pH-Controlled Self-Assembled Fibrillar Network Hydrogels: Evidence of Kinetic Control of the Mechanical Properties

Control of the nucleation and growth process in self-assembled fibrillary networks (SAFiN) with the goal of preparing physical hydrogels from low-molecular-weight gelators (LMWG) is well established but mainly for temperature-driven hydrogelators. In the presence of other stimuli, such as pH, the fundamental knowledge behind gel formation is still lacking. In particular, whether pH affects nucleation and growth of fibers and how this aspect could be related to the stability of the hydrogel are still a matter of debate. In this work, we establish a precise relationship between the pH change rate during micelle-to-fiber transition observed for stearic acid sophorolipids—a bolaform microbial glycolipid—and supersaturation. We show that SAFiN hydrogels are obtained for slow pH change rates when the supersaturation is low, whereas weak gels, or even phase separation through powder precipitation, are obtained upon fast pH change. Interestingly, these results are independent of the pH change method, may it be through manual variation using HCl or by using the internal hydrolysis of glucono-δ-lactone, the latter being currently acknowledged as a unique way to systematically obtain tough gels through internal pH change.