The effect of counterion size and valency on the increase in viscosity in polyelectrolyte solutions

Abstract

Entangled polymer solutions play an important role in many industries and applications, however the dynamics of these solutions are poorly understood. Here, the addition of salt to entangled polylectrolyte solutions (above cD) results in an increase in viscosity. The rheological properties of entangled xanthan solutions above the critical concentration cD are examined in a number of inorganic salt solutions. The effect of salt counterion size and valency on the magnitude of the viscosity increase is elucidated. A hypothesis that larger salt counterions produce higher viscosities is confirmed for both monovalent and divalent salts. Further, divalent salts are observed to produce higher viscosities than monovalent salts of similar ionic radius. Lastly, an alternative hypothesis incorporating ion bridging between polymer chains is proposed to explain the effect of counterion valency in the observed viscosity differences.