Study On The Structure And Properties Of Aqueous Solutions By Spectroscopy And Theory Model

Apparently, aqueous solutions can be widely found in nature and industrial processes and are playing important role in chemical and biochemical fields. The association behavior, which results from the intermolecular interaction, has been detected in many aqueous solution systems. Such special phenomenon not only induce the change in the microcosmic structure, but also result in changes of macroscopical properties of aqueous solutions such as density, viscosity, vapor-liquid equilibrium and so on, which finally affects the application of aqueous solutions in many fields. Actually spectroscopy is an accurate and convenient approach and can offer us abundant information about intermolecular interaction within solutions. Consequently in the present work, the spectroscopy coupled with theory and model was combined to investigate the microcosmic configuration as well as macroscopical properties of various aqueous solutions.We first focused on the local structure of aqueous solution of ionic liquids (ILs) using 1H NMR chemical shifts and the local composition (LC) model. Ionic liquids involving 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) as well as n-butylammonium nitrate (N4NO3) are chosen in this paper as representative of imidazolium-based and ammonium-based ILs. The concentration-dependent chemical shifts of H2O and ILs protons have been obtained by 1H NMR experiments respectively. Then the obtained local mole fraction can be used to analyze the local structure of systems. The results indicated that in IL-rich region the self-association of IL played the dominant role, while water preferred to interact with IL rather than self-association. Instead, in water-rich region the network of water molecules was established, since the self-association of water predominated. This conclusion was highly consistent with others obtained by other methods such as IR, MD simulation and so on.In order to explore the interaction between water and ILs more clearly, we measured the concentration-dependent 1H NMR chemical shifts of water in aqueous solution of ionic liquids, including N4NO3, EmimBF4 and N-methylimidazole acetate [Mim][CH3COO]. Then the effect of ILs on the water structure was analyzed and compared with that of organic compounds as well as inorganic salt. It was revealed that the influence of ionic liquids on the water structure showed two different behaviors:[Mim][CH3C00] leads to the increase ofδ(H2O); however the presence of N4NO3 or [Emim][BF4] makeδ(H2O) decrease gradually in water-rich region. Consequently it can be concluded that ionic liquids can be classified into kosmotropes (structure makers) or chaotropes (structure breaker) according to their effects on the water structure.Vapor-liquid-equilibrium (VLE) is one of the important macroscopical thermodynamic properties of aqueous solutions, which also results from intermolecular interactions. Hereby how to establish the relationship between spectroscopy and VLE data of aqueous solutions is an interesting and hot topic. Actually it has been demonstrated that both of the 1H NMR chemical shifts and Raman frequency shifts can be used to predict VLE data. Therefore, we attempt to make clear the relationship between IR and VLE data. Firstly, we proposed a local composition model which can correlate concentration-dependent C-H IR frequency. Then VLE data were predicted by using the parameter obtained from correlation coupled with one infinite dilute activity efficient.Since all of 1H NMR, Raman and IR spectroscopy can be employed to predict VLE data, we wonder whether there are relationships among these different spectroscopic properties. Based on this, we chose two sorts of solutions:N4NO3 ionic liquid/water system and organic compounds/water systems including DMF/water, acetone/water, methanol/water and ethanol/water. Then we performed the prediction among 1H NMR chemical shifts, Raman frequency shifts and IR frequency by using local composition model. The results demonstrated that Raman and IR spectroscopy, which are vibrational spectra, can be successfully predicted form each other. Moreover there are quantitative relationship between NMR and vibrational spectra.All in all, the theory, model and spectroscopy experiments are combined to investigate the structure and property of aqueous solutions. Such frame of studying on aqueous solutions has been established successfully in this paper. In addition, we found that there was quantitative relationship among different spectroscopic properties. We expect that these methods are applicable to investigate many other kinds of aqueous solutions.