Determination of the vaporisation enthalpies and estimation of the polarity for 1-alkyl-3-methylimidazolium propionate {[Cnmim][Pro](n = 2, 3)} ionic liquids

• Two ILs [Cnmim][Pro] (n = 2, 3) (1-alkyl-3-methylimidazolium propionate) were prepared.
• The vaporization enthalpies for the two ILs were determined using a thermogravimetric approach.
• A new scale of polarity, δμ, for the ILs was put forward.

Two propionic acid ionic liquids (PrAILs) [Cnmim][Pro] (n = 2, 3) (1-alkyl-3-methylimidazolium propionate) were prepared by the neutralisation method and characterised by 1H NMR spectroscopy and differential scanning calorimetry (DSC). The values of vaporisation enthalpies, ΔvapHmo, for two PrAILs [Cnmim][Pro] (n = 2, 3) were determined using a simple, convenient, and highly reproducible thermogravimetric approach. The enthalpy of vaporisation, ΔvapHmo, for ionic liquids (ILs) is assumed to be composed of two parts: one part corresponds with the induced energy, ΔvapHmo(n), and the other part corresponds with orientation energy from permanent dipole moment of ion pair in ILs, ΔvapHmo(μ). Using the values of ΔvapHmo, ΔvapHmo(n) and ΔvapHmo(μ), cohesive energy density, δ2 (δ   is the solubility parameter), the contribution of inducted energy, δn2, and the contribution of orientation energy, δμ2, were obtained. If a liquid only has δn then it is a non-polar liquid, and if a liquid not only has δn also has δμ then it is a polar liquid. Since the ion pairs in ILs have permanent dipole moment, ionic liquid has a certain polarity. Therefore, using δμ as the polarity scaling of ILs is very convenient because the values of δμ are very easy to calculate from the enthalpy of vaporisation and refractive index data.