Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (10): 1909046.doi: 10.3866/PKU.WHXB201909046

Special Issue: Frontiers in Colloid and Interface Chemistry

• Article • Previous Articles     Next Articles

Aggregation of Biodegradable Cationic Gemini Surfactants with Amide or Ester Groups

Yingxiong Wang1,2, Manli Deng1, Yongqiang Tang1,3, Yuchun Han1, Xu Huang1, Yanbo Hou1,4, Yilin Wang1,*()   

  1. 1 Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
    2 Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China
    3 School of Chemical Engineering, Xi'an University, Xi'an 710065, P. R. China
    4 Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, P. R. China
  • Received:2019-09-25 Published:2020-06-11
  • Contact: Yilin Wang E-mail:yilinwang@iccas.ac.cn
  • Supported by:
    国家自然科学基金(21633002)

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Abstract:

In the last thirty years, Gemini surfactants with various structures have been designed, synthesized, and demonstrated to show superior physicochemical properties. However, the utilization of non-degradable surfactants, including these Gemini surfactants, poses a threat to the environment; hence, degradable Gemini surfactants are desirable. Herein, biodegradable cationic Gemini surfactants with amide or ester groups in the hydrophobic chains or the spacer were synthesized. A monomeric surfactant containing an amide group and a Gemini surfactant with amide groups both in the hydrophobic chains and the spacer were synthesized for comparison. The effects of amide group location on the aggregation behavior of Gemini surfactants were studied systematically. The differences between the Gemini surfactants with amide groups and Gemini surfactants with ester groups were evaluated by comparing their aggregation behavior and hydrogen bonding formation. The Gemini surfactants with amide groups (C12A-Cn-AC12) in the chains showed much larger exothermic ΔHmic and more negative ΔGmic values than those of the corresponding monomeric surfactant C12A; besides, their critical micelle concentration (cmc) was more than one order of magnitude lower than that of C12A. The amide groups located in the hydrophobic alkyl chains promoted hydrogen bonding formation and self-assembly of the Gemini surfactants C12A-Cn-AC12. Moreover, 1H NMR spectra revealed that the co-effect of a short spacer and hydrogen bonding leads to slow exchange of the C12A-C2-AC12 molecules between the monomer and the aggregate. For the Gemini surfactant series C12-ACnA-C12, the amide groups notably increased the spacer length, and largest cmc value and smallest exothermic ΔHmic value were observed for C12-AC2A-C12 instead of C12-AC6A-C12. In C12-AC12A-C12, the spacer was long and sufficiently flexible to adopt a "U"-shaped conformation above the cmc, and it acted as the hydrophobic part of the surfactant, as confirmed by 1H NMR spectra. Among the Gemini surfactant with amide groups in both the spacer and the hydrophobic alkyl chains, C12A-AC6A-AC12 had a smaller cmc and I1/I3 ratio as well as more exothermic ΔHmic values than those of C12A-C6-AC12 and C12-AC6A-C12. 1H NMR spectra indicated that an ester-alcohol structural equilibrium exists during aggregation for the Gemini surfactants with ester groups. In addition, the Gemini surfactants with ester groups formed water-mediated hydrogen bonds in the aggregates. This water-mediated hydrogen bonding between ester groups was weaker than the direct hydrogen bonding between amide groups. Therefore, the Gemini surfactants with ester groups, C12E-C6-EC12 and C12-EC6E-C12, exhibited lower surface activity, a larger micelle ionization degree, higher micropolarity, and smaller exothermic ΔHmic and less negative ΔGmic values than their counterparts with amide groups, C12A-C6-AC12 and C12-AC6A-C12.

Key words: Cationic Gemini surfactant, Biodegradable, NMR, Micellization, Amide group, Ester group