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

Abstract

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 (ΔH) is from -23.30 to -7.80 kJ mol and the standard entropy change of the reaction (ΔS) is from -42.70 to -8.07 J mol K. The correlation between the degree of proton transfer and aqueous ΔpK or ΔH was investigated as well. Furthermore, in some special cases, ΔS, especially the entropy change of symmetry, also plays an important role in affecting the degree of proton transfer.

Experimental Evidence for Azeotrope Formation from Protic Ionic Liquids

“…A correct evaluation of acid and base strength should be done in the PIL medium, where the proton transfer equilibrium is established. In PILs the existence of neutral molecular species reduces ionicity (fraction of ions in the liquid that can effectively conduct electricity) and consequently the ionic conductivity ,. Recent progress has been made on the measurement of the proton transfer equilibrium in PILs: the results indicate that PILs are generally mostly ionic, but still contain a significant proportion of neutral species ,.…”

“…In PILs the existence of neutral molecular species reduces ionicity (fraction of ions in the liquid that can effectively conduct electricity) and consequently the ionic conductivity ,. Recent progress has been made on the measurement of the proton transfer equilibrium in PILs: the results indicate that PILs are generally mostly ionic, but still contain a significant proportion of neutral species ,. The extent of the acid‐base equilibrium in PILs adds complexity to the system regarding their properties and speciation, i. e. the molar fraction of all species (molecules, ions, and ion pairs) in equilibrium at a given set of conditions.…”

Effects of ionicity and chain structure on the physicochemical properties of protic ionic liquids

“…50 They found that the viscosity increased with increasing ionicity, and the conductivity was affected by the concentration and the diffusion constant of the ions. 47 From those reported viscosities (5.6 × 10 −3 and 1 × 10 −3 PaÁs at 353 K) and conductivities (0.4 and 0.1 SÁm −1 at 300 K), 5,32 the PIL with higher ionicity also has higher conductivity.…”

“…Also, the effects of conjugated structures, substituents, and entropies of symmetry for the cations on the ionicities have been studied. 47 In order to understand how ionicity influences the physicochemical properties of ILs, Watanabe and coworkers measured the thermodynamic property, viscosity, density, conductivity, and selfdiffusion coefficients of the anion and cation for 1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([Rmim] [(CF 3 SO 2 ) 2 N], R = methyl, ethyl, butyl, hexyl, and octyl). 48 They found that the ionic self-diffusion coefficient was negatively correlated with the viscosity, and the ratio of molar conductivities determined by electrochemical impedance spectroscopy and by PGSE-NMR with increasing alkyl chain length.…”

Understanding the Role of Protic Ionic Liquids (PILs) in Reactive Systems: Rational Selection of PILs for the Design of Green Synthesis Strategies for Allylic Amines and β‐Amino Esters

“…Several reports have demonstrated the presence of species other than the PILs that strongly affect their properties and performance as solvents . In many cases, the type of H‐bonded network that is present in the PIL causes a mixture of ions and neutral species, together with acid‐base pairs, leading to very particular properties . Besides, the molar relations between neutral and ionized species that conform the PIL may vary depending of the used synthesis procedure, as it was shown for example by D. Rogers and co‐workers for triethylamine and acetic acid (AcOH) and by D. Walsh and co‐workers for ammonium‐based triflates and trifluoracetates …”