Reversible pH-induced transformation of micellar aggregates between hemicylinders and laterally homogeneous layers at graphite-solution interfaces

pH-induced transformation between a hemicylindrical aggregate and a laterally homogeneous layer at the graphite-solution interface was demonstrated in micellar aggregates of dodecyldimethylamine oxide (C12DMAO) using atomic force microscopy (AFM). Nonionic C12DMAO (pH~8) and fully-ionized cationic C12DMAO (pH~1.5) both formed hemicylindrical aggregates on graphite, similar to aggregates formed by many other ionic (or nonionic) surfactants on graphite. However, a laterally homogeneous layer was observed in the case of nearly half-ionized C12DMAO around pH~4 (a 1:1 mixture of the nonionic and the cationic species). These results indicated that the surface curvature of the C12DMAO aggregates on graphite was the smallest around the degree of ionization =0.5, despite charging up the nonionic hemicylindrical aggregates. Using AFM images and the corresponding force curves, the transformation between the hemicylindrical aggregate and the laterally homogeneous layer was found to be reversible via a change in pH. The formation of the laterally homogeneous layer of nearly half-ionized C12DMAO is explained by hydrogen bond formation between the nonionic and the cationic headgroups.