A comparative analytical study for the different water pools present in alginate hydrogels: Qualitative vs. quantitative approaches

Alginate hydrogels have garnered significant attention due to their promising applications in the food, biomedical, and pharmaceutical industries. The detection and quantification of distinct water phases within these hydrogels offer valuable insights into their dynamic, absorptive, and mechanical properties. Despite being comprised solely of 2 wt % polymeric materials, the alginate hydrogels exhibit a highly porous morphology, characterized by distinct water pools exhibiting varying mobility and dynamic behaviors. These phases can be delineated as largely free water phase with high mobility, which occupies the macropores, and bound water with restricted mobility, which interacts with the fibrous polymeric structure. Water pools interacting with their surrounding environments possess variable crystal structures on variable freezing points, this could be easily detected using X-ray scattering techniques. A comparative study was conducted based on the information derived from each technique, with differential scanning calorimetry (DSC) yielding quantitative information for the water phases in alginate hydrogels (i.e., 58 % free and 42 % bound water in 0.75 wt % - 6 h aging sample), whereas cryogenic scanning electron microscopy (Cryo-SEM), wide and small-angle X-ray scattering (WAXS and SAXS), Fourier Transform Infrared (FT-IR), and rheology provided valuable qualitative insights. In this study, deep insights into the molecular structure of alginates were obtained including the alteration in morphology and macropore distribution, increase in the wall thickness, density, and mechanical properties upon increasing the Ca²⁺ concentration and aging period.