Xanthan: effect of molecular conformation on surface tension properties

Abstract

The effect of ionic strength, xanthan concentration and temperature on the molecular conformation of xanthan in aqueous solutions was determined by Potentiometrie titration. The surface tension of these aqueous xanthan solutions was determined by the drop weight and the Wilhelmy plate methods. The degree of xanthan ionization affected its molecular conformation with a transition point at ~0.85; below 0.85 the conformation was that of a helix, while above 0.85 that of a random coil. The ordered conformation was disrupted by increasing the electrostatic potential of the molecule. Temperature increases also disrupted the helix conformation; this effect was reduced by added salt. As expected, the surface tension properties of aqueous xanthan solutions were affected by its molecular conformation. Xanthan in the random coil conformation was more surface active than xanthan in the helix conformation. Wilhelmy plate determinations showed that the surface tension of xanthan solutions is time dependent. The equilibrium surface tension decreased and the adsorption rate increased significantly by increasing the xanthan concentration and by adding 0.1 N NaCl. Surface excess values calculated by the Gibbs equation suggested that the surface tension was affected by the proportion of molecular segments absorbed at the interface. The implications of these findings to industrial applications are discussed with particular reference to food emulsions.