Rheology and gelation of deacylated gellan polysaccharide with Na⁺ as the sole counterion

Abstract

The effect of NaCl on the formation, melting, and mechanical properties of Na⁺ gellan gels can be divided into five regions of rheological response.

• 1. 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.

• 2. 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.

• 3. 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.

• 4. T_o 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.

• 5. 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.