Self-assembly of didodecyldimethylammonium surfactants modulated by multivalent, hydrolyzable counterions

The self-assembly behavior of double-chained didodecyldimethylammonium (DDA(+)) surfactants with hydrolyzable phosphate (PO4(3-), HPO4(2-), and H2PO4(-)), oxalate (HC2O4(-) and C2O4(2-)), and carbonate (HCO3(-)/CO3(2-)) counterions was found to depend on both the counterion and its hydrolysis state, as determined by the pH of the system. Carbonate and phosphate ions at all hydrolysis states successfully stabilize an extended isotropic micellar solution region. These micelles are well-described as prolate ellipsoids which vary in size and aspect ratio depending on the surfactant concentration and hydrolysis state of the counterion. Both oxalate counterions form bilayer structures in dilute solution. The structures found with divalent oxalate, C2O4(2-), ions possessed very limited swelling capacity compared to the bilayer structures formed with monovalent oxalate, HC2O4(-), ions. The lamellar (Lα) phase was universally formed at sufficiently high surfactant concentrations for all hydrolyzable counterions. Two intermediate structures corresponding to a disordered mesh (Lα(D)) and tetragonal ordered mesh (T) phase were found to form with DDA2HPO4 prior to the Lα phase but not with other phosphate counterions.