Influence of pH on the Aggregation Morphology of a Novel Surfactant with Single Hydrocarbon Chain and Multi-Amine Headgroups

A novel single-chain surfactant with multi-amine headgroups, bis(amidoethyl-carbamoylethyl) octadecylamine (C18N3), was synthesized. Electronmicrographic study showed that in aqueous solution C18N3 formed small micelles (10−20 nm in diameter) at pH 2.0 and changed into much larger globule vesicles sized about 0.6−2.0 μm in diameter at pH 6.8. At pH 12.0 vesicles changed to a much larger continued lyotropic lamella structure. At pH = 2, the surface tension (γ)−concentration (C) curve at pH 2 was an ordinary one, having one critical micelle concentration at 2.9 × 10-3 mol L-1 at relatively high surface tension (52 mN m-1). However, two unique transition points were observed in the γ−C plot at pH = 6.8 and 10.5, showing higher surface activity that is believed to be associated with the micelle−bilayer structure transition. The protonation degree pKa's of the three amine headgroups were found to be 6.6, 10.6, and 10.9, respectively, indicating that a complete protonation state of the headgroups occurred at pH 2.0, which is consistent with the apparent surface areas of headgroup calculated according to Gibbs adsorption isotherm. Variation of sizes and morphologies of C18N3 in aqueous solution at different pH values suggest that our synthetic surfactant may have great potential applications as a template in fabricating drug delivery, biosensors, and biomolecular devices.