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 Ca2+ but destroyed by Fe3+. The gel strength of FC was enhanced by K+ and Ca2+, with G′ and G″ being highest at 50 mmol/L KCl and 25 mmol/L CaCl2, respectively. Water mobility was weakened after the addition of KCl and CaCl2 in accordance with the decrease in T23 relaxation time (free water, 100–1000 ms). After addition of KCl and CaCl2, 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 OH stretching vibration of the Ca2+ group and the blue-shift of the COS vibrations. The XRD results confirmed that the binding of Ca2+ 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.