Rheology and gelation of deacylated gellan polysaccharide with Na+ as the sole counterion
Small additions of salt, up to a total Na+ concentration of about 25 mM, have virtually no effect on the rheological properties of the ordered polymer at low temperature; the solutions remain fluid on cooling, and give mechanical spectra similar to those of entangled polysaccharide coils.
Slightly higher concentrations (about 40 mM Na+) induce formation of “weak gels” (i.e. solutions that remain pourable but give gel-like mechanical spectra). Network formation is attributed to Na+ ions promoting helix-helix aggregation by binding to carboxyl groups of the polymer and thus reducing charge density, and by suppressing residual electrostatic repulsion by nonspecific charge screening.
Na+ concentrations in the approximate range 100–300 mM give true, self-supporting, gels whose strength and setting point (T o) increase with increasing concentration of salt and which melt in two steps, attributed to dissociation of unassociated helices and cation-mediated aggregates, respectively.
To continues to increase with Na+ concentration over the range 300–750 mM, but the gel strength falls, suggesting destabilisation of the network structure by association of helices into a progressively smaller number of progressively larger aggregates.
At NaCl concentrations above about 750 mM, T o drops sharply. It is tentatively suggested that this reduction in the stability of the helix structure is of lyotropic origin.
Key wordsGellan polysaccharide Gelation Rheology Coil-helix transition Helix—helix transition
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