Abstract
A novel method was developed to determine hydration and dissolution
kinetics for well dispersed alginate powders added to agitated water.
A special propeller and a StressTech rheometer were used to monitor
the increase in viscosity as function of time. The shear stress values
obtained from the rheometer were used as a parameter for the viscosity,
and relative dissolution curves were plotted using shear stress/alginate
concentration standard curves, or directly using the shear stress
values as a parameter for dissolved alginate. These curves were fitted
to an exponential function that gave the rate constant for the dissolution
process, and a time constant related to the time necessary to hydrate
the alginate particles. The standard curves were linear between the
logarithm of the shear stress and the alginate concentration above
0.1 % (w/v). The alginate powders were usually dispersed in sugar.
For particle size 250 mum, an alginate/sugar ratio of 1:5 was found
sufficient for good dispersion. Increased particle size led to a
decrease in the dissolution rate, while raising the stirrer speed
increased dissolution rates. The rate constant increased with increased
temperature as given by Arrhenius' law, and the activation energy
for dissolution in water was found to be 23 kJ/mol, suggesting that
diffusion of alginate from the particle to the surrounding solution
was the rate determining step in the dissolution process. Further
evidence for this was obtained by comparing the dissolution rate
in solution of non-gelling ions (Na+, K+ and Mg2+) at different ionic
strengths. The decreased rate at higher ionic strength and in particular
the lower dissolution rates in excess of the divalent Mg2+-ion could
well be accounted for, in a qualitative manner, by the Nernst-Hartley
equation for diffusion of polyelectrolytes. With Ca2+-salts in the
solvent, partly hydrated particles were formed giving viscosity values
going through a maximum above a critical Ca2+-concentration. The
presence of a non-ionic polymer, polyethylene glycol, resulted in
a decrease in the dissolution rate. (C) 2003 Elsevier Science Ltd
All rights reserved.
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