Role of metal chlorides in the gelation and properties of fucoidan/κ-carrageenan hydrogels

Metal ions play a crucial role in forming hydrogels, and their effects on fucoidan (FUC): κ-carrageenan (KC) mixed gels were investigated. The results indicated that the FUC: KC mixed gels (FC) were promoted by K + and Ca 2+ but destroyed by Fe 3+ . The gel strength of FC was enhanced by K + and Ca 2+ , with G′ and G″ being highest at 50 mmol/L KCl and 25 mmol/L CaCl 2 , respectively. Water mobility was weakened after the addition of KCl and CaCl 2 in accordance with the decrease in T 23 relaxation time (free water, 100–1000 ms). After addition of KCl and CaCl 2 , the FC groups presented a typical three-dimensional network structure in contrast to the lamellar, disordered, and broken structure of FUC. Moreover, the FT-IR spectrum certified the enhancement of hydrogen bonds and the occurrence of electrostatic interactions during gel formation by the red-shift of the O H stretching vibration of the Ca 2+ group and the blue-shift of the C O S vibrations. The XRD results confirmed that the binding of Ca 2+ to FC was tighter than that of K + at the same charge content. These results provide a theoretical basis for understanding the interaction mechanism of FC with metal ions.