Surface-Induced Transformations for Surfactant Aggregates

Hexadecyltrimethylammonium cations (CTA+) form ordered structures at the interface between muscovite mica and aqueous solution. The structure is altered in the sequence bilayer → ordered cylinder → disordered cylinder → short cylinder → sphere by successive addition of Cs+, that is, more curved structures are observed at higher salt concentrations. Parts of the same sequence occur at the interface on addition of other alkali cations, but the structure is more curved in the order Li+ < K+ < Cs+, following the softness of the ion. On addition of salt, the changes in aggregate shape at the interface occur in the opposite sequence to those that occur in bulk solution, demonstrating the dominance of surface interactions in this system. Substitution of a counterion that has a greater binding affinity for the CTA+ ion, Br- for Cl-, leads to lower curvature structures at the same CTA+ and alkali cation concentrations. The effect of the counterion is similar to that observed in bulk, and is explained by a decrease...