Surface-induced phase behavior of alkyltrimethylammonium bromide surfactants adsorbed to mica, silica, and graphite

Using atomic farce microscopy (AEM) to study adsorption of alkyltrimethylam monium bromide surfactants to mica, silica, and graphite from aqueous solut ion, we find that the sharp Krafft transition in bulk is not accompanied by a similar change in morphology at the interface. Instead, interactions bet ween the solid substrate and the surfactant dictate an equilibrium morpholo gy that is usually similar above and below the Krafft temperature (T-K). Me chanical 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 AEM tip below T-K, consistent with slower molecular motion. Dependin g on the temperature, the formation of the equilibrium structures on mica a nd silica proceeds by different paths. Above T-K, where micelles are presen t in solution, adsorption proceeds via micelle-like structures, whereas bel ow T-K, adsorption occurs via growth of flat islands, which gradually coale sce. In some cases the adsorbed micelle intermediates were observed somewha t below T-K, probably because the negative surface potential allows cationi c micelles to form in the double layer or at the interface at monomer conce ntrations 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 cryst allization, pushing the surface transition point to lower or higher tempera tures, respectively. Graphite suppresses crystallization of the bulk struct ure and enhances crystallization of a different structure, whereas mica and silica enhance formation of a structure that is similar to the bulk crysta l. To test this hypothesis we modified the properties of one substrate, mic a, through adsorption of KBr. When KBr is introduced to solution, we observ ed 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 interacti on 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 mo nitor the growth of cylinder domains on increasing the temperature and the shrinkage of these domains on decreasing the temperature.