Solution viscosity and ion-specific crosslinking govern structure of alginate and pectin hydrogels: Correlating...

Hydrogels are widely utilized in biomedical, food, and pharmaceutical applications, where their functionality depends critically on their tunable mechanical and dynamic properties. These properties are governed by the solution viscosity and the nature of ionic crosslinking within their constituent polymers. This study elucidates the molecular and rheological relationships among polymer viscosity, molar mass, crosslinking ion identity, and their collective influence on structure-property behavior by integrating rheology, cryogenic scanning electron microscopy (Cryo-SEM), and solid-state NMR (ssNMR) spectroscopy. Sodium alginates of varying molar masses and viscosities, and two pectin sources (apple and citrus), were crosslinked using calcium (Ca²⁺) and zinc (Zn²⁺) ions under controlled conditions. Rheological analyses revealed that Ca²⁺-crosslinked alginates form rigid, solid-like networks (G' ≫ G″) through "egg-box" junctions in G-rich domains, while Zn²⁺-alginate gels remain softer and more fluid-like. In contrast, Zn²⁺-pectin hydrogels exhibit higher rigidity than their Ca²⁺ analogues, due to stronger Zn²⁺ coordination with galacturonic acid residues. Cryo-SEM analysis revealed compact, honeycomb-like porous architectures in Zn²⁺-crosslinked pectin hydrogels, indicating enhanced network density and mechanical integrity. Complementary ssNMR further demonstrated ion-specific structural organization, showing that Ca²⁺-crosslinked alginate forms fibrous, interpenetrating networks in which guluronic acid rich regions stiffen more readily than mannuronic acid rich ones. In contrast, Zn²⁺-crosslinked pectin exhibited stronger coordination with galacturonic acid carboxyl and hydroxyl groups, resulting in greater molecular immobilization and improved rigidity and stability. Moreover, increasing alginate molecular weight intensifies crosslinking efficiency and network stiffness. Collectively, these findings provide an insightful molecular-level framework for designing hydrogels with precisely tuned viscoelastic, thixotropic and dynamic properties, enabling their optimization for targeted functional and industrial applications.