Supramolecular membranes. Spontaneous assembly of aqueous bilayer membrane via formation of hydrogen bonded pairs of melamine and cyanuric acid derivatives

Amphiphilic hydrogen bond networks consisting of alkylated melamines and ammonium headgroup-appended cyanuric acids are stably dispersed in water as supramolecular membranes. Electron micrographs of these aqueous dispersions indicated the formation of supramolecular assemblies of mesoscopic dimension. Their aggregate morphologies, molecular orientation, and thermal characteristics are markedly dependent on the chemical structure of constituent molecules. Self-supporting multilayer films were obtainable by casting the aqueous dispersions, like the conventional aqueous bilayer. X-ray diffraction of the cast films indicated that hydrogen bonded pairs of ammonium-appended cyanuric acid and double-chained melamine adopted the bilayer structure. On the other hand, complementary pairs formed from single-chained melamines adopted partially or completely interdigitated bilayers. The long period of the former cast film (ca. 9 nm) is in good agreement with the thickness of disklike aggregates observed in electron microscopy, indicating that bilayer structures mediated by complementary hydrogen bonds are maintained in water. Thermal characteristics of aqueous dispersions were investigated by differential scanning calorimetry and spectroscopically by using 1,6-diphenyl-1,3,5-hexatriene as a fluorescence probe. The observed spectral characteristics indicate that supramolecular membranes display phase transition from a highly ordered state to a liquid crystalline phase. In addition, reversible dissociation and irreversible segregation of complementary pairs proceeded at higher temperatures. These supramolecular membranes are the first example of water-soluble supermolecules directed by complementary hydrogen bonds and constitute a new family of amphiphilically designed supramolecular assemblies.