title: Interactions of cationic trimeric, gemini and monomeric surfactants with trianionic curcumin in aqueous solution.
abstract: Interactions of trianionic curcumin (Cur(3-)) with a series of cationic surfactants, monomeric surfactant dodecyl trimethylammonium bromide (DTAB), dimeric surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) and trimeric surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD), have been investigated in aqueous solution of pH 13.0. Surface tension and spectral measurements indicate that the cationic surfactants display a similar surfactant concentration dependent interaction process with Cur(3-), involving three interaction stages. At first the three cationic surfactants electrostatically bind on Cur(3-) to form the surfactant-Cur(3-) complex. Then the bound and unbound cationic surfactants with Cur(3-) aggregate into surfactant-Cur(3-) mixed micelles through hydrophobic interactions above the critical micelle concentration of the surfactants (CMCC) in the presence of Cur(3-). Finally excess unbound surfactants self-assemble into micelles like those without Cur(3-). For all the three surfactants, the addition of Cur(3-) only decreases the critical micelle concentration of 12-6-12 but does not affect the critical micelle concentration of DTAB and DTAD. As the oligomeric degree of surfactants increases, the intermolecular interaction of the cationic surfactants with Cur(3-) increases and the surfactant amount needed for Cur(3-) encapsulation decreases. Compared with 12-6-12, either the weaker interaction of DTAB with Cur(3-) or stronger interaction of DTAD with Cur(3-) limits the stability or solubility of Cur(3-) in surfactant micelles. Therefore, gemini surfactant 12-6-12 is the best choice to effectively suppress Cur(3-) degradation at very low concentrations. Isothermal titration microcalorimetry, surface tension and (1)H NMR results reveal that 12-6-12 and Cur(3-) form a (12-6-12)2-Cur(3-) complex and start to form micelles at extremely decreased concentrations, where either 12-6-12 or Cur(3-) works as a bridge linkage and the resultant structure exhibits the characteristics of oligomeric surfactants.
authors.affiliation: Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. yilinwang@iccas.ac.cn.
authors.forename: Meina
authors.initials: M
authors.lastname: Wang
authors.forename: Chunxian
authors.initials: C
authors.lastname: Wu
authors.forename: Yongqiang
authors.initials: Y
authors.lastname: Tang
authors.forename: Yaxun
authors.initials: Y
authors.lastname: Fan
authors.forename: Yuchun
authors.initials: Y
authors.lastname: Han
authors.forename: Yilin
authors.initials: Y
authors.lastname: Wang