Equilibrium in Protic Ionic Liquids: The Degree of Proton Transfer and Thermodynamic Properties

The degree of proton transfer and thermodynamic parameters of six acetate-based protic ionic liquids (PILs) were measured using nuclear magnetic resonance spectroscopy and calculated by van't Hoff analytical method, respectively. The degree of proton transfer of these PILs at 298 K spread over a large range, which is from 39.6% (1-methylpyrrolidinium acetate, [MpyrH][AcO]) to 94.4% (1-butylimidazolium acetate, [BuimH][AcO]). The calculated standard enthalpy change of the reaction (Δ_rH_m^θ) is from -23.30 to -7.80 kJ mol^-1 and the standard entropy change of the reaction (Δ_rS_m^θ) is from -42.70 to -8.07 J mol^-1 K^-1. The correlation between the degree of proton transfer and aqueous ΔpK_a or Δ_rH_m^θ was investigated as well. Furthermore, in some special cases, Δ_rS_m^θ, especially the entropy change of symmetry, also plays an important role in affecting the degree of proton transfer.