A spectroscopic and viscometric study of the metal ion-induced gelation of the biopolymer chitosan

The gelation of the biopolymer chitosan in solution through crosslinking reactions with Pt(II) has been investigated by simultaneously monitoring both the viscosity of the system and the number of metal-amine bonds formed per chitosan chain, as probed spectroscopically. The variables studied in the investigations included the chitosan to Pt(II) concentration ratio, the pH of the reaction solution, and the concentration, molecular weight and degree of deacylation of chitosan. A kinetic model for the time dependence of the formation of crosslinks and a spectral model for the time evolution of the spectra were developed for the gelation process. These models included the effects of light scattering, steric hindrance, pH and variation of pK(a) as a function of the fraction of monomer units charged, and they were tested successfully against the experimental spectra. The time of gelation was found to decrease with increases in the following experimental parameters while holding all the other parameters constant: (1) concentration of Pt(II), (2) pH of the reaction solution, (3) molecular weight of chitosan, and (4) concentration of chitosan in solution. In accordance with percolation theory, the calculated crosslink/chain ratios at the time of gelation were found to be quite consistent for the various reaction conditions and to be consistent also with predictions of thresholds for percolation of rigidity by network theories. (C) 1997 Elsevier Science Ltd.