Rheological evidence for the silica-mediated gelation of xanthan gum

Abstract

In the present study, the conformational effects of the adsorbed polyelectrolyte xanthan gum on the stability of colloidal silica suspensions were investigated by monitoring the rheological responses. The conformational transition of xanthan molecules was induced by changes in pH, added salts, and annealing temperature. The suspension stability was probed by examining the rheological behavior such as shear viscosity versus shear rate and shear moduli versus oscillatory frequency. The oscillatory shear measurements provided useful information on the structural change in the suspension. The results showed that the silica particles dispersed in the xanthan solution remained stable at pH 9 for a long period, whereas at pH 2, the gel network structure formed within a few days. When the gel structure formed, the storage modulus was larger than the loss modulus over the entire frequency range considered here and exhibited a thixotropic behavior. The heat treatment temperature best for stabilizing the silica suspension was shown about 60 degrees C and the silica sol treated at this temperature sustained its stability for more than 6 weeks. (C) 1999 Academic Press.