Barium-Mediated Ionotropic Gelation of Pectin: From Binding Thermodynamics to Network Architecture

Abstract Although Ca2+-mediated ionotropic gelation of pectin has been extensively investigated, the role of alternative divalent cations in network formation remains poorly understood. Here, homogeneous bulk gelation and diffusion-controlled external gelation were used to investigate the effects of cation identity, molecular architecture, and pectin carboxylate availability on ionotropic gelation. CaCl2, BaCl2, and Ba(OH)2 were compared using isothermal titration calorimetry, rheology, X-ray micro-computed tomography, scanning electron microscopy, and solid-state NMR. Ba2+ exhibited stronger binding than Ca2+, consistent with cooperative supramolecular association, and produced more elastic networks, while Ba(OH)2 further enhanced gel formation through increased pectin carboxylate availability. External gelation generated distinct cation-dependent architectures, ranging from homogeneous porous structures to core−shell and layered morphologies. Solid-state NMR showed that Ca2+ induced a partial transition toward a 21 helix, whereas Ba2+ preserved the native 31 helix while increasing conformational order.