Ion association in [bmim][PF6]/naphthalene mixtures: An experimental and computational study

DEL PÓPOLO, M.G.; MULLAN, C.L.; HOLBREY, J.D.; HARDACRE, C.; BALLONE, P.

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

AMER CHEMICAL SOC

Año: 2008 vol. 130 p. 7032 - 7041

0002-7863

Mixtures of room temperature ionic liquids (IL) with neutral organic molecules provide a valuable testing ground to investigate the interplay of the ionic and molecular-dipolar state in dense Coulomb systems at near ambient conditions. In the present study, the viscosity η and the ionic conductivity σ of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6])/naphthalene mixtures at T = 80°C have been measured at 10 stoichiometries spanning the composition range from pure naphthalene to pure [bmim][PF6]. The viscosity grows nearly monotonically with increasing IL mole fraction (x), whereas the conductivity per ion displays a clear peak at x ≈ 15%. The origin of this maximum has been investigated using molecular dynamics simulations based on a classical force field. Snapshots of the simulated samples show that the conductivity maximum is due to the gradual transition in the IL component from an ionic state at high x to a dipolar fluid made of neutral ion pairs at low x. At concentrations x < 0.20 the ion pairs condense into molecular-thin filaments bound by dipolar forces and extending in between nanometric droplets of IL. These results are confirmed and complemented by the computation of dynamic and transport properties in [bmim][PF 6]/naphthalene mixtures at low IL concentration. © 2008 American Chemical Society.