A thermo-sensitive supramolecular hydrogel derived from an onium salt with solution-gel-crystal transition properties

The gelation ability of melamine was evaluated under various acidic conditions, and the related gelator aggregates were investigated with scanning electron microscopy, transmission electron microscopy, single-crystal X-ray diffraction, thermo-gravimetric analysis, differential scanning calorimetric analysis, rheological experiments, 1H nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy and powder X-ray diffraction. It was found that melamine interacted strongly with a number of organic/inorganic acids in water, forming thermal-reversible supramolecular hydrogels with different critical gelator concentrations (CGCs). The CGCs of gelators successively made with salicylic acid, m-hydroxybenzoic acid and p-hydroxybenzoic acid decreased because of the steric hindrance, whereas those related to oxalic acid dihydrate and orthoboric acid presented higher values due to the lack of a phenyl ring in the molecular structures. More interestingly, a unique onium salt, 2,4,6-triamino-1,3,5-triazin-1-ium benzoate dihydrate (TTIBD), was formed via the Lewis acid-base reaction of benzoic acid and melamine. It crystallized in the monoclinic space group C2/c (Z = 8) with lattice parameters a = 21.477 (3) angstrom, b = 10.2253 (14) angstrom, c = 12.3312 (17) angstrom and beta = 98.717 (3)degrees. The formed hydrogel not only exhibited thermo-sensitive characteristics and solid-like behavior, but also showed a solution-gel crystal transition, being an amorphous-to-crystalline phase transition. By increasing the concentration from 0.04 to 0.12 mol L-1, the gel-solution transition temperature increased from 23.0 to 49.5 degrees C, and the gel-crystal transition time decreased from 430 to 253 min, but both leveled off upon increasing the concentration. The TTIBD crystal along with the corresponding hydrogel was self-assembled via hydrogen bonds and pi-pi stacking interactions.