Langevin behavior of the dielectric decrement in ionic liquid water mixtures

We present large scale polarizable simulations of mixtures of the ionic liquids 1-ethyl-3-methylimidazolium trifluoromethanesulfonate and 1-ethyl-3-methylimidazolium dicyanamide with water, where the dielectric spectra, the ion hydration and the conductivity were evaluated. The dielectric decrement, the depression of the dielectric constant of water upon addition of ions, is found to follow a universal functional of Langevin type. Only three physical properties need to be known to describe the complete range of possible concentrations, namely the dielectric constant of pure water, of pure ionic liquid and the linear slope of the dielectric decrement at low ionic liquid concentrations. Both the generalized dielectric constant, as well as the water contribution to the dielectric permittivity follow the functional dependence. We furthermore find that a scaling of van der Waals parameters upon addition of polarizable forces to the force field is necessary to correctly describe the frequency dependent dielectric conductivity and its contribution to the dielectric spectrum, as well as the static electric conductivity, which is also treated in the framework of a pseudolattice theoryThis work was funded by the Austrian Science Fund FWF in the context of Project No. P23494 and by the Cost Action CM 1206: “Exchange on ionic liquids”. Funding from Spanish Ministry of Economy and Competitiveness (Projects MAT2014-57943-C3-1-P and MAT2017-89239-C2-1-P) is gratefully acknowledged. Moreover, this work was funded by the Xunta de Galicia (AGRUP2015/11 and GRC ED431C 2016/001). E. H. is recipient of a DOC Fellowship of the Austrian Academy of Sciences at the Institute of Computational Biological ChemistryS