Surface-Induced Phase Behavior of Alkyltrimethylammonium Bromide Surfactants Adsorbed to Mica, Silica, and Graphite

Using atomic force microscopy (AFM) to study adsorption of alkyltrimethylammonium bromide surfactants to mica, silica, and graphite from aqueous solution, we find that the sharp Krafft transition in bulk is not accompanied by a similar change in morphology at the interface. Instead, interactions between the solid substrate and the surfactant dictate an equilibrium morphology that is usually similar above and below the Krafft temperature (T K). Mechanical properties, tested by pushing an AFM tip though the adsorbed film, do change near the T K. In general, the film is more resistant to passage of the AFM tip below T K, consistent with slower molecular motion. Depending on the temperature, the formation of the equilibrium structures on mica and silica proceeds by different paths. Above T K, where micelles are present in solution, adsorption proceeds via micelle-like structures, whereas below T K, adsorption occurs via growth of flat islands, which gradually coalesce. In some cases the adsorbed micelle intermediates were observed somewhat below T K, probably because the negative surface potential allows cationic micelles to form in the double layer or at the interface at monomer concentrations below the critical micelle concentration. We hypothesize that the absence of a distinct structural transition near T K at the surface of the solids is due to strong interactions that either suppress or enhance crystallization, pushing the surface transition point to lower or higher temperatures, respectively. Graphite suppresses crystallization of the bulk structure and enhances crystallization of a different structure, whereas mica and silica enhance formation of a structure that is similar to the bulk crystal. To test this hypothesis we modified the properties of one substrate, mica, through adsorption of KBr. When KBr is introduced to solution, we observed a temperature-dependent structural transition from a flat adsorbate to a cylindrical adsorbate. We propose that KBr weakens the ability of mica to template crystal formation at the interface in two ways: by adsorption of K+ to mica in competition with alkyltrimethylammonium ions, and by interaction of Br- with the surfactant in competition with mica anions. The cylinder/flat transition occurs over a time scale of minutes, and we are able to monitor the growth of cylinder domains on increasing the temperature and the shrinkage of these domains on decreasing the temperature.