A novel method for measuring hydration and dissolution kinetics of alginate powders

摘要

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 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 f 1:5 was found sufficient for good dispersion. Increased particles 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 solutionw as the rate determining step in the dissoluiton process. Further evidence for this was obtained by comparing the dissolution rate in solution of non-gelling ions (Na~+, K~+ and Mg~(2+)) at different ionic strengths. The decreased rate at higher ionic strength and in particlar the lower dissolution rates in excess of the divalent Mg~(2+)-ion could well be accounted for, in a qualitative manner, by the Nernst-Hartley equation for diffusion of polyelectrolytes. With Ca~(2+) -sales in the solvent, partly hydrated particles were formed giving viscosity values going through a maximum above a critical Ca~(2+)-concentration. The presence of a non-ionic polymer, polyethylene glycol, resulted in a decrease in the dissolution rate.