Acta Phys. -Chim. Sin. ›› 2013, Vol. 29 ›› Issue (07): 1401-1407.doi: 10.3866/PKU.WHXB201304282

• THERMODYNAMICS, KINETICS, AND STRUCTURAL CHEMISTRY • Previous Articles     Next Articles

Direct Evidence for the Energy of Weak Interactions between Surfactant Molecules Using High Sensitivity Isothermal Titration Calorimetry

LOU Peng-Xiao1, WANG Yu-Jie2,3, BAI Guang-Yue1, FAN Chao-Ying1, Wang Yi-Lin3   

  1. 1 Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Engineering, Henan Normal University, Xinxiang 453007, Henan Province, P. R. China;
    2 School of Chemistry and Engineering, Henan Institute of Science and Technology, Xinxiang 453003, Henan Province, P. R. China;
    3 Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
  • Received:2013-02-18 Revised:2013-04-27 Published:2013-06-14
  • Contact: BAI Guang-Yue E-mail:baiguangyue@htu.cn
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (21273061) and Beijing National Laboratory for Molecular Sciences, China.

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Abstract:

Calorimetry is a direct experimental method that can be used to study the thermodynamics of weak interactions between surfactant molecules, allowing the energetic parameters of such interactions to be obtained. In this work, a nano-isothermal titration calorimeter with a thermostat (TAM III) was used to evaluate the thermodynamic behavior of molecular self-assemblies of single and mixed surfactants in aqueous solution. Electrical calibration of this instrument showed that its precision is better than ±0.09%. The accuracy of the system was tested by measuring the reaction heat of tris-hydroxymethylaminomethane (Tris), employed often as a calorimetric standard substance, with hydrochloric acid. The resulting value ((-47.48±0.12) kJ·mol-1) agreed well with that in the literature. We then determined the critical micelle concentration (cmc) and enthalpy of micellization for dodecyltrimethylammonium bromide (DTAB) with a“head-and-tail”structure, which were consistent with reported values, as well as reliable results for sodium cholate (NaC) with a rigid steroid skeleton composed of hydrophilic and hydrophobic surfaces. Furthermore, for the mixed system of oppositely charged surfactants (DTAB/NaC), the mixed cmc and enthalpy of mixed micellization were also obtained in NaC- and DTAB-rich regions. A stronger synergistic effect was observed between the two types of surfactants in the NaC-rich region than in the DTAB-rich one. Conductivity measurements allowed the thermodynamic behavior of the mixed system (DTAB/NaC) to be discussed in detail.

Key words: Isothermal titration calorimetry, Intermolecular interaction, Thermodynamics, Surfactant, Enthalpy of interaction, Conductivity