Small-angle X-ray scattering and rheological characterization of alginate gels. 1. Ca-alginate gels

Ca-alginate gels were studied by small-angle X-ray scattering and rheology to determine relations between chemical composition and concentrations of t he alginate and the elasticity and structure of the gels. The gels were pre pared by in situ release of Ca2+ from either Ca-EGTA or CaCO3 with total Ca 2+ concentration in the range 5-30 mM. Alginates with low (39%), intermedia te (50%), and high (68%) fractions of alpha-L-GulA originating from the bro wn algae Ascophyllum nodosum, Laminaria hyperborea leaf, and Laminaria hype rborea stipe, respectively, were employed. Two to three different degrees o f polymerization for each chemical composition were used in the experiments . The excess small-angle X-ray scattering for the alginates in solution yie lded linear cross-sectional Guinier plots, and the cross-sectional radius o f gyration, R-g,R-c, was determined to be 3.1-4.6 Angstrom. The SAXS profil es of the alginate gels depended on the alginate concentration, Ca2+ concen tration, and the alginate composition. The SAXS data suggested that dimeriz ation of chain segments was the principal association mode at low fractiona l Ca2+ saturation of guluronic acid of the alginate. Increasing the fractio nal Ca2+ saturation of guluronic acid, either by the concentrations or sele ction of alginate source, yielded coexisting lateral association modes, as manifested in a curvature in the cross-sectional plots. The coexisting junc tion zone multiplicities occur because of a delicate balance between the bl ock length distribution of the alpha-L-GulA residues, polymer concentration , and Ca2+. These results are quantitative extensions of the "egg-box" mode l used to describe ionotropic gelation of alginate and hence enhance the un derstanding of the structure-function relationship of alginate gels.