An amphiphilic tris-urea compound (1) containing hydrophilic resorcinol units was designed and synthesized. Compound 1 formed supramolecular hydrogels in basic buffers, such as glycine-NaOH, phosphate-NaOH, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)-NaOH, and borate-NaOH. The optimum pH range of the buffer solution for gelation was 10-11 and insoluble suspensions or solutions were formed when the pH was outside this range. When the borate-NaOH buffer was used, supramolecular hydrogels were formed over a wide pH range (7.5-11.0). The thermal stabilities and viscoelastic properties of the supramolecular hydrogels were determined from the gel-to-sol phase transition temperatures and rheological properties, respectively. The supramolecular hydrogel formed from compound 1 and the borate-NaOH buffer exhibited a pH-responsive reversible gel-to-sol phase transition property. Gel-to-sol phase transition could be achieved by adding NaOH and regelation of the sol was realized by adding an appropriate amount of boric acid. Increasing the amount of the acid resulted in a gel-to-sol phase transition.
Keywords: gel; phase transition; self-assembly; supramolecular chemistry; urea.