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

Abstract

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.