Rheological and kinetic study of the ultrasonic degradation of xanthan gum in aqueous solutions

The effectiveness of ultrasound to degrade the molecular weight of xanthan gum in aqueous solutions was investigated for sonication times up to 60 min at 20 °C and for polymer concentrations up to 0.1 g/dl. The Huggins equation was found to be applicable to the intrinsic viscosity of xanthan gum prior to sonication, while a truncated form was found to be adequate for estimating the intrinsic viscosity of the degraded xanthan. To better understand the influence of salting-in and salting-out salts (classified on the basis of the Hofmeister series) on degradation, xanthan-gum solutions were pre-mixed with 0.1, 10−2, 10−3, or 10−4 M NaCl or Na2SO4, prior to ultrasonication. A kinetic model was developed and successfully applied to quantify and predict the degradation rates and efficiency. The various reaction rate constants and reaction orders were found to correlate with the different salt species and concentrations used, suggesting that salting-in and salting-out salts could increase or inhibit ultrasonic degradation by adjusting the molecular conformation of the xanthan.