DISSOCIATION OF DIMERIZED XANTHAN IN AQUEOUS-SOLUTION

Aqueous solutions of low-molecular-weight sodium xanthan with a degree of pyruvation of about 1 3 were heated at 95°C for different time periods td between 2·5 and 15 h, mixed with aqueous sodium chloride to an NaCl concentration of 0·01 or 0·1 m and subjected to viscosity, optical rotation and sedimentation equilibrium measurements at 25°C. The measured specific rotations showed that, upon addition of aqueous NaCl to 0·1 m, Na xanthan disordered in water at 95°C became ordered to a conformation locally similar to that of each chain in the double helix of the polysaccharide in 0·1 m aqueous NaCl at 25°C. However, both viscosity and molecular weight decreased considerably below the values for the double-helical dimer as td increased. No substantial degradation of the sample was found to occur within td = 9 h. It was concluded from data analysis that at least 70% of the dimers dissociated into monomers when pure water solutions were heated at 95°C for 9 h. © 1990.