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The influence of water on the structural and transport properties of model ionic liquids.

TitleThe influence of water on the structural and transport properties of model ionic liquids.
Publication TypeJournal Article
Year of Publication2010
AuthorsSpohr, HV, Patey, GN
JournalJournal of Chemical Physics
Pagination234510/1 - 234510/13
Date Published2010///
ISBN Number0021-9606
Keywordswater structural transport property model ionic liq

Mol. dynamics simulations are used to investigate the influence of water on model ionic liqs. Several models, where the ions vary in size, and in the location of the charge with respect to the center of mass, are considered. Particular attention is focused on the variation in transport properties (diffusion coeffs., shear viscosity, and elec. cond.) with water concn. An effort is made to identify the underlying phys. reasons for water’s influence. The results for our model ionic liqs. fall loosely into two categories, depending on the mol. characteristics of the constituent ions. If the ion size disparity is not too large (cation:anion diam. ratio .ltorsim. 2:1), and if the ion charge location is such that directional ion pair bonds are relatively weak, then we find that the ionic diffusion coeffs. and the elec. cond. increase, and the viscosity decreases with increasing water concn. This agrees with what is commonly obsd. exptl. for room temp. ionic liqs. (RTILs). For these systems, we do not find changes in the equil. structure that can account for the strong influence of water on the transport properties. Rather, by varying the mol. mass of water in our simulations, we demonstrate that the dominant effect of water can be dynamical in origin. In RTIL-water mixts., the mol. mass of water is generally much less than that of the ions it replaces. These lighter water mols. tend to displace much heavier counterions from the ion coordination shells. This reduces caging and increases the diffusivity, which leads to higher conductivities and lower viscosities. For models with a larger ion size disparity (3:1), or in charge-off-center systems, where strong directional ion pairs are important in the pure ionic liq., the behavior can be quite different. In these systems, the diffusion coeffs. and elec. cond. can still display conventional behavior and increase when water is added even though the reasons for this can be more complex than in the simpler cases noted above. However, in these systems the viscosity can increase, sometimes quite steeply, with increasing water concn. We trace this unusual behavior to the formation of assocd. structures, extended anion-water chains that can weave among the cations in the size disparate case, and strongly bound cation-water-anion clusters in the charge-off-center systems. (c) 2010 American Institute of Physics. [on SciFinder(R)]