Proton Transfer in Protic Ionic Liquids
Final Report Abstract
Protic ionic liquids have the potential to overcome the drawback of slow charge transport in aprotic ionic liquids via transfer of protons between proton acceptors and donors. Earlier studies have shown that the prototypical typical protic ionic liquid 1-methylimidazolium acetate is only partially ionic and thus contains both proton acceptors and donors, required for fast proton transport. Despite the rather high conductivity of this ionic liquid the contribution of such proton transport, in addition to conventional diffusive transport of ions, has remained elusive and also the factors that govern the mobility of protons have not been fully established. In this project we aimed at unravelling charge transport and proton dynamics in 1-methylimidazolium based ionic liquids. Using experimental and computational dielectric spectroscopy, nuclear magnetic resonance spectroscopy, and transient spectroscopy on photoacids dissolved in the ionic liquids, we studied ion dynamics over wide length and timescales. Our results show that the overall charge carrier mobility strongly depends on the immediate environment, which we assess using solutions of the ionic liquids in molecular solvents. Depending on the nature of the solvent, ion dynamics can be both enhanced or restricted. Our results further show that proton dynamics are determined by the balance between available proton acceptors and donors, which we demonstrate using ionic liquid mixtures to enhance the degree of protonation of the ionic liquid cation. Transient spectroscopy experiments demonstrate that not only methylimidazolium and acetate can act as proton acceptor, but also acetic acid. The participation of acetic acid in proton transport allows for proton hopping along hydrogen-bonded chains, a mechanism that resembles Grotthuss-type proton transport in water where protons reach unprecedented mobilities. Yet, our results also revealed that the mobility of a large fraction of ions in this ionic liquid is restricted due to constraints imposed by the structure of the ionic liquids. These constraints are evidenced by marked contributions of hindered ion translations to the dielectric response at low frequencies. Together, our results highlight that proton transport in protic ionic liquids relies on the subtle balance between protonated and deprotonated molecular species together with their interaction with their immediate environment. The observation that protonation of acetic acid is an active part in proton transport may help explaining high conductivities for acetic acid containing ionic liquids and could allow for engineering proton hopping in protic ionic liquids. Further, our findings offer routes towards optimizing proton transport in these systems, e.g. via altering protonation equilibria, using molecular solvents as additives, and reducing constraints for ionic dynamics imposed by solvation cages.
Publications
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Emulating proton transfer reactions in the pseudo-protic ionic liquid 1-methylimidazolium acetate. Physical Chemistry Chemical Physics, 24(16), 9277-9285.
Jacobi, Richard; Joerg, Florian; Steinhauser, Othmar & Schröder, Christian
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Polarizable molecular dynamics simulations on the conductivity of pure 1-methylimidazolium acetate systems. Physical Chemistry Chemical Physics, 24(25), 15245-15254.
Joerg, Florian & Schröder, Christian
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Charge transport in protic ionic liquids: Effect of protonation state in 1-methylimidazolium – acetate/trifluoroacetate mixtures. Journal of Molecular Liquids, 390, 122975.
Sutter, Johannes; Haese, Constantin; Graf, Robert & Hunger, Johannes
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Comparative analysis of dielectric spectra in protic ionic liquids: Experimental findings and computational molecular decomposition. Journal of Molecular Liquids, 396, 123834.
Joerg, Florian; Sutter, Johannes; van Dam, Laurens; Kanellopoulos, Konstantinos; Hunger, Johannes & Schröder, Christian
