Chain conformation of xanthan in solution as influenced by temperature and salt addition

The capillary viscometry and Fourier transform infrared spectroscopy in attenuated total reflection (ATR-FTIR) were used to investigate the conformation of xanthan gum (XG) chains in aqueous solutions, in the absence/presence of NaCl and in the temperature range from 25 to 90 °C. The content of acetate and pyruvate groups per side of XG was determined by 1H NMR spectroscopy. The viscometric parameters (the intrinsic viscosity, [η], and the hydrodynamic interaction parameter, B) were determined by modeling the experimental data according to the new Wolf model. The viscosity for XG solution in the absence/presence of NaCl as temperature rises has an ascendent evolution which takes place with slope variations. These changes were attributed to conformational transition of the chains from the ordered to disordered form affecting the polymer-polymer and polymer-solvent interactions. Salt addition diminishes the electrostatic repulsions between COO− groups, the ordered conformation is stabilized and the temperature at which conformation changes occur is shifted to higher values (above 55 °C for XG solution with 10−2 mol/L NaCl). The ordered-disordered transition of the chains was also observed in ATR–FTIR spectra. Significant shifts in the regions 1750–1550 cm−1, corresponding to vibration of COO− groups, indicate changes in the hydrogen bonding network.