Ion Association in [bmim][PF₆]/Naphthalene Mixtures: An Experimental and Computational Study

Abstract

Abstract: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] [PF 6]. 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.

Interaction of Room Temperature Ionic Liquid Solutions with a Cholesterol Bilayer

“…From time to time, we observe molecularly thin whiskers protruding from the IL film into the water and consisting of a chainlike arrangement of neutral cation-anion pairs. Previous studies have shown that in ILs, these structural motifs arise from the association of ions into neutral polar molecules 22 bound by dipolar forces into chainlike aggregates. In most cases, whiskers emerge and return to the IL film, thus giving rise to closed dipolar loops.…”

Three commentaries on the nano-segregated structure of ionic liquids

“…Somewhere in the middle we must have another type of percolation limit, where the polar network starts to disrupt and starts to form isolated ionic aggregates. One of the first studies to address this problem [19] studied solutions of [C 4 mim][NTf 2 ] in naphthalene and concluded that as the concentration of the ionic liquid diminished, the polar network started to change from a globular to a more filamentous . From left to right: associative solute (water) that interacts mainly with the polar network; dipolar solute (acetonitrile) that interacts at the polar-non-polar interface; and non-polar solute (n-hexane) that interacts mainly within the non-polar domains.…”

Imidazolium Salt Ion Pairs in Solution

“…This conductivity maximum was attributed to the gradual transition of the IL component from an ionic state at higher x to a dipolar fluid built by neutral contact ion pairs at low x. [37] Some conductivity measurements were supported by molecular dynamics (MD) simulations. The electrical conductivity has been determined for mixtures of the IL EMI·NTf 2 with chloroform.…”