Mechanochemical Reaction of Calcium Alginate Hydrogel due to a Counterion Exchange

Stress relaxations of calcium alginate hydrogel (CaAlg gel) rods induced by adding other counterions were investigated by measuring their tensile stresses. The stress decreased upon adding Ba2+ and H+, whereas it increased upon adding Pb2+. The relaxation time, equilibrium elastic modulus and spontaneous elastic modulus were obtained. The relaxation times were analyzed using a theory involving counterion diffusion accompanied by an ion-exchange reaction under the assumption that the rate-determing step is the diffusion of added counterions (M2+ and M+) in the gel. The relaxation time was found to be a function of the concentration and diffusivity of the added counterions, the radius of the gel rod, the concentration of carboxylate ions and the counterion-selectivity coefficient () of the alginate. By fitting the observed relaxation times with theoretical ones, the optimal values of a were obtained: = 100, = 20 and = 3 × 103. The rate-determing step of stree relaxation was confirmed to be the counterion diffusion process in CaAlg gel.