Study On The Condensed Structures,Properties And Equations Of State Of Imidazolium-based Ionic Liquids Under High Pressure

Ionic liquids are liquids that are composed entirely of ions near room temperature.Ionic liquids have attracted increasing attention due to their unusual properties,which allow their applications as solvents for green processing,highly energetic materials,drug delivery,and novel electrolytes for energy applications,etc.However,their industrial applications may be harmful to the environment due to the toxicity of some ionic liquids,so it is necessary to study its potential recovery and purification methods.The best way of recovery and purification for ionic liquids may be crystallization because of its non-volatility.Generally speaking,there are two ways to realize the crystallization of ionic liquids.One way is crystallization from the melt upon cooling,and the other way is crystallization from hot saturated solution upon cooling or crystallization by evaporation of the solvent.Considering that the effect of pressure is equivalent to that of temperature to some extent in thermodynamics,compression may also induce the crystallization and solidification of ionic liquids.In this paper,we focus on imidazolium-based ionic liquids which have wide application now.The research objects and methods are extended based on the previous studies.The pressure-induced crystallization and solidification of ionic liquid from the melt and solution have been studied systematically using in-situ spectroscopy and synchrotron radiation X-ray diffraction measurement.Their condensed structures and phase transitions behavior have also been investigated under high compression,which will promote the basic research and application of ionic liquids under high pressure and provide a new idea for the recovery and purification of ionic liquids.Furthermore,the studies of the properties and equation of state of ionic liquids are limited within a narrow range of pressure,and the methods to gain the properties of ionic liquids have been developed in this paper.Their properities have been obtained and the form of the equation and related thermodynamic parameters are determined in an extended range of pressure,which will be helpful to enrich the physical properities and better understand the thermodynamic properties and phase behavior of ionic liquids.The specific contents are as follows.?1?The crystallization and solidification of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate?[C₂mim][CF₃SO₃]?fromthemelt,1-ethyl-3-methylimidazolium hexafluorophosphate?[C₂mim][PF₆]?from methanol solution and mixed solution of 1-butyl-3-methylimidazolium hexafluorophosphate?[C₄mim][PF₆]?and 1-butyl-3-methylimidazolium tetrafluoroborate?[C₄mim][BF₄]?have been investigated under high pressure using in-situ Raman spectroscopy.For[C₂mim][CF₃SO₃],two crystalline phases formed at about 1.3 and 1.7 GPa with increasing pressure at lower compression rate of about 0.3 GPa/h;however,the amorphous phase solidified with superpressurized glass above 3.3 GPa at higher compression rate of about 1.2 GPa/h.Compression rate might make great contribution to kinetically driven phase transitions,and it was necessary to be considered for pressure-induced phase transitions of ionic liquids using diamond anvil cell.For[C₂mim][PF₆]in methanol solution,the original crystal and the newly grown crystal in the solution showed different phases under high pressures up to 1.4 GPa;however,the crystal obtained by recrystallization showed similar phase to that of the former newly grown crystal.Our findings realized the crystallization of ionic liquids from solution under high pressure,which might facilitate the development of an effective way for crystallization and purification of ionic liquids under high pressure.For mixed solution of[C₄mim][PF₆]and[C₄mim][BF₄]by different mole ratios,two crystalline phases formed near 0.6⁰.9 GPa and 2.4².7 GPa,which showed the preference of gauche-trans?GT?conformers and an unknown cation conformer when the mole ratio of the mixed solution is 8:1.However,the mixed solution with mole ratio of 1:1 did not crystallize up to 2.4 GPa.Compared with the amount of[PF₆]^-,a small amount of[BF₄]^-did not change the structure of[C₄mim]⁺under high pressure,and[C₄mim][BF₄]which was difficult to crystallize participated in the pressure-induced crystallization process.Besides,equal amount of[BF₄]^-inhibited the packing of ions of the mixture,and[C₄mim][PF₆]which was easy to crystallize did not crystallize.?2?Thevitrificationof1-ethyl-3-methylimidazoliumethylsulfate?[C₂mim][EtOSO₃]?and[C₄mim][BF₄]have been studied by in-situ Raman spectroscopy combined with the broadening of the sharp ruby R₁ fluorescence line.[C₂mim][EtOSO₃]solidified from the melt to a superpressurized glass at about 2.4GPa,which was different from the glassy state at low temperature.The hydrostatic properties of[C₄mim][BF₄]were firstly studied systematically by synchrotron X-ray diffraction measurements,ruby fluorescence measurement and quantitative measurements of pressure gradients.And the results suggested that[C₄mim][BF₄]had hydrostatic limit up to about 6 GPa and quasi-hydrostatic to about 21 GPa,which indicated that this kind of ionic liquid can be used as a new kind of hydrostatic pressure transmitting medium in high pressure experiment.?3?Thecondensedstructureof1-alkyl-3-methylimidazolium bis?trifluoromethylsulfonyl?imide?[C_nmim][NTf₂]??n=2,6?under high pressure has been investigated by in-situ Raman spectroscopy and synchrotron X-ray diffraction measurements.[C₂mim][NTf₂]experienced two phase transitions and two crystalline phases formed at about 0.6 GPa and 2.0 GPa,when the pressure was increased in steps at lower compression rate of about 0.4 GPa/h.The difference of the structure between the two crystalline phases was small,which might result from the change of intermolecular interaction.When the pressure was increased to 2.5 GPa in a single step,[C₂mim][NTf₂]did not crystallize,even after long enough time.[C₆mim][NTf₂]underwent glass transition when the pressure was increased at lower compression rate of about 0.4 GPa/h.The change of cation conformers under high pressure was more obvious than that of anion conformers,which was related to the flexibility of alkyl chain of the cation.?4?The properties of imidazolium-based ionic liquid under high pressure were obtained in an extended range of pressure,and the common forms of equation of state were used to fit the PVT data of ionic liquids firstly within 3 GPa.The viscosity of1-ethyl-3-methylimidazolium tetrafluoroborate?[C₂mim][BF₄]?was obtained up to1.3 GPa utilizing the falling-ball method in the diamond anvil cell.According to the correlation between the ratios of Raman intensity and the concentrations,a novel and simple method of quantitative measurement of solubility under high pressure was provided,and the solubility of[C₂mim][PF₆]in methanol up to 2 GPa was obtained.The densities of[C₄mim][PF₆]and[C₄mim][BF₄]were obtained by using diamond anvil cell and the piston cylinder respectively,and the refractive index of[C₄mim][BF₄]was further obtained under high pressure.For[C₄mim][BF₄],The PVT data based on two experiment methods and literature were fitted or extrapolated with the Tait equation.The fitting results of them were approximate,which suggested that the density data of relatively lower pressure could be extrapolated to predict the density of higher pressure using the Tait equation.Meanwhile,the related parameters of equation of state were determined and the isothermal compression rate?_Twas further obtained.For[C₄mim][PF₆],the liquid and solid state were respectively fitted using the Tait equation and the second-order Birch-Murnaghan equation,and the related parameters and thermodynamic parameters were determined.