Static light scattering studies on chitosan solutions: From macromolecular chains to colloidal dispersions

The neutralization of a homogeneous series of chitosans with degrees of acetylation (DA) varying between 1% and 71% was followed stepwise by static light scattering. The determination of the gyration radii (R-G,R-z) and the second virial coefficients (A(2)) allowed us a better understanding of the chitosan behavior during the neutralization in aqueous solutions. On increasing pH, a critical value was evidenced both by a drop Of R-G,R-z and a fall of A(2). The values of the critical pH were also shown as highly dependent on the polymer concentration and the ionic strength of the media. On increasing DA, the range of pH where chitosan was apparently soluble enlarged although the second virial coefficient decreased with both the increase of DA and the degree of neutralization. Working at the critical pH, just before precipitation occurred, transmission electronic microscopy allowed us to evidence a sol/gel transition responsible for the formation of a stable colloidal dispersion. The average particle sizes increased with the degree of acetylation: from 100 nm for DA = 1% to 300 nm for DA = 51%. These aggregates were probably assembled both by hydrophobic interactions and hydrogen bonding due to the neutralization of the ammonium groups into -NH2.