Sodium Xanthan Research Articles

Second virial coefficient for charged rods: sodium xanthan in aqueous sodium chloride

Second virial coefficient for charged rods: sodium xanthan in aqueous sodium chloride

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 t d 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 t d increased. No substantial degradation of the sample was found to occur within t d = 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.

Thermally induced conformational change of xanthan in 0.01 m aqueous sodium chloride

Light-scattering and viscosity measurements were made on five sonicated samples of sodium xanthan in 0.01M aqueous NaCl at 25 and 80°, temperatures at which the polysaccharide is known to take on certain ordered and disordered conformations, respectively. For two of these samples, measurements were also made at different temperatures between 25 and 80°. From the data obtained for weight-average molecular weight M w, radius of gyration, and intrinsic viscosity, the following conclusions were derived. ( 1) The ordered conformation of Na xanthan in 0.01M aqueous NaCl at 25° is essentially the same dimerized, double helix as that found previously in 0.1M aqueous NaCl. ( 2) When the temperature is raised from 25 to 80°, the dimer does not dissociate to single chains, but its statistical radius decreases, or remains unchanged, depending on its M w. ( 3) The disordered conformation at 80° is well represented by a model in which four wormlike chains are joined together by a very short double-helical segment. ( 4) The thermally induced, order-disorder conformational change of xanthan in 0.01 m aqueous NaCl is the process of partial melting of the double helix.

Viscosity behaviour and persistence length of sodium xanthan in aqueous sodium chloride

Zero-shear-rate intrinsic viscosities [eta] of sodium xanthan in aqueous NACl at 25 degrees C were determined for five samples ranging in weight- average molecular weight from 2 x 10(5) to 4 x 10(6) at salt concentrations Cs between 0.005 and 1 M, at which the polysaccharide maintains its double-helical structure. The measured [eta] for every sample was almost independent of Cs, in contrast to usual observations on flexible polyelectrolytes. The persistence length q of sodium xanthan was determined as a function of Cs by use of the theory of Yamakawa et al. for [eta] of an unperturbed worm-like cylinder, and from its Cs dependence the intrinsic persistence length q(o) ( = q at infinite ionic strength) was estimated to be 106 nm. This q(o) value was roughly twice as large as that of double-stranded DNA, indicating a high intrinsic rigidity of the xanthan double helix. The electrostatic contribution ( = q - q(o)) to q was only about 10% even at the lowest Cs of 0.005 M. Thus, it was concluded that above Cs = 0.005 M, the double- helical structure of sodium xanthan is hardly stiffened by electrostatic interactions between charged groups.