Order–Disorder conformation change of xanthan in 0.01M aqueous sodium chloride: Dimensional behavior

Weight‐average molecular weights Mw, second virial coefficients, and z‐average radii of gyration 〈S2〉 were determined by light scattering as a function of temperature T for four sodium salt samples of xanthan in 0.01M aqueous NaCl, in which the polysaccharide undergoes an order–disorder conformation change with increasing T. The data for 〈S2〉 and Mw at 25 and 80°C, the lowest and highest temperatures studied, confirmed the previous conclusion that the predominant conformation at the former T, i.e., in the ordered state, is a double helix, while that at the latter T, i.e., in the disordered state, is a dimerized coil expanded by electrostatic repulsions between charged groups of the polymer. As T was increased from 25 to 80°C, 〈S2〉 sigmoidally decreased or increased depending on the dimer's molecular weight. This temperature dependence of 〈S2〉 and that determined elsewhere for a high molecular weight sample were found to be described almost quantitatively by a simple dimer model in which the double helix melts from both ends, when the double‐helical fraction in the dimer at a given T estimated previously from optical rotation data was used.