Physico-Chemical Properties Study Of Allyl Ionic Liquids In Water And Methanol Solutions At Dilute Region

Since the end of last century,ionic liquids have been widely concerned by researchers due to their excellent properties,and have been tried to apply in various fields,which is considered as a kind of"green solvent"because of the potential to replace traditional volatile organic solvents.One of the most attractive features of ionic liquids is the ability to modify their structure,by modifying the structure of cations or choosing different types of anions,to improve some aspects of their properties and make them suitable for specific applications.Imidazole is widely used as the cationic part of ionic liquids due to its low cost and simply to modify the length,number and functional groups of alkyl side chains on its nitrogen atoms.Among many types of anions,dicyandiamide anions have the advantages of low symmetry and high charge delocalization,ionic liquids composed of it usually have ideal melting point,dynamic viscosity and conductivity.In addition,adding flexible substituents,such as allyl group,to cations of ionic liquids can not only improve the fluidity and conductivity,but also improve other properties of ionic liquids.In practice,ionic liquids are often mixed with some organic solvents to adjust the properties of the system,reduce the application cost,and thus expand its application scope.Early studies mostly focused on the synthesis of ionic liquids and used it as reaction medium,however,with the continuous development of subsequent applications of ionic liquids,people pay more and more attention to their physicochemical properties,based on the basic properties,such as density,viscosity,conductivity and refractive index,the relevant parameters and energy can be calculated to study the interactions in the system,which is of great significance to the design and application of ionic liquids.In this paper,we synthesized two allyl functionalized ionic liquids1-allyl-3-ethylimidazoledicyandiamidesalt[AEim][DCA]and1-allyl-3-ethylimidazol tetrafluoroborate[AEim][BF₄],and prepared dilute solutions from its with water or methanol,respectively.The Physical and Chemical properties of the dilute solutions are explored.The details of this paper are as follows:1.Two allyl functionalized ionic liquids,[AEIM][DCA]and[AEIM][BF₄]were synthesized from 1-ethylimidazole,3-bromo-1-propene and sodium dicyandiamide or sodium tetrafluoroborate by a two-step reaction.The structure of the product was characterized by ~1H NMR and ¹³C NMR,and it was determined that the structure of the product was consistent with the structure of the target ionic liquid.In addition,the water content of the dried product was analyzed by Karl Fischer method,which showed that the purity of the product was more than 99%.2.The ionic liquids synthesized in this paper,[AEim][DCA]and[AEim][BF₄],are water-soluble ionic liquids,so the standard addition method（SAM）is adopted to eliminate the influence of residual moisture after drying on the experiment.The densityρ,viscosityη,conductivityσand refractive index n _Dvalues of[AEim][DCA]and[AEim][BF₄]with different water contents have been measured in the temperature range of 288.15 K～313.15 K at atmospheric pressure,and they are linearly fitted as a function of water content to derive the values ofρ,η,σand n _Dof[AEim][DCA]and[AEim][BF₄]at the water content are 0 ppm in the same temperature range.On this basis,theρ,η,σand n _Dvalues of the two pure ionic liquids were compared and discussed.3.According to the gravimetric method,ionic liquids[AEim][DCA]and[AEim][BF₄]were prepared into dilute solutions with water and methanol,respectively,and the mass molar concentration of ionic liquids m₁ranged from 0.1000to 1.0000 mol·kg^-1.Theρ,η,σand n _Dvalues of pure solvent and dilute solution were measured at 288.15 K～313.15 K and atmospheric pressure.The results show that the methods and instruments used in the determination ofρ,ηand n _Dvalues are reliable.4.Based on the measuredρdata of dilute solution,the excess molar volume V^Eof the them at different temperature were calculated,the V^Eand mole fraction of ionic liquid x₁were fitted by Redlich-Kister polynomial equation.The positive or negative values and their variation trends of V^Eof different systems were compared,and the effects of temperature and concentration of ionic liquid on them were analysed.The results show that the interaction between different molecules is weaker than the that between the same molecules in the system of ionic liquid and water,however,there may be hydrogen bond interaction between different molecules in the systems of ionic liquids with methanol,and effective filling caused by the large difference between the molecular volume of solute and solvent.5.Based on theηdata,the viscosity B coefficient of ionic liquids in four dilute solutions were calculated by Jones-Dole empirical equation,and the influence of ionic liquids on the structure of water and methanol solvents was discussed base on it.The results show that there is certain interaction between solute molecules and solvent molecules in the four ionic liquid systems studied in this experiment,and the strength of it between ionic liquid and methanol molecules is greater than that between ionic liquid and water molecules,which is confirmed by the results obtained from V^E.The Eyring’s transition state theory and relative viscosityη_rare introduced into the viscosity values of the dilute solutions,and the Gibbs free energy of excess viscous flow activationΔG^≠Eof the dilute solutions were derived and calculated based on it,the obtainedΔG^≠Ewere are fitted with x₁by Redlich-Kister polynomial equation,and the fitting result is good.The results about interaction force obtained from theΔG^≠Eare agreement with that obtained from viscosity B coefficient,which indicates that theΔG^≠Eis reasonable.In addition,the activation enthalpy and entropy of the excess viscous flow are calculated according to the relationship betweenΔG^≠Eand temperature,the results show that theΔG^≠Emainly comes from the contribution of activation enthalpy.6.Based on theσdata,the molar conductivityΛ_mwas calculated and the limit molar conductivityΛ₀was obtained by fitting theΛ_mwith concentration of ionic liquid according to Onsager equation.And theΛ_mof the four dilute solutions decreases with the increase of ionic liquid content and increases with the increase of temperature,theΛ₀also increases with the increase of temperature.The results show that in the dilute solution studied in this paper,although the cations and anions of ionic liquids are solvated by water and methanol,interactions between ions are weakened,but has not been completely destroyed,and its strength increases with the increase of concentration of ionic liquids,which lead to ion association,relaxation and electrophoresis effect between them.In addition,theηandσof the system were discussed by Walden rule,and the results showed that the ionic liquids studied in this paper have structural damage characteristic on the original solvent system.7.Based on the n _Ddata,the molar refractive index R_Dis calculated,and the dispersion force in the system is discussed base on it.The results show that there is a certain degree of dispersion force between the two ionic liquids with water or methanol solvents at higher temperatures.In addition,for the same ionic liquid,the dispersion force between it with methanol solvent molecules is greater than that between it with water solvent molecules.