Self-Assembly of Cationic Surfactants on a Graphite Surface Studied by STM

The self-assembly of hexadecyltrimethylammonium bromide (C 16 TAB) and dodecyltrimethylammonium bromide (C 12 TAB) from ethanol solution that was allowed to evaporate was imaged on graphite with a scanning tunneling microscope (STM). The STM images showed that these cationic surfactants could form ordered parallel stripes with approximately twice the length of a surfactant molecule, displaying that the cationic surfactants were arranged in a head-to-head configuration. The high-resolution image indicated that three bright spots amid a stripe could correspond to three methyl groups of a couple of head-to-head molecules. This observation provided direct support for interpretation of the model proposed from atomic force microscope studies concerning the first adsorbed layer at solid−aqueous interfaces.