Thermal and viscoelastic properties of xanthan gum/chitosan complexes in aqueous solutions

The viscoelasticity and thermal properties of aqueous solutions of blended xanthan gum (XA) with chitosan were investigated in order to study the electrostatic interaction established between two polysaccharides. Storage modulus G’, loss modulus G” and zero shear rate viscosity η_O attain a maximum at a chitosan concentration C _max. The above results indicate that the junction between XA and chitosan is formed in a concentration range lower than C _max and the viscoelasticity of systems increases with increasing concentration. In a concentration range higher than C _max, junction formation may not occur effectively since the excess amount of chitosan completely screens anions of XA. The chain rigidity of XA decreases by the screening of the repulsive interaction between anions on XA chains. The ineffective junction formation and the decrease of XA chain rigidity may cause the decrease of viscoelasticity of systems with increasing concentration. The value of C _max decreases with increasing molecular mass of chitosan. From melting enthalpy of the above system measured by DSC, the amount of non-freezing water (W _nf) was evaluated. W _nf shows a minimum at the concentration C _max. This fact suggests that hydrophobic fields increased by junction structure formation through ion-complexation between XA and chitosan molecules.