Steady shear and dynamic rheological properties of xanthan gum solutions in viscous solvents

An extensive examination of the first normal stress difference and linear v iscoelastic properties of xanthan gum solutions has been conducted in relat ion to molecular theories in the literature. The first normal stress differ ence, storage modulus, and loss modulus are reported for 0.01-0.04% w/w sol utions of xanthan gum in high viscosity (wheat syrup and water) solvents. T he average length of the xanthan molecules used was determined by light sca ttering to be (1.25 +/- 0.05)X 10(3) nm. The storage and loss moduli obtain ed show a frequency dependence consistent with theories that included a rel axation time spectrum, while the first normal stress difference exhibits de pendence on shear rate consistent with theories of suspensions of rigid par ticles. Both the first normal stress difference and the solute contribution to the storage modulus were found to vary linearly with concentration and with the solvent viscosity to the power of 2/3. Extensional viscosity measu rements of a xanthan gum solution are in good agreement with the prediction s for semi-dilute suspensions of rigid rods. (C) 1999 The Society of Rheolo gy. [S0148-6055(99)00403-4].