Promotion and inhibition of xanthan ‘weak-gel’ rheology by calcium ions

The effect of calcium ions on the rheology of xanthan has been studied by low-amplitude oscillatory measurements. Solutions of Na+ xanthan were prepared at a fixed concentration of 5 g/l (∼7.5 mN w.r.t. COO−), with incorporation of 10 mN NaCl to ensure adoption of the ordered conformation, and with CaCl2 concentrations varied between 0 and 100 mN. Measurements were made at 5 °C after cooling (at 1 °C/min) from the disordered state at 90 °C. G′, G″ and the slope of log η∗ vs. log ω rose steeply as calcium ion concentration was increased to 7.5 mN (∼100% stoichiometric equivalence), but then dropped sharply between 7.5 and 15 mN, before continuing to increase monotonically at higher concentrations of Ca2+; converse behaviour was observed for tan δ. Thus the solutions pass through states of maximum and minimum gel-like character at ∼100% and ∼200% stoichiometric equivalence of Ca2+ (i.e. at Ca2+:COO− ratios of ∼1:2 and ∼1:1, respectively). Closely similar results were obtained for Me4N+ xanthan in the presence of 10 mN Me4NCl and 4 M urea. The anomalous reduction in gel-like character at Ca2+ concentration between stoichiometric and twice stoichiometric is tentatively ascribed to partial replacement of intermolecular site-binding of calcium ions by binding to individual carboxyl groups, to maximise the degree of complexation.