the formation of cation-base interactions at the expense of existing cation-anion hydrogen-bonding interactions, and are compensated for by the additional imidazolecation interactions. Imidazole-imidazole correlations were not anticipated, but appear consistent with recent work by Yaghini et al 32 where similar results were observed from vibrational spectroscopy and MD simulation.…”

“…et al 32 have recently studied [1−ethylimidazolium][NTf 2 ] + imidazole mixtures using IR spectroscopy and MD simulation. The results were interpreted in terms of extensive cation-anion, cation-imidazole and imidazole-anion hydrogen-bonding.…”

<strong>Thermal characterization of ethylammonium nitrate </strong>

“…Since crystallization is usually a very slow process in ILs, the dynamic nature of this experiment does not allow a full crystallization of the dense ionic material to take place in the cooling part of the cycle. This cold crystallization after a Tg is typically observed in glass-forming liquids when cooled rapidly enough to prevent crystallization, as it has previously been reported for some based on pyrrolidinium-and methylimidazolium-based ILs [10][11][12][13]. The glass transition is not directly observed here, since EAN's glass transition temperature (previously reported to be -91 ºC [6]) lies clearly below our lowest studied temperature.…”

The influence of hydrophilicity on the orientational dynamics and structures of imidazolium-based ionic liquid/water binary mixtures

“…These hydrogen bonding acceptors prefer to coordinate with the hydrogen atoms in the imidazolium rings in the Emim + cations, leading to the formation of multiple hydrogen bonds or a hydrogen bond network within the RTIL matrices. 14,17,21,53 It should be noted that the hydrogen atoms in the imidazolium rings do not coordinate with the F atoms in the NTf 2 − anions via hydrogen bonding interactions due to their lower electronegativities in the anionic framework. However, these hydrogen atoms tend to form hydrogen bonds with the F atoms in the BF 4 − anions but with decreased hydrogen bond directionality and strength.…”

“…Previously, RTIL/water mixtures have been studied using a broad range of methods including optical heterodynedetected optical Kerr effect (OHD-OKE) spectroscopy, 12,14,15 fluorescence spectroscopy, 16 nuclear magnetic resonance spectroscopy, [17][18][19][20][21][22] dielectric spectroscopy, 23 and theoretical and computational calculations. 17,[24][25][26][27][28] In this paper, we use OHD-OKE spectroscopy in conjunction with atomistic molecular dynamic simulations to better understand the interplay between microstructures and dynamics of RTIL/water binary mixtures. New experimental results are combined with previously published data on RTIL/water mixtures to provide a broader understanding of hydrophilicity in RTILs.…”