Gel–sol–gel′ evolution triggered by formic acid

Abstract

Supramolecular “gel A” based on β-cyclodextrin as the gelator and K2CO3 as the salt bridge can be activated by the addition of HCOOH to evolve into another “gel B”. In the process, the release of CO2 dissociated the network of the “gel A” bridged by K2CO3 and the new network of the “gel B” can be reorganized with the help of the newborn HCOOK. The two gels, “A and B”, were investigated by optical microcopy, scanning electron microscopy, small- and wide-angle X-ray scattering, Fourier transform-infrared spectroscopy and X-ray diffraction. The “gel B” with greater elastic modulus (G′, 1×105Pa) could endure higher applied stress (the yield point 300Pa) than the “gel A” (G′, 3×104Pa; the yield point 180Pa). The “gel B” also exhibited the higher phase transition temperature (161.75°C) than that (34.4°C) of the “gel A” in differential scanning calorimetry. The influence of analogous salts on the macroscopic aspect of the samples was also reported. The reason behind the gel–sol–gel′ phase transition was the salt bridge evolution. This work is the first report on gel evolution with phase reorganization triggered by chemical additive (HCOOH), which may be of great significance to develop drug controlled release materials, and other more complex stimuli-responsive materials.