Acetonitrile Boosts Conductivity of Imidazolium Ionic Liquids

Abstract

We apply a new methodology in the force field generation (Phys. Chem. Chem. Phys.2011, 13, 7910) to study binary mixtures of five imidazolium-based room-temperature ionic liquids (RTILs) with acetonitrile (ACN). Each RTIL is composed of tetrafluoroborate (BF(4)) anion and dialkylimidazolium (MMIM) cations. The first alkyl group of MIM is methyl, and the other group is ethyl (EMIM), butyl (BMIM), hexyl (HMIM), octyl (OMIM), and decyl (DMIM). Upon addition of ACN, the ionic conductivity of RTILs increases by more than 50 times. It significantly exceeds an impact of most known solvents. Unexpectedly, long-tailed imidazolium cations demonstrate the sharpest conductivity boost. This finding motivates us to revisit an application of RTIL/ACN binary systems as advanced electrolyte solutions. The conductivity correlates with a composition of ion aggregates simplifying its predictability. Addition of ACN exponentially increases diffusion and decreases viscosity of the RTIL/ACN mixtures. Large amounts of ACN stabilize ion pairs, although they ruin greater ion aggregates.

Imidazolium Salt Ion Pairs in Solution

“…[90] In the case of ILs with the BMI cation and anions containing CN groups, MD simulations on the mixtures with water exhibit increased cation-anion coordination for ILs with more CN groups in the anion structure especially at lower IL molar fractions. [91] Chaban et al [36] devised MD simulations for the transport properties in binary mixtures of the ILs containing 1-alkyl-3-methylimidazolium cations and tetrafluoroborate anions with acetonitrile. Especially in the case of low mole fractions of the ILs EMI·BF 4 and BMI·BF 4 , the structural analysis indicated the existence of ion pairs and smaller ion aggregates being responsible for the boost-up of the electric conductivity in these mixtures as observed experimentally.…”

“…The concentration dependence in the radial distribution functions for distances between the cation's and anion's centre of mass exhibit increasing first peak amplitudes upon dilution of the ILs with acetonitrile. [36] Blocking the C(2)ÀH proton at the imidazolium cations by a methyl group, it is expected that the cation's hydrogen-bond donation to the anion and solvent molecules is significantly reduced. Using ion pairs of ILs containing the BMMI cation and anions such as chloride, prolinate, formiate, and bicarbonate, stable contact ion pairs have been maintained in chloroform, transient ion pairs in DMSO, and instantaneous ion pair dissociation has been observed in water.…”

“…Moreover, the radial distribution functions show that the molecular solvent stabilizes contact ion pairs, whereas the ion clusters of larger size are destroyed, as suggested by molecular dynamic simulations. [36] Therefore, the conductivity of imidazolium ILs correlates with a composition of ion aggregates in solution. Note that neat imidazolium ILs display apparently the opposite behaviour.…”

Proton transfer and esterification reactions in EMIMOAc-based acidic ionic liquids

“…5 mL of the vapor in the headspace was injected (splitless) onto the column. The oven temperature was ramped from 35 C to 130 C during the chromatography. Ions with expected m/z were extracted and chromatographic peak areas were used to calculate the vapor pressure.…”

“…31,34 On the other hand, IL solution conductivities increase when a non-ionic molecular compound (nonelectrolyte or very weak acid) such as acetonitrile, methanol, or water is mixed with ILs, even though the overall ion concentration reduced. 29,32,35,36 The addition of formic acid and other carboxylic acids affects the conductivity and the viscosity of the IL solutions in a manner similar to non-electrolytes or weak electrolytes. Based on the above empirical discussion, the effects of carboxylic acids addition, including formic acid, to the ILs are most likely dilution effects.…”