Small-angle X-ray scattering and rheological characterization of alginate gels. 2. Time-resolved studies on ionotropic gels

Abstract

Gelation was observed by time-resolved small-angle X-ray scattering and rheology on 10 mg/ml Ca-alginate gels prepared by in situ release of Ca 2+ from CaEGTA or CaCO 3 with total Ca 2+ concentration in the range 10–20 mM. This was carried out for alginates having a fraction of α- l-GulA (G) of F G=0.39 and 0.68, respectively, obtained by the selection of alginates isolated from two different brown algae, Ascophyllum nodosum and Laminaria hyperborea stipe. Correlation between the rheological data and SAXS data shows that a large fraction of the lateral association precedes the formation of a continuous network through the sample cell. Following the initial association of chain segments in junction zones, the analysis using two-component broken rod model indicates the formation of larger bundles, and that the relative weight of these bundles increases with increasing time. The molecular model for the bundles is proposed by associating 2–16 units (G-blocks) composed of 14 (1→4) linked residues of α- l-GulA in parallel according to the available crystallographic data. The storage modulus increases as the bundles composed of associated alginate chains grow during the gel formation. The gel elasticity is mainly sustained by single chains in the alginate sample with a low fraction of α- l-GulA. The alginates with a high fraction of α- l-GulA associate into thicker bundles which join to form a network. Here the gel elasticity seems to be due to the flexible joints between bundles, since the fraction of single chains is extremely low.