Phase Transition And Properties Of Several Imidazole Ionic Liquids At High Pressure

In this paper,the emerging green organic compounds—imidazole ionic liquids are studied as the research object,using high pressure diamond anvil cell(DAC)technology and relative supporting experimental equipments.In this paper,the high-pressure induced structural transitions and properties of three imidazoles ionic liquids—1-ethyl-3-methylimidazolium chloride([EMIM]Cl),1-butyl-3-methylimidazolium chloride([BMIM]Cl)and 1-ethyl-1-methylimidazolium bromine([BMIM]Br)are studied in detail by high pressure in-situ Raman spectroscopy,infrared spectroscopy,synchrotron radiation XRD and mass spectrometry.The investigations have shown that 1-ethyl-3-methylimidazolium chloride undergoes at least four successive structural transitions at around 5.8,9.3,15.8,and 19.1 GPa,respectively.On the basis of the structural transition induced by the change of the alkyl chain caused by high pressure,the ion stacking method determind by the hydrogen bonds and the chemical reaction caused by the high pressure are also the important reasons leading to the structural transitions,which are put forward in this paper.At the same time,the process of converting the cation of the sample from non-planar(Np)to planar(P)induced by high pressure is investigated in detail.Since the initial[EMIM]CI is amorphous and the conventional heating recrystallization method does not crystallize the amorphous[EMIM]Cl,DAC-loaded sample is crystallized by treating it at 150 ? for 12 hours in a furnace before cooling it down to room temperature for 2 hours therein.During this process,the cation has completely transformed from the Np conformation to the P conformation,which indicates that the cation in the plane is more favorable for the crystal structure.In addition,a sudden emergence of photoluminescence was observed at around 20 GPa whether[EMIM]Cl heated or not.And with pressure,the intensity of photoluminescence continues to increase.New peaks appear in the Raman and infrared spectroscopy at this pressure.Therefore,we speculate that the sample begins to polymerize at this pressure,which is further confirmed by mass spectrometry experiments.The photoluminescence is indeed caused by high-pressure induced by cationic polymerization.The[EMIM]Cl undergoes four successive structural transitions with all of the new structures are orthogonal phase and differet space group.In the study of[BMIM]Cl and[BMIM]Br,our work focuses on the conformation isomerization of cations in these two ionic liquids.The theoretical calculations show that the[BMIM]+ cations have four conformation of TT,TG,GT and GG.The[BMIM]Cl and[BMIM]Br are TT and GT conformation under ambient conditions,respectively.In the experiment,we find that the cations of[BMIM]Cl at atmospheric pressure are TT conformation.And when the pressure reache 3.4 GPa,the TT,TG and GG conformations coexist.However,the[BMIM]Br maintains the GT conformation at both atmospheric pressure and high pressures under 13.7 GPa.In addition,the space group of[BMIM]Cl and[BMIM]Br changes with the change of the cationic conformation.The[BMIM]Cl is monoclinic symmetry at ambient condition and undergoes structural transition when the pressure reaches 3.4 GPa.It is converted into an orthorhombic symmetry.The[BMIM]Br keeps orthogonal symmetry structure in the same pressure range.However,the cations of[BMIM]Cl totally transform into the GT conformer,if the DAC-loaded sample is treated at 150 ? for 12 hours in a furnace before being cooled down to room temperature.The cations maintain the GT conformer with pressure.However,the[BMIM]Br maintains the GT conformation under the same heat treatment.When[BMIM]Cl and[BMIM]Br have the same conformation,they have the same structure—orthorhombic system.Therefore,the conformation of the cations determines the stacking of the anions and cations,and then determinds the crystalline structure of the[BMIM]Cl and[BMIM]Br.