Direct evidence of gel–sol transition in cyclodextrin-based hydrogels as revealed by FTIR-ATR spectroscopy

The phase transition from gel to liquid suspension in cyclodextrin (CD)-based hydrogels is in depth monitored by using Fourier transform infrared spectroscopy in attenuated total reflectance geometry.
Cyclodextrin nanosponges (CDNS) synthesized by polymerization of CD with the cross-linking agent ethylenediaminetetraacetic dianhydride at different cross-linking agent/CD molar ratios have been left to evolve from gel phase into liquid suspension upon gradual increase of the hydration level. Measurements of the changes occurring in the vibrational dynamics of the system during this transition provide direct evidence of the gel–sol progress of the CNDS hydrogel, by accounting for the connectivity pattern of water molecules concurring to the gelation process. The experimental results clearly indicate that the increase of the hydration level is accompanied by the corresponding increase of the population of H2O molecules engaged in high-connectivity hydrogen-bond networks. The water tetrahedral arrangement is thus dominant above a characteristic cross-over hydration level, experimentally determined for all the investigated samples. The observation of this characteristic cross-over point for the CDNS hydrogel and its correlation with other parameters of the system (e.g. the absorption ability of CDNS and elasticity of the polymer matrix) is, once again, modulated by the cross-linking agent/CD molar ratio. The latter seems indeed to play a key role in defining the nano- and microscopic properties of nanosponge hydrogels.