Liquid crystalline phase in xanthan gum (XG)/H₂O/H₃PO₃ and XG/H₂O/H₃PO₄ tertiary systems: a thermal and rheological study

This study investigates the difference in phase transition and rheological behavior between xanthan gum (XG)/H₂O/H₃PO₃ (XWP₃) and XG/H₂O/H₃PO₄ (XWP₄) tertiary systems using polarized optical microscopy, Fourier transform infrared spectroscopy, light transmission detection, and rheometry. The results show that the LC (liquid crystal) phase formation in the XWP₄ system was more strongly suppressed than that in the XWP₃ system because the former exhibited stronger interactions between acid and XG molecules. With respect to the transition from LC to I (isotropic) phase at high temperature, the transition time of the XWP₄ system was found to be much shorter than that of the XWP₃ system. The transition time, also called the relaxation time, was measured by observing the annealing time and fitted using the VFT expression. The activation energies E for this transition in XWP₄ and XWP₃ systems are 3.0 and 4.7 kJ/mole, respectively, indicating that the XWP₃ system exhibits stronger intermolecular attraction and is more sensitive to variation in temperature. In the rheological tests, as the temperature of the XWP₄ system increased from 25 to 95 °C, the viscosity in the transitional region declined consistently, while in the XWP₃ system, the decline proceeded through three stages owing to the shifting tautomers of the H₃PO₃. In the LC region, the viscosity normally fell as the shear rate or temperature increased but increased with the heating rate or XG concentration. Most interestingly, all of the results herein demonstrated that the viscosity of the XG solution in the LC region followed a power law with an index of roughly 0.08, which was found to be independent of the type of acid, concentration of acid, and XG concentration.