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

Abstract

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) Å, b = 10.2253 (14) Å, c = 12.3312 (17) Å and β = 98.717 (3)°. 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 °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 π–π stacking interactions.