Local Structure Evolution
and its Connection to Thermodynamic and Transport Properties of 1-Butyl-3-methylimidazolium
Tetrafluoroborate and Water Mixtures by Molecular Dynamics Simulations
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Abstract
Our recently developed improved united atom force field
shows a good quality to reproduce both the static and transport properties
of neat ionic liquids (ILs). Combined with the TIP4P-Ew water model,
the force field is used to simulate the mixture of 1-butyl-3-methylimidazolium
tetrafluoroborate ([C<sub>4</sub>mim][BF<sub>4</sub>]) and water without
further optimization to adjust any cross parameters. Liquid densities
of the mixture are well predicted over the entire concentration range
at temperatures from 298.15 to 353.15 K. Simulations also reproduce
the positive values of excess volumes and excess enthalpies, as well
as their increase with temperature. The simulated viscosities are
in good agreement with experimental values, especially in the water-rich
region. We found three distinct regions by analyzing the concentration
dependent self-diffusion coefficients via Stokes–Einstein (SE)
relation, indicating the mixture experiences significant microheterogeneity
with the adding of water. This observation is well connected to the
structure features obtained in simulations, such as radial distribution
functions (RDFs), spatial distribution functions (SDFs) and water
clustering analysis. At the water mole fraction (<i>x</i><sub>2</sub>) less than 0.2, most of the water molecules are isolated
in the polar cation–anion network in ionic liquids. With the
increase of <i>x</i><sub>2</sub> from 0.2 to 0.8, large
water cluster forms and eventually percolates the whole system. When <i>x</i><sub>2</sub> > 0.8, ionic liquids show a moderate degree
of aggregation (with maximum around 0.9 to 0.95) before the cations
and anions are fully dissolved in water