Atomic Force Microscopy Observation of the Nanostructure of Tetradecyltrimethylammonium Bromide Films Adsorbed at the Mica/Solution Interface

An investigation was performed by atomic force microscopy (AFM) using two measuring modes on the nanostructure of adsorbed films of tetradecyltrimethylammonium bromide (C 14 TAB) formed at the solution/mica interface. Cylindrical micelle-like aggregates, quite similar to those reported by Manne and Gaub ( Science 1995, 270, 1480−1482), were found in noncontact AFM observations based on the electrostatic repulsive force between mica substrate with adsorbed C 14 TAB and AFM probe. The cylindrical micelle-like structure was observed only above the critical micelle concentration (cmc) of C 14 TAB (3.5 mM), and the diameter of the cylinder was ca. 5−6 nm. In contrast, a hexagonal lattice-like structure with a 1.0−1.2 nm lattice spacing, which differs from that for mica substrate, was found in a wide range of surfactant concentrations covering those above and below the cmc when contact mode AFM observations were carried out in the adhesive force region. The adsorbed state of C 14 TAB on the mica substrate was discussed based on these experimental findings.