Determination of the Ion Association Constant in Binary Solutions of (Methanol + Phosphonium Ionic Liquid) and (Methanol + Imidazolium Ionic Liquids)

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Raymond, Colby
Ionic Liquids , Low Concentration Chemical Model
Ionic liquids (ILs) are an exciting area of research since they encompass many of the ideals of the green chemistry initiative. Moreover, they can be easily tailor-made through ion exchange chemistry to create a desired ion pair. Trihexyltetradecylphosphonium chloride (PIL-Cl) is widely known for its electrolytic properties but its physicochemical properties have not been thoroughly characterized. Similarly, long chain (>C_6) imidazolium ILs also have not been extensively characterized, in contrast to the shorter chained (C­_2 – C_6) analogs. The purpose of our work is to determine the association constants of binary solutions composed of methanol (MeOH) + IL using several ILs that include PIL-Cl, 1-hexyl-3-methylimidazolium chloride (C_6MIM-Cl), 1-decyl-3-methylimidazolium chloride (C_10MIM-Cl) and 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C_10MIM-SA). Conductivity has been measured for solution concentrations of < 1 mmolal to ~13 mmolal over a temperature range of 20.0oC to 40.0oC in 5.0o increments. The conductivity data was analyzed in the framework of Barthels Low concentration Chemical Model (LcCM) to calculate the IL association constants in MeOH. In addition to conductivity measurements, we also determined the association constant for PIL-Cl using the voltaic response of a chloride ion-selective electrode to assess the utility and validity of this approach.