Vibrational signatures of the water behaviour upon confinement in nanoporous hydrogels

Abstract

The fundamental question of how the reorganization of the hydrogen-bond (HB) network of water is influenced by the combination of nano-confinement and hydrophobic/hydrophilic solvation effects is addressed here using a spectroscopic study of water absorbed in a model, pH-sensitive polysaccharide hydrogel. The effects of temperature, hydration level and pH on the vibrational dynamics associated with the water molecules and the polymer skeleton are disentangled and analysed by a complementary and combined 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 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 kinds of interactions, i.e. hydrophobic vs. hydrophilic.

Guest–matrix interactions affect the solvation of cyclodextrin-based polymeric hydrogels: a UV Raman scattering study

“…The latter parameter has also been found to affect the thermoactivation of NS hydrogels, 13,29 which at high temperatures display stronger solvation of the more hydrophobic parts of the polymer backbone. 29 This effect has been efficiently monitored by following the temperature dependence of the dephasing time associated with the vibrational modes involving specific moieties present in the polymer matrix of NS.…”

“…[10][11][12] Besides these practical aspects, hydrogels are interesting model systems for studying the role of hydrophobic and hydrophilic interactions in determining the gelling behaviour of biomacromolecules. This has been recently shown in the case of polysaccharidebased hydrogels, which are characterized by the presence in their polymeric backbone of both hydrogen bond donor and acceptor groups, [13][14][15] around which water can be rearranged in different ways. It is well known that important chemical and physical properties of hydrogels are determined, at the molecular level, by both structural features of the hydrogel matrix (e.g., the density of cross-links) and intermolecular interactions such as hydrophobic associations and hydrogen bonds.…”

“…36 UV Raman scattering investigations of NS hydrogels loaded with Caf were carried out as a function of temperature and guest concentration with the aim of studying the effect of guest-matrix interactions on the solvation behaviour of NS polymers. 13,29 This process is particularly informative on the structural rearrangements of the hydrophobic/hydrophilic groups of NS and on the breaking/formation of specific non-covalent interactions between chemical groups of Caf and the polymer skeleton of NS. The study was performed on two types of NS, which were obtained by varying during the synthesis the molar ratio of cyclodextrin to the cross-linker.…”

“…13,29 In brief, b-cyclodextrin (b-CD) was dissolved in anhydrous DMSO and in the presence of anhydrous Et 3 N was reacted with the cross-linking agent pyromellitic anhydride (PMA) at room temperature for 3 hours under intensive stirring. PMA was added at molecular ratios of 1 : 4 and 1 : 8 with respect to the monomer b-CD.…”

“…[23][24][25][26] Additional uses of NS in agriculture 27 and environmental control 28 are also reported in the literature. A description of the gelling behaviour of NS hydrogels in terms of their structural and dynamical properties has been provided in some recent works, [13][14][15]29 mainly via the analysis of vibrational spectra. These studies suggest that the hydration of NS and the viscosity of the gel phase can be tuned by controlling specific factors during the synthesis of NS 15,30 and by regulating the hydration level 14 and the pH of the system.…”