Application of solubility theory in bi-component hydrogels of melamine with di(2-ethylhexyl) phosphoric acid

Di(2-ethylhexyl) phosphoric acid (P) and melamine (M) were used to produce a supramolecular PM complex at a 1 : 1 molar ratio, which then formed a metastable hydrogel in pure water and stable mixed-hydrogels in organic–water mixtures. According to SEM, the morphologies of the aggregates depended on the nature of the solvent where they were formed. Elongated fibers were observed in hydrogels, while rod-like and thicker fibers were dominant in mixed-hydrogels. FTIR and NMR spectroscopies revealed that hydrogen bonding within the PM complex was the main driving force for the self-aggregation of the gelator. The gelation tests showed that the Flory–Huggins interaction parameter (χ) can serve as a model to predict the gelation behavior of a known gelator in a range of untested solvents. Hildebrand solubility parameters involved in the model were estimated by using established polymer solubility theory (Fedors' method). In very general terms, the estimated χ value largely reflects solvent–gelator interactions, which should become a useful model allowing us to identify a gelator for any suitable solvent.