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Structural and dynamical properties of ionic liquids: competing influences of molecular properties.

TitleStructural and dynamical properties of ionic liquids: competing influences of molecular properties.
Publication TypeJournal Article
Year of Publication2010
AuthorsSpohr, HV, Patey, GN
JournalJournal of Chemical Physics
Pagination154504/1 - 154504/12
Date Published2010///
ISBN Number0021-9606
Keywordsdiffusion viscosity cond ionic liq ion pair size disparity, structural dynamical property ionic liq ion pair size disparity

Room temp. ionic liqs. differ from molten salts in many ways and our work concs. on two distinguishing features. These are large cation-anion size disparities and at least one ionic species where the center of mass and the center of charge do not coincide. In earlier work, we examd. the influences of these features in isolation on simple spherical models. This paper extends this work to ionic liq. models where both features are present, and where the characteristic distance σ+-’ detg. the strength of the Coulombic attractions is unconstrained. We consider the interplay among these mol. features and elucidate their relative importance to the behavior of ionic liqs. Particular attention is focused on the transport properties. We find that size disparity, charge location, and σ+-’ can all have large (often competing) effects. In our models, size disparity and small charge displacements lead to weakly bound, directional ion pairs, and the resulting asym. ion-counterion distribution gives rise to increased diffusion coeffs., consequently lower viscosity, and increased cond. These observations are analogous to effects reported in the literature, and we see similarities between the directional ion pairs in our models and directional cation-anion pairing through weak hydrogen bonding in room temp. ionic liqs. In our models, large charge displacements lead to strongly bound, long-lived, directional ion pairs, and in this regime the trends noted above are reversed, increased viscosities, and decreased conductivities are obsd. Recently, creating more strongly hydrogen bonded, directional ion pairs has been put forward as possible means of achieving larger viscosity redns. The trend reversal that we observe suggests that this might not work in practice. (c) 2010 American Institute of Physics. [on SciFinder(R)]