Abstract
In-situ gelation of semidilute xanthan solutions with trivalent chromium,
aluminum or iron ions was studied by rheology and UV-spectroscopy.
Measurements of the elastic modulus of xanthan gel cylinders prepared
by dialysis against the complexing ion at pH values from 2 to 6 indicate
that monomeric species of the ion are ineffective, whereas dimeric
or higher oligomeric species are effective in crosslinking the polysaccharide.
When chromium was used as the crosslinking species, the dependence
of the gelation rate on the ionic concentration followed a power
law with a coefficient of 1.7. The gelation time and the gelation
rate were found to extrapolate to zero at 1 mm Cr for 2.5 mg/ml xanthan.
The limiting concentration of xanthan needed for gelation with 5
mm Cr(III) at 20-degrees-C was estimated as 0-35 mg/ml. This critical
xanthan concentration is close to the overlap concentration c* estimated
from the experimentally determined intrinsic viscosity eta using
c* = 1.4/eta. An apparent activation energy for crosslinking of
xanthan was calculated as E(a) = 42 kJ/mol and E(a) = 108 kJ/mol
for C rand Al ions, respectively. The fractal dimensionality of xanthan-Cr
at the sol-gel transition was estimated as 1.3 applying the Chambon-Winter
criterion for gelation, thus indicating that this gelation criterion
is applicable also to stiff-chain polysaccharides such as xanthan.
Users
Please
log in to take part in the discussion (add own reviews or comments).