Robustness of the Hydrogen Bond and Ion Pair Dynamics in Ionic Liquids to Different Parameters from the Reactive Flux Method

S Gehrke and B Kirchner, JOURNAL OF CHEMICAL AND ENGINEERING DATA, 65, 1146-1158 (2020).

DOI: 10.1021/acs.jced.9b00529

Both hydrogen bonding and ionic interactions have an extreme influence on the properties of all kinds of materials. Ionic liquids mostly possess both of them. In the present work, we calculate with the aid of the reactive flux theory the dynamics of hydrogen bonds as well as of the ionic network for different 1-butyl-3-methylimidazolium-based ionic liquids. To apply this method, we carry out molecular dynamics simulations over broad temperature and simulation parameter ranges. Hydrogen bond lifetimes vary with temperature in the range of 10 ps (400 K)-60 ps (300 K) for the more fluid systems to 60 ps (400 K)-2000 ps (300 K) for the more viscous systems. The ion pair dynamics behave differently. While the more fluid systems show ion pairs which possess a factor of 4 longer lifetimes than the hydrogen bonds, the ion pair lifetimes of the more viscous systems actually are a factor of 0.6 smaller and thus faster than the hydrogen bond lifetimes. Although the choice of the hydrogen bond angle is less sensitive, a deviation in the distance criterion of up to 5% from the radial pair distribution functions' minimum leads to a change of less than 10% in the calculated lifetime. The ion pair dynamics are less sensitive with respect to the variation of the geometrical parameter. Interestingly, for both kinds of dynamics the impact on the activation energy is even less prone to the different cutoff values than the lifetime. In fact, the cutoff for the distance criteria causes the activation energies to remain almost unchanged. We find that the stability of the method is sufficient if the simulation time is at least a factor of 10 longer than the corresponding lifetime.

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