Acta Phys. -Chim. Sin. ›› 2011, Vol. 27 ›› Issue (08): 1913-1918.doi: 10.3866/PKU.WHXB20110713

• SOFT MATTER • Previous Articles     Next Articles

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

FU Su-Zhen, CHEN Qing-De, SHEN Xing-Hai   

  1. Beijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation Chemistry Key Laboratory of Fundamental Science, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
  • Received:2011-04-12 Revised:2011-05-03 Published:2011-07-19
  • Contact: SHEN Xing-Hai E-mail:xshen@pku.edu.cn
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (20871009) and Fundamental Research Funds for the Central Universities, China.

Abstract:

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 (ΔHmic), the Gibbs free energy (ΔGmic), and the entropy (ΔSmic) 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 ΔGmic, favoring the formation of micelles. Furthermore, by combining with the aqueous solution data for [Cnmim]X (X=Cl-, 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 of β-cyclodextrin (β-CD) caused an increase in its cmc and a decrease in ΔHmic and ΔSmic; while the presence of KBF4 caused a decrease in cmc and ΔHmic, and an increase in ΔSmic.

Key words: Ionic liquids, 1-Alkyl-3-methylimidazolium tetrafluoroborate, Isothermal titration calorimetry, Fluorescence, Conductivity

MSC2000: 

  • O645