Vibrational signatures of the water behaviour upon confinement in nanoporous hydrogels

The fundamental question of how the reorganization of hydrogen-bond (HB) network of water is influenced by the combination of nano-confinement and hydrophobic/hydrophilic solvation effects is here addressed by a spectroscopic study of water absorbed in model, pH-sensitive polysaccharide hydrogel. The effect of temperature, hydration level and pH on the vibrational dynamics associated to the water molecules and polymer skeleton are disentangles and analysed by a complementary and joint use of UV Raman scattering and IR spectroscopy. The experimental data give evidence that the solvation effects in the hydrogel matrix are essentially dominated by the hydration of the more hydrophobic parts of the polymer network, while the effect of pH on the HB reorganization of confined water molecules is found to be similar to that induced by cooling of the system. A tentative explanation of these results has been provided in terms of interplay between different kind of interactions , i.e. hydrophobic vs hydrophilic