Real-time monitoring the order-disorder conformational transition of xanthan gum

The viscoelastic behavior of xanthan gum was carefully investigated in aqueous solutions (between 0.1% and 3% wt.) and in presence of extra-salt (NaCl) by following different thermal procedures. Thus, by a gradual rise of temperature, a sharp increase of the viscoelastic moduli was highlighted between 55 degrees C and 70 degrees C, depending on the polymer and salt concentration. This behavior is due to a conformational transition between an ordered state (helix) to a disordered conformation (random coil) which allows xanthan chains to interact each other forming a network structure. In order to investigate the thermal history influence on the macromolecular conformation, homogeneous xanthan solutions of different concentrations were kept at 4 degrees C and used for rheological measurements at different constant temperatures. The conformational transition observed in oscillatory shear measurements occurs after a characteristic storage time at a given temperature. It was noted that the transition time decreases as the temperature is higher. The results obtained in frequency sweep experiments before and after the conformational changes show a strong influence of the thermal history on the viscoelastic parameters. A fast cooling at 4 degrees C determines a rearrangement of xanthan chains that tend to adopt again the ordered state which is the stable conformation at low temperatures. This process evolves in time and is influenced by salt content. The reported experimental observations should be considered in all thermal treatments applied to xanthan solutions because the properties are strongly influenced by the ordered or disordered state adopted as a function of temperature and also influenced by salt addition. (C) 2020 Elsevier B.V. All rights reserved.