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The influence of molecular ion characteristics on room temperature ionic liquid structure and transport properties Spohr, Heidrun Veronika


Room temperature ionic liquids (RTILs) are pure, organic salts, that are liquid at ambient temperatures. RTILs are highly customizable, with a wide choice of ion types and varying substituents. The customizability of RTILs also poses the greatest challenge, rational design of liquid properties from molecular structure. The influence of RTIL ion characteristics on liquid structure and transport properties is systematically investigated employing molecular dynamics simulations. The characteristics investigated include size disparity, ion charges that are displaced from the center of mass, and variation of the cation-anion charge separation σ'(+−). Different simple spherical ionic liquid models are developed that isolate each of these characteristics. Statistical mechanical analysis shows that more size disparate models have decreased coordination numbers, the diffusion coefficients and electrical conductivity increase, and the viscosity decreases. The effects of size disparity can be canceled to a large extent by decreasing σ'(+−). An increasing displacement of the ion charge leads to the formation of directional ion pairs of increasing strength. Weak pairing results in non-uniform ion distributions and reduced caging, with similar liquid property trends as size disparate systems. A large charge displacement and short σ'(+−) leads to increasing numbers of strong, long-lived directional ion pairs that dominate the liquid behavior. Increasing viscosities and decreasing electrical conductivities are observed in this regime. The temperature behavior of the ionic liquid models deviates from linear Arrhenius behavior, especially for the conductivity. The relationship between diffusion and viscosity conforms to the fractional Stokes-Einstein equation. The qualitative conclusions of our calculations suggest the utilization of ions with moderate charge displacement, and large size disparities, for desired low-viscosity RTILs with large ion mobilities. Mixtures of water with ionic liquid models generally show increased diffusion coefficients and electrical conductivities, and decreased viscosities. The increased mobility of the ions can be mainly ascribed to dynamical effects due to light water replacing heavy ions in the ion coordination shell. We observe deviating behavior when water can form strong, directional interactions with at least one ion, in the case of small, or of strongly charge displaced ions. In both cases the viscosity increases with increasing water concentration.

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