Interactions of cationic trimeric, gemini and monomeric surfactants with trianionic curcumin in aqueous solution

Abstract

Interactions of trianionic curcumin (Cur³⁻) 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 Cur³⁻, involving three interaction stages. At first the three cationic surfactants electrostatically bind on Cur³⁻ to form the surfactant–Cur³⁻ complex. Then the bound and unbound cationic surfactants with Cur³⁻ aggregate into surfactant–Cur³⁻ mixed micelles through hydrophobic interactions above the critical micelle concentration of the surfactants (CMC_C) in the presence of Cur³⁻. Finally excess unbound surfactants self-assemble into micelles like those without Cur³⁻. For all the three surfactants, the addition of Cur³⁻ 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 Cur³⁻ increases and the surfactant amount needed for Cur³⁻ encapsulation decreases. Compared with 12–6–12, either the weaker interaction of DTAB with Cur³⁻ or stronger interaction of DTAD with Cur³⁻ limits the stability or solubility of Cur³⁻ in surfactant micelles. Therefore, gemini surfactant 12–6–12 is the best choice to effectively suppress Cur³⁻ degradation at very low concentrations. Isothermal titration microcalorimetry, surface tension and ¹H NMR results reveal that 12–6–12 and Cur³⁻ form a (12–6–12)₂–Cur³⁻ complex and start to form micelles at extremely decreased concentrations, where either 12–6–12 or Cur³⁻ works as a bridge linkage and the resultant structure exhibits the characteristics of oligomeric surfactants.