Partition and diffusion of sodium and chloride ions in soft charged foam: The effect of external salt concentration and mechanical deformation

The partition and diffusion characteristics of an acrylic acid/acrylamide hydrogel, copolymerized in the pores of a polyurethane foam with sodium and chloride ions, were studied by radiochemical methodologies. The hydrogel foam swells by 51%, 80%, and 260% relative to its raw state under bath salt concentrations of 2.0, 1.0, and 0.15 M, respectively. The corresponding partition coefficients are 1.13, 1.29, and 1.99 for sodium (Na⁺) and 0.89, 0.85, and 0.65 for chloride (Cl^-). The diffusion coefficients are independent of bath concentration and increase linearly with hydration towards their values in water. Deformation affects partition and diffusion solely by dilatation, which determines the swelling and hydration. Comparison of the hydrogel foam with cartilage and intervertebral disc shows considerable similarities and suggests that the same mechanisms control their function.