A smart supramolecular hydrogel exhibiting pH-modulated viscoelastic properties

The macroscopic viscoelastic properties of a physical hydrogel are reversibly modulated by tuning the microscopic hydrogen-bonding interactions with pH. The hydrogel forms at a rather low concentration of the multi-pyridyl-based gelator, N, N', N-tris(3-pyridyl)trimesic amide. The yield stress of the hydrogel is greatly enhanced from 10 to 769 Pa by changing the pH from the amide N-H bond and the nitrogen on the pyridyl group (N-H center dot center dot center dot Py). Fourier transform (FT) IR spectroscopy indicates that hydrogen bonds of N-H center dot center dot center dot Py are partially broken because the pyridyl groups are partly protonated at pH5.0. This condition leads to a highly branched and homogeneous fibrillar network, which is confirmed by X-ray diffraction (XRD) measurements and field emission scanning electron microscopy (FESEM) images. Highly branched fibrillar networks create more compartments and greatly increase the interfacial tension that is required to hold the solvent in the gel thereby increasing the yield stress to 769 Pa. By further increasing the acidity of the hydrogel to pH < 3.0, the gel becomes a sol. Both the change in the viscoelastic properties and the sol-gel transition are reversible and controllable in the material.