Aggregation of a Cationic Gemini Surfactant with a Chelating Molecule and Effects from Calcium Ions

The aggregation behavior of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) with chelating molecule ethylenediaminetetraacetic acid (EDTA) and the effects of calcium bromide (CaBr₂) on the structure and morphology of the aggregates in the mixture have been investigated by surface tension, isothermal titration microcalorimetry, electrical conductivity, ζ potential, dynamic light scattering, cryogenic transmission electron microscopy, freeze-fracture transmission electron microscopy, and ¹H NMR techniques. It was found that the electrostatic attraction between the carboxyl groups of EDTA and the headgroups of 12-6-12 leads to the formation of oligomeric-like surfactant EDTA(12-6-12)₂ at an EDTA/12-6-12 molar ratio of 0.50. The critical aggregation concentration of the EDTA(12-6-12)₂ complexes is much lower than that of 12-6-12, and the complexes form loose, large network-like premicellar aggregates and then transfer into small micelles with an increase in concentration. Moreover, the addition of CaBr₂ induces the transition from the loose aggregates and micelles to vesicles owing to the coordination interaction between the calcium ion and EDTA and the electrostatic interaction between EDTA and 12-6-12. The work reveals that as a bridging molecule between the calcium ion and the gemini surfactant, the chelating molecule greatly promotes the assembly of the gemini surfactant and strengthens the molecular packing in the presence of calcium ions.