Studies On The Aggregation Behavior Of Ionic Liquids In Solutions

As a class of novelty media and functional materials, room temperature ionic liquids (ILs) have been developed rapidly in recent years. ILs exhibit many unique physicochemical properties, such as nonflammability under ambient conditions, negligible volatility, high thermal stability, large liquid range, strong ability to dissolve many chemicals, and fine-tuning ability for the physical and chemical properties. Thus ILs have 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), in the present work, we have systematically studied the effect of alkyl chain length of cation, anionic structure, ring type of the cations, added salts and organic solvents on the aggregation behavior of ionic liquids in aqueous solutions. The major content is as follows:1. Densities and conductivities for aqueous solutions of ionic liquids [Cnmim]Br (n=4, 6, 8, 10, 12) and [C4mim][BF4] have been mearsured at 298.15K as a function of ionic liquid concentrations. Based on the pseudophase model of micellization, the critical aggregation concentrations of the ILs and the related physicochemical properties have been determined. It is suggested that the aggregation of ILs is driven by alkyl chain-ion inductive and hydrocarbon-hydrocarbon interactions. In addition, the average aggregation number of [Cnmim]Br (n=8, 10, 12) have been determined by the steady-state fluorescent quenching technique. It is shown that [C6mim]Br and [C4mim][BF4] form aggregates at the concentrations of above critical aggregation concentration, and the aggregation of [C8mim]Br, [C10mim]Br and [C12mim]Br is strong enough to form micelles. Therefore, the alkyl chain length of cation can be tailored to switch the aggregation behavior of ILs.2. Experimental densities and conductivities for aqueous solutions of [C8mim]X (X= [NO3], [CH3COO], [CF3COO], [CF3SO3], and [ClO4]), 4m-[C8pyr]Br and [C8mpyrr]Br have been determined at 298.15 K as function of ionic liquid concentrations. Average aggregation numbers of the ILs have been determined by the steady-state fluorescent quenching technique. At the same time, morphology and size distribution of the ILs aggregates have been determined by transmission electron microscopy (TEM) and the dynamic light scattering (DLS). The effect of anionic structure and ring type of the cations on the aggragtion behavior of ILs in aqueous solutions was examined. It was found that the ability of these anions to promote aggregation formation follows Hofmeister series. The ring type of the cations also has significant effect on the aggregation behavior of the ILs. So the aggregation behavior of ionic liquid in aqueous solutions can be regulated and controlled by the anionic structure and ring type of the cations.3. Thirteen sodium salts (NaCl, NaBr, NaI, CH3CO2Na, NaSCN, NaNO3, NaBrO3, NaClO3, C6H5COONa, Na2CO3, Na2SO4, Na2C4H4O6, Na3CH5O7) and two bromides (FeBr3 and AlBr3) were selected to study the salt effect on the aggregation of [C10mim]Br in water. Results of onductivity, fluorescence, and dynamic light scattering shown that salt-in effect occured for FeBr3 and AlBr3, while salt-out effect was observed for the other salts investigated. The ability of these anions to promote aggregation formation follows Hofmeister series. Therefore, the aggregation degree of ILs can be controlled by selecting a right salt at a right concentration.4. [C12mim]Br has been chosen as a mold ionic liquid to study the effect of ethylene glycol, glycerol, dimethyl sulfoxide, N,N-dimethyl formamide, formamide andβ-CD on the aggregation behavior of ILs in aqueous solutions by conductivity, fluorescent and the dynamic light scattering measurements. It was found that these additives reduced the ability of aggregation of [C12mim]Br in aqueous solutions. However, different mechanism forβ-CD has been suggested from the other organic additives. Therefore, the aggregation degree of ILs can be controlled by selecting a right organic additive at a right concentration.5. Conductivities for the dilution solution of [Cnmim]Br(n=4, 6, 8, 10, 12) in water, [Cnmim]Br(n=4, 6, 8) in methanol (1-propanol, 1-pentanol, acetonitrile, and acetone), and [C4mim][BF4] in water (acetonitrile, and acetone) have been measured precisely at 298.15K. The data were analysized by Lee-Wheaton conductivity equation in terms of ionic association constant and the limiting molar conductance. The effect of alkyl chain length of cations, anionic structure, and the properties of solvents on the ionic association constant and the limiting molar conductance was comprehensively discussed. The information of ionic association of the ILs is obtained.