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 but destroyed by Fe. The gel strength of FC was enhanced by K and Ca, with G' and G″ being highest at 50 mmol/L KCl and 25 mmol/L CaCl, respectively. Water mobility was weakened after the addition of KCl and CaCl in accordance with the decrease in T relaxation time (free water, 100-1000 ms). After addition of KCl and CaCl, 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 Ca group and the blue-shift of the COS vibrations. The XRD results confirmed that the binding of Ca 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.