化学学报 ›› 2013, Vol. 71 ›› Issue (06): 897-905.DOI: 10.6023/A13010080 上一篇    下一篇

研究论文

尾链含对硝基苯醚基团的阳离子Gemini表面活性剂的胶束化热力学

黄旭a,b, 韩玉淳b, 王毅琳a   

  1. a 中国科学院化学研究所 北京 100190;
    b 北京宝洁技术有限公司 北京 101312
  • 收稿日期:2013-01-16 出版日期:2013-06-14 发布日期:2013-03-21
  • 通讯作者: 王毅琳,yilinwang@iccas.ac.cn;Tel.: 010-82615802;Fax:010-82615802 E-mail:yilinwang@iccas.ac.cn
  • 基金资助:

    项目受国家自然科学基金(No. 21025313, 21021003)资助.

Thermodynamic Investigation on Micellization of Cationic Gemini Surfactants with Nitrophenoxy Groups in Hydrophobic Chains

Huang Xua,b, Han Yuchunb, Wang Yilina   

  1. a Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190;
    b P&G Beijing Technology Co., Ltd., Beijing 101312
  • Received:2013-01-16 Online:2013-06-14 Published:2013-03-21
  • Supported by:

    Project supported by the National Natural Science Foundation of China (No. 21025313 and 21021003).

利用等温滴定微量量热法和电导法研究了具有不同疏水链长并且疏水链尾部含有对硝基苯醚基团的Gemini表面活性剂胶束化过程的热力学, 分别利用相分离模型和质量作用模型对观察焓与浓度的关系曲线进行拟合, 获得了胶束化过程的热力学参数. 两种模型获得的胶束化焓一致, 均为较大负值, 而吉布斯自由能却相差较大. 用相分离模型得到的胶束化过程的热容变化均为负值, 并随链长增加绝对值增大, 胶束化过程总的热容变化主要来自非极性的烷基链去水合产生的热容变化, 证明处于疏水链末端的对硝基苯醚基团在表面活性剂形成胶束后依然与水相接触. 质量作用模型获得的胶束聚集数随疏水链长增加逐渐下降, 这是由具有长疏水链的表面活性剂形成预胶束所导致.

关键词: Gemini表面活性剂, 胶束化, 等温滴定微量量热, 相分离模型, 质量作用模型

Isothermal titration microcalorimetry (ITC) is a powerful technique for acquiring thermodynamic information on the micellization process of surfactants. In typical ITC experiments, consecutive injections of small volume of a surfactant micellar solution into water contained in a sample cell are performed. The sample cell is maintained at a constant temperature and the heat of processes occurring during dilution is monitored for each injection and plotted as a function of surfactant concentration in the cell. The interpretation of experimental data obtained by ITC is necessarily based on models describing micellization process. So far, there are two main models to the thermodynamic analysis of the micellization process: the mass action model, considering micelles and unassociated monomers to be in an association-dissociation equilibrium and the phase separation model, which regards micelles as a separate phase at critical micelle concentration (CMC) and assumes that the micellization process is strongly cooperative. The phase separation model is the simplest model for describing micelle formation assuming that this process is akin to a separate phase-precipitation. Based on these two models, an ITC curve of observed enthalpy change versus surfactant concentration allows the determination of CMC and the enthalpy of micellization (ΔHmic) of a surfactant. Other thermodynamic parameters related to micellization, namely the free energy (ΔGmic), the entropy (ΔSmic) and the heat capacity of micellization (ΔCp,mic) can be calculated from the experimentally determined CMC and ΔHmic. In this paper, ITC and electrical conductivity were employed to investigate the micellization process of cationic gemini surfactants, N,N,N',N'-tetramethyl-N,N'-bis[10-(4-nitrophenoxy)alkyl]-1,6-hexanediammonium dibromide (Nm-6-mN, with m=8, 10 and 12, which are the numbers of carbon in the hydrocarbon chains), in aqueous solutions. Both phase separation model and mass action model were used to obtain a series of thermodynamic parameters. The results show that the obtained ΔHmic values based on the two models are very close, however, the obtained ΔGmic values based on the two models are not consistent. In addition, the ΔCp,mic of the micellization process is mainly from the dehydration contribution of hydrophobic alkyl chains of the surfactants, which means the nitrophenoxy group located in the hydrophobic chain still contacts with water after the micellization. Furthermore, the micellar aggregation number n can be obtained by employing the mass action model. The micellar aggregation number n decreases with the increase of the hydrophobic chain length. The reason is that the surfactant with longer hydrophobic chains prefers to form premicelles, leading to the decrease of the average aggregation number.

Key words: Gemini surfactant, micellization, isothermal titration microcalorimetry, phase separation model, mass action model