Kinetic study of chitosan-tripolyphosphate complex reaction and acid-resistive properties of the chitosan-tripolyphosphate gel beads prepared by in-liquid curing method

Chitosan gel beads were prepared using an in-liquid curing method by the ionotropic crosslinking with sodium tripolyphosphate. Crosslinking characteristics of the chitosan-TPP beads were improved by the modification of in-liquid curing mechanism of the beads in TPP solution. Chitosan gel beads cured in pH value lower than 6 were really ionic-crosslinking controlled, whereas chitosan gel beads cured in pH values higher than 7 were coacervation-phase inversion controlled accompanied with slightly ionic-crosslinking dependence. According to the result, significantly increasing the ionic-crosslinking density of chitosan beads could be achieved by transferring the pH value of the curing agent, TPP, from basic to acidic. The swelling behavior of various chitosan beads in acid appeared to depend on the ionic-crosslinking density of the chitosan-TPP beads that were deeply affected by the curing mechanism of the beads. The mechanism of chitosan-TPP beads swollen in weak acid was chain-relaxation controlled, while the mechanism of chitosan-TPP beads swollen in strong acid seem to be not only chain-relaxation but also chain-scission controlled. Chitosan-TPP beads prepared in acidic TPP solution decreased the chain-scission ability due to the increase of ionic crosslinking density of the beads. By the transition of curing mechanism, the swelling degree of chitosan-TPP beads was depressed, and the disintegration of chitosan-TPP beads would not occur in strong acid. The mechanism of ionic-crosslinking reaction of chitosan beads could be investigated by an unreacted core model, and the curing mechanism of the chitosan beads is mainly diffusion controlled when higher than 5% of chitosan was employed.