Formation of physical hydrogels with terpyridine-containing carboxylic acids

Terpyridine-, diphenylbipyridine-, and triphenylpyridine-containing carboxylic acids were prepared, and the physical hydrogels formed from their sodium salts were studied. When terpyridine-containing carboxylic acids were dissolved by heating in aqueous NaOH solutions and then cooled, thermally-reversible transparent hydrogels were formed. The gelation ability was characterized by minimum gel concentrations. The structurally related diphenylbipyridine and triphenylpyridine-containing carboxylic acids failed to form hydrogels. Critical micelle concentrations (CMCs) of the sodium salts were determined according to emission spectra. Under an appropriate condition, terpyridine- and triphenylpyridine-containing carboxylic acids formed highly viscous fluids instead of hydrogels. The kinematic viscosity, determined with a Ubbelohde viscometer, increased with an increasing concentration of carboxylic acids. The increase in viscous fluids was associated with the formation of rod-like micelles. Transmission electron micrographs revealed numerous fibrous aggregates with diameters of several nanometers. The formation of hydrogels occurs on the following process: an increase in the concentration beyond CMCs brings about the transformation of spherical micelles to rod-like micelles, then the elongated rod-like micelles become juxtaposed and interlocked, which immobilizes the liquid.