Gel-sol evolution of cyclodextrin-based nanosponges: role of the macrocycle size

The effect of the macrocycle size on the gel-sol evolution of cyclodextrin-based hydrogel is here investigated by using Fourier transform infrared absorption in attenuated total reflectance geometry (FTIR-ATR). Different types of nanosponges obtained by polymerization of a- and b-cyclodextrin (CDNS) with an activated derivative of ethylenediaminetetraacetic acid have been progressively hydrated in order to follow the evolution of these systems from a gel state to a liquid suspension. The in deep analysis of the high-frequency vibrational dynamics of the hydrogel during its gel-sol evolution revealed that the microscopic origin of this phenomenon is strictly connected to different hydrogen bond environments in which water molecules confined in the pores of nanosponges can arrange. By following a well consolidated approach, the OH stretching band of water, clearly observed in the high-frequency range of the vibrational spectra of nanosponges hydrogel, has been decomposed into sub-bands assigned to different arrangements of water molecules at various degrees of cooperativity. A comparison of the diagrams obtained for homologous CDNS prepared from a- and b-CD shows how the size of cyclodextrin macrocycle allows to efficiently modulate the gelation points at constant cyclodextrin/crosslinker molar ratio n.