FTIR-ATR analysis of the H-bond network of water in branched polyethyleneimine/TEMPO-oxidized cellulose nano-fiber xerogels

The present paper reports a detailed experimental vibrational analysis, performed by Fourier transform infrared spectroscopy in attenuated total reflectance geometry (FTIR-ATR), of water confined in the pores of cellulose nano-sponges (CNSs), prepared using TEMPO oxidized and ultra-sonicated cellulose nano-fibers (TOUS-CNFs) as three-dimensional scaffolds, and branched polyethyleneimine (bPEI) as the cross-linking agent. The analysis was carried out by varying hydration and cross-linker amount, with the aim of achieving a deep understanding of how the hydrogen bond (H-bond) scheme developed by engaged water molecules can play a role in the water adsorption process already observed at macroscopic level, furnishing at the same time evidence of a nano-porous network for CNSs. In particular, the combined investigation of the FTIR-ATR spectra of CNSs hydrated with H₂O and D₂O allowed for the selective analysis of vibrational modes of entrapped water molecules, namely O–H stretching and HOH bending modes. As main result, a destructuring effect of hydration on the H-bond pattern of interfacial water molecules is revealed, associated to structural modifications of the bPEI/TOUS-CNFs network previously detected by small angle neutron scattering (SANS) technique. It turned out to be more relevant for low bPEI amounts. In addition, a supercooled behavior of entrapped water molecules is detected, supporting the idea of a nano-confinement for water in these systems. The obtained information can be very helpful in view of all the possible applications of bPEI-TOCNF sponges as efficient adsorbent materials, especially for water remediation.