Streaming Potentials of Hyaluronic Acid Hydrogel Films

The streaming potentials of hyaluronic acid (HA) hydrogel films are measured and theoretically interpreted by systematically varying the HA concentration and the streaming electrolyte pH and ionic strength. While Donnan potentials are expected to vanish with sufficient added salt, apparent ζ-potentials from the Helmholtz-Smoluchowski interpretation remain of the order -20 mV. To theoretically interpret these data, we derived an electrokinetic model (valid in the Debye-Hückel regime) that accounts for ionic and hydrodynamic permeability of the gels. The films could then be ascribed an effective acid dissociation constant pK_a ≈ 4.2, specific HA charge ≈-0.1e mmol g^-1, and Brinkman/hydrodynamic permeability $$ ∼ $$S^1/3, where $$ is the Brinkman length for HA solutions in the as-prepared reference state and S is the hydrogel swelling ratio. At an ionic strength of 10 mmol L^-1, for example, the HA surface potentials are only ψ_D/2 ≈ -8 mV, where ψ_D is the Donnan potential, considerably lower than ζ-potentials furnished by the Helmholtz-Smoluchowski interpretation. This insight significantly changes how the films are expected to interact with other surfaces and colloids via Derjaguin-Landau-Vervey-Overbeek-type forces. Our analysis furnishes formulas for the swelling ratio S and hydrodynamic permeability $$, expressed explicitly as simple power-law functions of the as-prepared HA concentration c_ha (wt %), consistent with independent assessments of the HA solution permeability and polyelectrolyte-hydrogel swelling theory. These may prove valuable for extrapolating the results to other combinations of ionic strength, pH, and HA and cross-linking concentrations.