BEHAVIOR OF NATIVE XANTHAN IN THE BIPHASIC REGION - A NA-23(+) COUNTERION NMR-STUDY

The isotropic phase and the biphasic region of salt-free native xanthan were investigated using Na-23 NMR techniques. Visual inspection of the samples between crossed polarizers demonstrates the formation of anisotropic domains in the biphasic region beyond 9 g/L, and it was found to coincide with the observation of quadrupolar splitting (QS) of the Na-23 NMR signal. Before QS and birefringence are observable, the first indication of domain formation is a strong increase of the fast component of the apparent transverse relaxation rate at 5 g/L. Hence, the boundary concentration of the isotropic phase, a limit for further conformational analysis by NMR, can now be established with more accuracy. The QS of the Na-23 NMR signal depends on magnetic field strength, orientation, and time. A sample which was rapidly concentrated from the isotropic phase to the onset of the biphasic region displayed an increased relaxation rate, which decayed toward equilibrium in ca. 50 days. For the first time, using NMR, we were able to monitor the development of this slow equilibration. In the biphasic region, the QS reaches equilibrium after 1.5 year. Many observations of apparent hysteresis and irreversibilities in the literature may be related to such a slow equilibration.