Interactions of Divalent and Trivalent Metal Counterions with Anionic Sulfonate Gemini Surfactant and Induced Aggregate Transitions in Aqueous Solution

Interactions of multivalent metal counterions with anionic sulfonate gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)-propane (C12C3C12(SO3)2) and the induced aggregate transitions in aqueous solution have been studied. Divalent metal ions Ca(2+), Mg(2+), Cu(2+), Zn(2+), Mn(2+), Co(2+), and Ni(2+) and trivalent metal ions Al(3+), Fe(3+), and Cr(3+) were chosen. The results indicate that the critical micelle concentration (CMC) of C12C3C12(SO3)2 is greatly reduced by the ions, and the aggregate morphologies of C12C3C12(SO3)2 are adjusted by changing the nature and molar ratio of the metal ions. These metal ions can be classified into four groups because the ions in each group have very similar interaction mechanisms with C12C3C12(SO3)2: (I) Cu(2+) and Zn(2+); (II) Ca(2+), Mn(2+) and Mg(2+); (III) Ni(2+) and Co(2+); and (IV) Cr(3+), Al(3+) and Fe(3+). Cu(2+), Mg(2+), Ni(2+), and Al(3+) then were selected as representatives for each group to further study their interaction with C12C3C12(SO3)2. C12C3C12(SO3)2 interacts with the multivalent metal ions by electrostatic interaction and coordination interaction. C12C3C12(SO3)2 forms prolate micelles and plate-like micelles with Cu(2+), vesicles and wormlike micelles with Al(3+) or Ni(2+), and viscous three-dimensional network structure with Mg(2+). Moreover, precipitation does not take place in aqueous solution even at a high ion/surfactant ratio. The related mechanisms have been discussed. The present work provides guidance on how to apply the anionic surfactant into the solutions containing the multivalent metal ions, and those aggregates may have potential usage in separating heavy metal ions from aqueous solutions.