Self-assembly of anionic pyrene derivatives with cationic surfactants bearing a tetradecyl chain

The association of 1,3,6,8-pyrenetetrasulfonate (S4P) or 8-hydroxypyrene-1,3,6-trisulfonate (S3POH) with cationic surfactants possessing tetradecyl substituent was studied in aqueous solution at 298 K. Two types of nanoparticles were produced depending on the mixing ratio of the components. The self-assembly to negatively charged larger nanoparticles at low surfactant excess was always exothermic, whereas the formation and size diminution of the positive nanoparticles in the presence of larger amount of surfactants were practically thermoneutral. The alteration of the fluorescence spectrum upon self-organization of the constituents indicated p-p interaction between the pyrene moieties within the nanoparticles. In tetradecyltrimethylammonium bromide solution, S4P induced the formation of not only nanoparticles but also mixed micelles composed of supramolecular and conventional amphiphiles. A pH-responsive entropy-controlled transformation of nanoparticles to such mixed micelles was achieved when 1-methyl-3-tetradecyl-imidazolium cation had more than 19-fold molar excess over S3POH.