Physico-chemical properties of aqueous solutions of xanthan: an n.m.r. study

The conformations of xanthan in aqueous solution as a function of temperature have been studied. Measurements of optical activity indicate that the conformational transition, induced by varying the polymer concentration, is analogous to that induced by changes in ionic strength and pH. Within a certain range of concentrations, the low-temperature conformation has a molecular-weight-dependent stability, which shows the usual sigmoidal melting profile with increase in temperature. The 13C-n.m.r. data reflect the increase of the mobility of C-1 and the side-chain carbon atoms in the transition-temperature region. The 23Na relaxation behaviour changes on melting the ordered xanthan conformation. At least two correlation times are needed in order to describe the field-strength dependence of the longitudinal and transverse 23Na relaxation. At 25 degrees, a value of 6.8 ns is obtained for the largest correlation time for the fluctuation of the electric-field gradient. The high-temperature conformation also generates correlation times of the order of ns. From 17O relaxation measurements, a reduction of the mobility of water molecules in the presence of xanthan chains is also observed.