The Impact of Multivalent Counterions, Al<sup>3+</sup>, on the Surface Adsorption and Self-Assembly of the Anionic Surfactant Alkyloxyethylene Sulfate and Anionic/Nonionic Surfactant Mixtures

The impact of multivalent counterions, Al<sup>3+</sup>, on the surface adsorption and self-assembly of the anionic surfactant sodium dodecyl dioxyethylene sulfate, SLES, and the anionic/nonionic surfactant mixtures of SLES and monododecyl dodecaethylene glycol, C<sub>12</sub>E<sub>12</sub>, has been investigated using neutron reflectivity, NR, and small angle neutron scattering, SANS. The addition of relatively low concentrations of Al<sup>3+</sup> counterions induces a transition from a monolayer to well-defined surface bilayer, trilayer, and multilayer structures in the adsorption of SLES at the air−water interface. The addition of the nonionic cosurfactant, C<sub>12</sub>E<sub>12</sub>, partially inhibits the evolution in the surface structure from monolayer to multilayer interfacial structures. This surface phase behavior is strongly dependent upon the surfactant concentration, solution composition, and concentration of Al<sup>3+</sup> counterions. In solution, the addition of relatively low concentrations of Al<sup>3+</sup> ions promotes significant micellar growth in SLES and SLES/C<sub>12</sub>E<sub>12</sub> mixtures. At the higher counterion concentrations, there is a transition to lamellar structures and ultimately precipitation. The presence of the C<sub>12</sub>E<sub>12</sub> nonionic cosurfactant partially suppresses the aggregate growth. The surface and solution behaviors can be explained in terms of the strong binding of the Al<sup>3+</sup> ions to the SLES headgroup to form surfactant−ion complexes (trimers). These results provide direct evidence of the role of the nonionic cosurfactant in manipulating both the surface and solution behavior. The larger EO<sub>12</sub> headgroup of the C<sub>12</sub>E<sub>12</sub> provides a steric hindrance which disrupts and ultimately prevents the formation of the surfactant−ion complexes. The results provide an important insight into how multivalent counterions can be used to manipulate both solution self-assembly and surface properties.