| Abstract [eng] |
Ionic liquids are ionic materials characterized by low melting temperature. Because of unique physical properties, including negligibly low volatility, high eletrical conductivity and viscosity, ionic liquids are used in organic synthesis, investigation of nanomaterials, electrochemistry. One of the most researched aspects of ionic liquids are their intermolecular structure, notable for its nanoheterogeneity. Ionic liquid and water mixtures are even more complex molecular systems. One of the open scientific problems related with intermolecular structure of ionic liquid-water systems: formation of nanoscale water pockets in these mixtures. Objective of this study is to extract information about intermolecular structure of 1-butyl-3-methylimidazolium nitrate [C4mim][NO3] ionic liquid and its various composition mixtures with water by performing molecular dynamics (MD) and quantum mechanics / molecular mechanics (QM/MM) simulations. Research was started by preparation of three molecular ensembles of pure [C4mim][NO3] for MD modelling. NPT simulations were performed using different force field parameters published in scientific articles. The most suitable force field was selected by calculating densities and analyzing their dependence on temperature. Then NVT and NPT simulations were carried out for four different systems: pure ionic liquid and its three mixtures with water (water molar fractions are 20 %, 50 % and 80 %). Radial distribution functions and coordination numbers for atoms of NO3– ions and H2O molecules around three hydrogen atoms of imidazole ring of C4mim+ ion were calculated. Furthermore, angular distribution analysis was performed and probabilities of formation of different-sized water clusters were evaluated. Finally, QM/MM simulations for C4mim+ ions and H2O molecules were performed. 1H NMR (nuclear magnetic resonance) shielding constants and chemical shifts were calculated from QM/MM results and their values were compared with experimental data of analogous [C4mim][NO3]-water mixtures. Radial and angular distribution analysis provided the information that both NO3– and H2O directionally coordinate around all three hydrogen atoms of imidazole ring, but the strongest coordination is at 2nd position of imidazole ring. Several different orientations of NO3– ir H2O coordinating to imidazole ring were observed. Directional coordination at cation is weakened by the increased water molar fraction in [C4mim][NO3]-water mixtures. In the case of high water content in mixtures, intermolecular structure and physical properties are reliant on anion-water and water-water interactions rather than cation-anion and cation-water interactions. Water pockets were not observed in [C4mim][NO3]-water mixtures, depending on the results of water clusterization analysis. When water molar fraction is lower, small clusters of few water molecules predominate and when water molar fraction reaches 80 percent, nearly all of water molecules form a continuous network. 1H NMR parameters of C4mim+ ion and water, that were calculated using QM/MM modelling, were not accurately evaluated, based on experimental data and trends when water molar fraction is increased. Nevertheless, QM/MM modelling methodology can be improved in several different ways to achieve better accordance with experimental data. |
