Relationship between the molecular structure of alginates and their gelation in acidic conditions

The study reports examined the influence of chemical composition of blocks along alginate chains on the ability to form a gel in acidic conditions. For that purpose, GG (where G is α‐ L ‐guluronic acid) and MM (where M is β‐ D ‐mannuronic acid) blocks of high purity were obtained with good yield from algae recovered along Madagascar coasts; the molecular weight distribution and M/G ratio were determined. The alginates extracted from Sargassum species (SG1 and SG4) are particularly rich in G units. From the dependence of viscosity on pH for solutions of each type of block, it is established that cooperative interchain interactions based on hydrogen bond networks occur for GG blocks when the pH is lower than 3.41 with an optimum at pH = 2.68; MM blocks phase‐separate at pH ≤ 1.93. For a purified commercial alginate (M/G = 1.3) a gel is formed at pH < 3.05 and modulus G ′ becomes maximum at pH = 2.28. From these data, it is concluded that gelation in acidic conditions is mainly based on GG blocks forming junction zones. It is shown that acid gels obtained from pre‐formed gels in the presence of calcium are stronger than when formed directly from the sodium salt form. In addition, the degree of swelling of these gels in the acid form is lower (50% lower) than in the calcium form; this is attributed to the screening of electrostatic repulsions in the gel when the pH is around the intrinsic p K of carboxylic groups and, eventually, to the establishment of hydrogen bond interactions involving M units.