The Study On The Ionicity And Equilibrium Of Ammonium Acetate Protic Ionic Liquid

There is a chemical equilibrium between neutral molecular and ionic species in protic ionic liquids (PILs) due to the proton transfer may be less than complete. Consequently, it is important to have techniques that enable a measure of the amount of the ionic and molecular species. However, there is currently no standard method for determining the ionicity of PILs as short of method to distinguish the ionic and molecular species and to do quantitative works, let alone the equilibrium constant. In this work, the specific ionicity and equilibrium properties of alkylammonium acetate (ethylammonium acetate and n-propylammonium acetate), dialkylammonium acetate (diethylammonium acetate and di-n-propylammonium acetate), and trialkylammonium acetate (triethylammonium acetate and tri-n-propylammonium acetate) are obtained by the Attenuated Total Reflection Fourier Transform Infrared spectroscopy, as well as equilibrium constant.Molecular species are directly observed in the IR spectra over the range1200-1800cm-1, and the ratio of ionic and molecular species can be quantitatively calculated by the area of characteristic absorption peaks, which calibrated based on the NaAc/HAc solution. The results show that over90%ionic species are found in alkylammonium acetate and dialkylammonium acetate, but no more than20%ionic species in trialkylammonium acetate, indicating that the structures of cations play a more important role in the ionicity of ammonium acetates while ApKa values go close. In other words, the more alkyl substituent in cation makes it more difficult for the proton to transfer from the anion to cation.It is interesting to find that both chemical and phase equilibriums are existing in triethylammonium acetate (ionicity<20%) prepared by mixing equimolar amounts of acetic acid and ammonium as it is separated into two layers (rich amine layer and rich PIL layer) which are characterized by1H NMR, respectively. The triethylammonium acetate could be obtained only in the form of the down layer of the equimolar synthesized solution. The relationship between chemical and phase equilibriums explored through in-situ infrared spectroscopy, pH titration, and conductivity titration measurements illustrates that the single triethylammonium acetate layer could be separated into two layers with both chemical and phase equilibrium again when the mole ratio of added triethyamine to theoretical triethylammonium acetate reaches to0.12. Furthermore, we apply the ionicity data to calculate the equilibrium constant.