Rheological evidence for the silica-mediated gelation of xanthan gum

In the present study, the conformational effects of the adsorbed polyelectr olyte xanthan gum on the stability of colloidal silica suspensions were inv estigated by monitoring the rheological responses. The conformational trans ition of xanthan molecules was induced by changes in pH, added salts, and a nnealing temperature. The suspension stability was probed by examining the rheological behavior such as shear viscosity versus shear rate and shear mo duli versus oscillatory frequency. The oscillatory shear measurements provi ded useful information on the structural change in the suspension. The resu lts showed that the silica particles dispersed in the xanthan solution rema ined stable at pH 9 for a long period, whereas at pH 2, the gel network str ucture formed within a few days. When the gel structure formed, the storage modulus was larger than the loss modulus over the entire frequency range c onsidered here and exhibited a thixotropic behavior. The heat treatment tem perature best for stabilizing the silica suspension was shown about 60 degr ees C and the silica sol treated at this temperature sustained its stabilit y for more than 6 weeks. (C) 1999 Academic Press.