Ionic Association and Solvation of the Ionic Liquid 1-Hexyl-3-methylimidazolium Chloride in Molecular Solvents Revealed by Vapor Pressure Osmometry, Conductometry, Volumetry, and Acoustic Measurements

Abstract

A systematic study of osmotic coefficient, conductivity, volumetric and acoustic properties of solutions of ionic liquid 1-hexyl-3-methylimidazolium chloride ([C(6)mim][Cl]) in various molecular solvents has been made at different temperatures in order to study of ionic association and solvation behavior of [C(6)mim][Cl] in different solutions. Precise measurements on electrical conductances of solutions of [C(6)mim][Cl] in water, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and acetonitrile at 293.15, 298.15, and 303.15 K are reported and analyzed with Barthel's low-concentration chemical model (lcCM) to obtain the limiting molar conductivities and association constants of this ionic liquid in the investigated solvents. Strong ion pairing was found for the ionic liquid in 2-propanol, 1-butanol, and 1-propanol, whereas ion association in acetonitrile, methanol and ethanol is rather weak and in water the ionic liquid is fully dissociated. In the second part of this work, the apparent molar volumes and isentropic compressibilities of [C(6)mim][Cl] in water, methanol, ethanol, acetonitrile, 1-propanol, 2-propanol, and 1-butanol are obtained at the 288.15-313.15 K temperature range at 5 K intervals at atmospheric pressure from the precise measurements of density and sound velocity. The infinite dilution apparent molar volume and isentropic compressibility values of the free ions and ion pairs of [C(6)mim][Cl] in the investigated solvents as well as the excess molar volume of the investigated solutions are determined and their variations with temperature and type of solvents are also studied. Finally, the experimental measurements of osmotic coefficient at 318.15 K for binary solutions of [C(6)mim][Cl] in water, methanol, ethanol, 2-propanol, and acetonitrile are taken using the vapor pressure osmometry (VPO) method and from which the values of the solvent activity, vapor pressure, activity coefficients, and Gibbs free energies are calculated. The results are interpreted in terms of ion association, ion-dipole interactions, and structural factors of the ionic liquid and investigated organic solvents. The ionic liquid is solvated to a different extent by the molecular solvents, and ionic association is affected significantly by ionic solvation.

Ionic association and conductance of ionic liquids in dichloromethane at temperatures from 278.15 to 303.15 K

“…The data are consistent with the classical ionic association theory of electrolytes [28]. Similar behavior of other ionic liquids in various solvents was also observed [1,3,[29][30][31][32][33][34]. In turn, in very low-permittivity tetrahydrofuran (ε r = 7.58), ionic liquid (1-butyl-2,3-dimethylimidazolium tetrafluoroborate [bmmim][BF 4 ]) form not only ion pairs but also triple ions [29].…”

Self-Assembly of Charged Oligomeric Bilayers at the Aqueous/Organic Interface Regulated by Ion-Pair Associations

“…There were also electrostatic repulsive interactions between the positively charged headgroups at the oil-aqueous interface, for both the monolayer and the bilayer. If all the electrolytes had been dissociated in the aqueous phase, then the electrostatic interactions between the headgroups would have been effectively screened at the Debye lengths given above (< 1 nm), however, it is known for the similar MIM (+) :hexane molecule that the ionizable MIM (+) surface sites were not fully dissociated, but were only partially neutralized due to specific binding of Cl (-) anions, with a dissociation constant Kd = 0.275 M. 23 This resulted in a Langmuir-type adsorption profile as a function of the ionic strength. At equilibrium, the effective charge density at the interface was regulated by the law of mass action: 14 MIM (+) + Cl (-) ⇋ MIM-Cl at the oilaqueous interface.…”

Imidazolium Salt Ion Pairs in Solution

“…ÞÞ tematic study of the electric conductance for the HMI·Cl ion pair in several lower alcohols, acetonitrile, and water, this association constant has been shown to follow an almost exponential increase with the inverse of the solvent's dielectric constant. [39] The conductance in mixtures of BMI·PF 6 , BMI·BF 4 and BMI·CF 3 CO 2 with several solvents (ethanol, ethanol + water, acetonitrile, ethyl acetate, THF, acetone, pyridine, dichloromethane, and chloroform) also indicate an increasing tendency of aggregation in lower dielectric environments. [40,41] The ion pair association constants for BMI·Cl and BMI·BF 4 have been established from conductivity measurements in DMSO and methanol.…”