Characterisation of xanthan gum solutions using dynamic light scattering and rheology

The ‘weak gel’ formation mechanism of xanthan gum solutions has been examined using rheological and light scattering methods. Significant differences are observed between the critical overlap concentration (c∗) measured using both techniques. The effect of shear on c∗ for semi-flexible molecules such as xanthan is demonstrated. The approximate theories of Doi and Edwards (1984). The Theory of Polymer Dynamics. Oxford: Clarendon Press) for rigid rods and De Gennes (1979). Scaling Concepts in Polymer Solutions. London: Cornell University Press) for random coils are used to explain the differences observed between rheological and light scattering measurements of c∗. A second concentration regime parameter due to aggregation, c∗∗, is observed in dynamic light scattering results (DLS). The parameter c∗∗ has been previously observed for xanthan systems. Light scattering measurements in various electrolytes suggest that the use of DLS is able to distinguish between the transition from truly dilute behaviour to semi-dilute behaviour (c∗) and the critical concentration for aggregation (c∗∗).