Studies On The Aggregation And Microstructure Of Imidazolium Ionic Liquids In Solutions

In recent years, room temperature ionic liquids (ILs), as a class of novel media and functional materials, have been developed rapidly. Owing to their low vapor pressure and negligible volatility, ionic liquids have been used as environmentally benign solvents to replace the conventional organic solvents, and found wide applications in chemical synthesis, catalysis, biochemistry, extraction and separation science, and material preparation. As a part of the project supported by the National Natural Science Foundation of China (No. 20873036), the major content of the present work is as follows.1. Ten kinds of organic solvents, including ethylene glycol, dimethylsulfoxide, diethylene glycol, triethylene glycol, formamide, acetonitrile, methanol, ethanol, 1-propanol and acetone, have been selected to study the influence of organic additives on the aggregation behavior of [C12mim]Br in water by conductivity, fluorescence and dynamic light scattering techniques. It is showed that all the studied organic additives weaken aggregation capability of the IL in water, which can be explained by the solvophobic power of alkyl chains in mixed solvents. Solvophobic parameters (Sp) have been used directly to measure the solvophobic power of alkyl chains of the IL. A smaller Sp value disfavors aggregation of the IL in solutions. Therefore, a right solvophobic parameter can be selected to modulate the aggregation behavior of the IL in water.2. The concentrations for the transition of aggregates structures in aqueous solution of ionic liquids [Cnmim]Br (n= 6, 8, 10, 12, 14) have been determined in the wide concentration range by fluorescence, surface tension and 1H NMR at 298.15K. At the same time, morphology and size distribution of the ILs aggregates have been determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The molecular packing parameter theory has been used to predict the microstructure of ILs aggregates. The effect of alkyl chain length of cation and concentration of the ILs on the microstructure of aggregates was also examined. It is found that alkyl chain length dose not affect the morphology of the aggregates but affect the size of the aggregates with the increase of the IL concentration, the structure of the aggregates changes from spheres micelle to cylinders micelle and vesicles for ionic liquids [C10mim]Br, [C12mim]Br and [C14mim]Br.3. 1H NMR, dynamic light scattering, one-dimensional IR spectra and two-dimensional IR correlation analysis have been used to study the effects of systems temperature and IL concentration on the microstructure of [C₄mim][BF₄] in water. The experimental results show that the size of the aggregates increases with increasing IL concentration and decreasing systems temperature. As the IL concentrations is lower than its critical aggregation concentration, the hydrogen bonding interaction between water molecules and the anion part of IL is prior to hydrogen bonding interaction between water molecular and the C-H group on the imidazolium ring during the process of temperature decreasing. However, when the IL concentrations is above its critical aggregation concentration, interaction of water molecule with anion and cation of the IL decrease, while the hydrogen bonding interaction between anion and cation increase with decreasing temperature. Therefore, the aqueous IL solution exhibits a liquid-liquid phase separation phenomenon with an upper critical solution temperature.