离子液体1-烷基-3-甲基咪唑四氟硼酸盐在水溶液中的聚集行为

Aggregation Behavior of 1-Alkyl-3-methylimidazolium Tetrafluoroborate Ionic Liquids in Aqueous Solutions

采用等温滴定量热法、静态荧光猝灭法和电导法系统研究了典型离子液体1-烷基-3-甲基咪唑四氟硼酸盐([Cnmim][BF4])在水溶液中的聚集行为,获得了胶束形成的临界胶束浓度(cmc),摩尔焓变(ΔHmic),摩尔吉布斯自由能变(ΔGmic),摩尔熵变(ΔSmic)以及不同浓度时[Cnmim][BF4]胶束的平均聚集数等基本参数.发现这类离子液体的聚集为熵驱动,阳离子的烷基链越长,ΔGmic越负,聚集更容易发生.此外,结合[Cnmim]X(X=Cl-,Br-)的相关研究发现,阳离子相同时,体积越大和疏水性越强的阴离子与头基的结合能力越强,能有效地降低头基之间的静电排斥,降低cmc,利于胶束的形成.对于[C12mim][BF4],添加剂β-环糊精(β-CD)的加入可使cmc增大,ΔHmic和ΔSmic减小,而KBF4则可使cmc和ΔHmic减小,ΔSmic增大.

The aggregation behavior of 1-alkyl-3-methylimidazolium tetrafluoroborate （[Cnmim][BF4]） ionic liquids in aqueous solutions was investigated by isothermal titration calorimetry （ITC）, fluorescence quenching, and conductivity methods. The critical micelle concentration （cmc）, the changes of the enthalpy （△Hmic）, the Gibbs free energy （△Gmic）, and the entropy （△Smic） for the micelle formation and the mean aggregation number of the micelles at different concentrations were obtained. We found that entropy was the principle driving force leading to the micellization of these types of ionic liquids. The increase in alkyl chain length led to a decrease in △Gmic, favoring the formation of micelles. Furthermore, by combining with the aqueous solution data for [Cnmim]X （X=CI-, Br-）, the effect of anions on the aggregation behavior of the ionic liquids was investigated. The increase in the hydrophobicity and the volume of the anions favored the formation of micelles when the ionic liquids contained identical cations. This is believed to result from the decrease in electrostatic repulsion between the head groups in the micelles because of the relatively strong binding between the anions and the cations. With respect to [C12mim][BF4], the addition ofβ-cyclodextrin （β-CD） caused an increase in its cmc and a decrease in AHm,c and △Smic; while the presence of KBF, caused a decrease in cmc and △Hmic, and an increase in △Smic.