Investigating the transport of excess protons in a protic ionic liquid via polarizable molecular dynamics simulations
Márta G?DÉNY1, Florian JÖRG1, Christian SCHRÖDER1, Maximilian KOVAR1
1University of Vienna, Wien, Austria
Ionic liquids (IL) have been proposed as alternative, non-flammable, electrolytes for batteries. While the conductivity of aprotic ILs is in general too low for this purpose, due to their viscosity; protic ionic liquids show a higher conductivity. Protons have a high mobility, and with reversible proton transfers, charge transfer is no longer bound to mass transfer.
In order to investigate the mechanisms and time scales of proton transfer in a protic IL, polarizable molecular dynamics simulations were carried out on a 1-methylimidazolium acetate based system. Excess protons were introduced by the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid.
Extensive research had already been done by our research group on the system, including the development of the polarizable force field, characterization of the equilibrium concentrations of charged and neutral species, as well as proton transfer probabilities in the 1-methylimidazolium acetate IL, and the development of the python module protex which makes the simulation of proton transfers possible.
In this work, the force field parameters of the photoacid were optimized. Subsequent simulations of the photoacid and the IL in methanol were set up in a way to mimic that of our experimental partners, who measured transfer times via pump-probe experiments. Thus a direct comparison of transfer times, conductivities, etc. was possible.
Proton transfer was enabled by protex, using a dual topology approach: in regular intervals legible protons were transferred with a certain probability, by changing the parameters of the involved molecules to that of the corresponding (de)protonated species. Only protons within a certain distance to an acceptor were transferred.
Subsequently, radial distribution functions, conductivity and diffusion coefficients were calculated. The mean squared displacement of the excess proton, coordination numbers, and the time it takes until the proton reaches an acetate, give insight into the mechanism of the proton transport. Thus, the rate limiting step, possible aggregations, as well as the type of transport (e.g. Grotthus-like or vehicular) can be analyzed.