title: Synthesis and aggregation behavior of a hexameric quaternary ammonium surfactant.
abstract: A star-shaped hexameric quaternary ammonium surfactant (PAHB), bearing six hydrophobic chains and six charged hydrophilic headgroups connected by an amide-type spacer group, was synthesized. The self-assembly behavior of the surfactant in aqueous solution was studied by surface tension, electrical conductivity, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and NMR techniques. The results reveal that there are two critical aggregate concentrations during the process of aggregation, namely C(1) and C(2). The aggregate transitions are proved to be caused by the changes of the surfactant configuration through hydrophobic interaction among the hydrocarbon chains. Below C(1), PAHB may present a star-shaped molecular configuration due to intramolecular electrostatic repulsion among the charged headgroups, and large aggregates with network-like structure are observed. Between C(1) and C(2), the hydrophobic interaction among the hydrophobic chains may become stronger to make the hydrophobic chains of the PAHB molecules curve back and pack more closely, and then the network-like aggregates transfer to large spherical aggregates of ∼100 nm. Beyond C(2), the hydrophobic interaction may become strong enough to cause the PAHB molecular configuration to turn into a pyramid-like shape, resulting in the transition of the spherical large aggregates to spherical micelles of ∼10 nm. Interestingly, the PAHB displays high emulsification ability to linear fatty alkyls even at very low concentration.
authors.affiliation: Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China.
authors.forename: Yaxun
authors.initials: Y
authors.lastname: Fan
authors.forename: Yanbo
authors.initials: Y
authors.lastname: Hou
authors.forename: Junfeng
authors.initials: J
authors.lastname: Xiang
authors.forename: Defeng
authors.initials: D
authors.lastname: Yu
authors.forename: Chunxian
authors.initials: C
authors.lastname: Wu
authors.forename: Maozhang
authors.initials: M
authors.lastname: Tian
authors.forename: Yuchun
authors.initials: Y
authors.lastname: Han
authors.forename: Yilin
authors.initials: Y
authors.lastname: Wang
mesh_terms: D008958:Models, Molecular; D015394:Molecular Structure; D000644:Quaternary Ammonium Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D013501:Surface-Active Agents / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D000644:Quaternary Ammonium Compounds; D013501:Surface-Active Agents