Diffusion of ions in a calcium alginate hydrogel-structure is the primary factor controlling diffusion

The diffusion of solutes has been evaluated in an alginate hydrogel as a function of its structure. The role of solute and gel charge on the diffusion measurements were of particular interest. Diffusion coefficients were measured using fluorescence correlation spectroscopy as a function of solute charge and size, bulk solution ionic strength and pH, and gel density. Diffusion coefficients of fluorescent dextrans with hydrodynamic radii up to 6 nm were reduced by 30% in a 1.8% (w/w) hydrogel whereas they were reduced by only 2% in a 0.2% (w/w) hydrogel. The role of ionic strength was examined for various concentrations (0.1-100 mM) and compositions of ions (Na(+), Ca(2+) or mixtures thereof). The diffusion coefficient of a small charged probe (rhodamine 6G, R6G(+)) did not change significantly with increasing ionic strength when sodium was used as the counter ion. The diffusion coefficient was only moderately influenced by the charge of solutes (from +1 to -2). Similarly, pH variations from 3 to 9 had little impact on the diffusion coefficients of R6G(+) in the gel. On the other hand, the addition of Ca(2+) had a significant impact on gel compactness, which led to a significant reduction in solute diffusion. For the calcium alginate hydrogels, structural modifications resulting from Ca binding were much more important than electrostatic effects due to modifications of the gel Donnan potential.