Interaction mechanism in sol-gel transition of alginate solutions by addition of divalent cations

The interaction mode between divalent cations and alginate in aqueous solutions during sol-gel transition has been experimentally examined at different temperatures with the help of viscosity and density measurements. Comparison of temperature effects on the viscosity behaviour of alginate solution by addition of cupric and calcium cations confirmed different interaction modes existing between the Ca-alginate systems and the transition metal Cu-alginate systems. Different interaction mechanisms were proposed for the Cu-alginate systems at different temperature regions. Intra-molecular cross-linking clusters were found to pass through a conformational transformation with increasing temperature. This transformation of the interaction mode is not reversible when the solution is cooled. Visible and UV spectroscopic investigations suggest that the coordination of cupric ions mainly occurs to carboxyl groups, and the stoichiometric ratio f is proportional to the fraction of cross-links formed in the sol-gel transition process.