X-Ray Analyses of Complexes Formed between Surfactants and Aromatic Compounds. I. A Common Structural Pattern of the Complexes

The cationic surfactants such as hexadecyl-, tetradecyl-, dodecyl-, and decyltrimethylammonium bromides form complexes with a variety of aromatic compounds in aqueous solutions. Well-defined crystals were obtained if the aromatic compounds had hydrogen donor moieties. The structures of ten kinds of the complex crystals were analyzed by X-rays. All the crystal structures had a common pattern of complex formation, in which the aromatic compounds are sandwiched by the two alkyl chains of the surfactant molecules. There are two modes of stacking when the complexes with the common pattern are piled up to form the crystalline lattice. The common pattern of complex formation is responsible for the high viscoelasticity of the aqueous solutions containing cationic surfactants and aromatic compounds.