Investigations On The Physical Properties And Microscopic Structures In Ionic Liquids Solutions

The physicochemical properties of ionic liquid (IL) solutions are determinated by their microstructure. Therefore, the study of the structure-activity relationship of IL solutions has theoretical significance and practical applications. It is known that spectroscopy and experiment are two accurate and convenient approaches to study the microstructure and interaction of solutions. Consequently in the present work, the physicochemical properties of IL solutions were measured and used to investigate the microstructure and interaction. On the other hand,1H NMR spectroscopy was employed to investigate the microstructure of IL solutions as well as the physicochemical properties. The following is the main results obtained in this work:Densities and viscosities were determined for the binary mixtures of n-butylammonium acetate (N4AC) or n-butylammonium nitrate (N4NO3) IL with water, methanol, ethanol, n-propanol, and n-butanol at temperatures (293.15,298.15,303.15,308.15, and313.15) K. The thermal expansion coefficient, molecular volume, and standard entropy of N4AC and N4NO3were deduced from the density results, respectively. Excess molar volumes NE and viscosity deviations Δη of the above mixtures were obtained and fitted with the Redlich-Kister equation. The results indicate that VE are all negative and have minimum values when the mole fraction of IL is about xIL≈0.3. The hydrogen bonding between IL and water is more remarkable in N4AC/water and N4NO3/water mixtures. The packing efficiency is dominant in N4AC/alcohol and N4NO3/alcohol mixtures, which decreases with the increasing of carbon number of alcohols. VE of N4AC/water mixtures is more negative than that of N4NO3/water mixtures at the same condition. Δη of the above mixtures are all negative and have minimum values when the mole fraction of IL is about χIL≈0.7. Δη of mixtures containing N4Ac is more negative than that of mixtures containing N4NO3. The absolute value of Δη decreases with the increasing of temperature.Spectroscopic and physicochemical properties of IL solutions are both results from their microstructure. Hereby how to establish the relationship between spectroscopic and some physicochemical properties is an interesting topic. In this work,1H NMR chemical shifts of N4NO3/water,1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4)/water,[Bmim]BF4/ethanol, and1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4)/water mixtures were correlated by local composition model. The viscosities were predicted by using the parameters obtained from1H NMR spectroscopy with satisfactory results. Thus, the relationship between1H NMR spectroscopy and viscosity is established.The relative chemical shifts (A8) were put forward to investigate the microscopic structure of [Emim]BF4during the dilution process with water. The results reveal that the variations of the microscopic structure of three aromatic protons are inconsistent. The strength of H-bond between water and three aromatic protons follows the order:(C2)H…O>(C4)H…O>(C5)H…O. More water position around the (C6)H than that around the (C7)H or (C8)H.Some special phenomena of IL solutions can be illustrated by their microstructure. In this study, the microstructure of ethyl acetate+ethanol+[Bmim]BF4mixtures were carefully investigated by1H NMR spectroscopy to reveal how [Bmim]BF4affected the vapor-liquid equilibria (VLE) property of the azeotropic mixtures. Results from1H NMR spectroscopy studies indicate that the interactions between (C2)H of the cation and O-H functional group of ethanol significantly contributes to enhance the relative volatility of ethyl acetate to ethanol, which is also proven by the VLE experimental and1H NMR spectroscopic results of ethyl acetate+ethanol+1-buty1-2,3-dimethylimidazolium tetrafluoroborate ([Bmmim]BF4) mixtures.In general, microstructural and physicochemical properties of IL solutions are investigated. The relationships between1H NMR and viscosity are established. The mechanism of how IL affects the VLE of ethyl acetate+ethanol azeotropic mixtures is illustrated by1H NMR spectroscopy. We expect that1H NMR spectroscopy can be applied to investigate the structure-activity relationship of other IL solutions.