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 (similar to 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 degrees C after cooling (at 1 degrees C/min) from the disordered state at. 90 degrees C. G', G'' and the slope of log eta* vs. log omega rose steeply as calcium ion concentration was increased to 7.5 niN (similar to 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 delta. Thus the solutions pass through states of maximum and minimum gel-like character at similar to 100% and similar to 200%, stoichiometric equivalence of Ca2+ (i.e. at Ca2+:COO- ratios of similar to 1:2 and similar to 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. (c) 2007 Elsevier Ltd. All rights reserved.