E.V. Korchagina

Chitosan and its hydrophobic derivatives: Preparation and aggregation in dilute aqueous solutions

This review surveys the main methods of preparing chitosan and its hydrophobic derivatives and their influence on the microstructure of polymers. The experimental data on the aggregation of these poly� mers in dilute aqueous solutions are summarized. Basic factors affecting aggregation are analyzed, and its general regularities are revealed. It is shown that, in the case of both chitosan and its hydrophobic derivatives, the formation of aggregates is governed by the competition of attraction of associating groups promoting aggregation and their repulsion arising from the presence of charged units and counterions hindering aggre� gation. Various aggregate models that were proposed for chitosan derivatives with different mainchain lengths and different contents of associating groups are discussed.

Multichain aggregates in dilute solutions of associating polyelectrolyte keeping a constant size at the increase in the chain length of individual macromolecules.

Multichain aggregates together with individual macromolecules were detected by light scattering in dilute aqueous solutions of chitosan and of its hydrophobic derivatives bearing 4 mol % of n-dodecyl side groups. It was demonstrated that the size of aggregates and their aggregation numbers increase at the introduction of hydrophobic side groups into polymer chains. The key result concerns the effect of the chain length of individual macromolecules on the aggregation behavior. It was shown that for both unmodified and hydrophobically modified (HM) chitosan, the size of aggregates is independent of the length of single chains, which may result from the electrostatic nature of the stabilization of aggregates. At the same time, the number of macromolecules in one aggregate increases significantly with decreasing length of single chains to provide a sufficient number of associating groups to stabilize the aggregate. The analysis of the light scattering data together with TEM results suggests that the aggregates of chitosan and HM chitosan represent spherical hydrogel particles with denser core and looser shell covered with dangling chains.

Effects of hydrophobic substituents and salt on core-shell aggregates of hydrophobically modified chitosan: Light scattering study

In this study we examine two methods of enhancement of aggregation of hydrophobically modified chitosan in dilute aqueous solutions: by increasing the content of n-dodecyl substituents, favoring hydrophobic association, and by increasing the amount of added low molecular weight salt, screening the electrostatic repulsion between similarly charged aggregating chains. By static and dynamic light scattering it was demonstrated that at the growth of the content of hydrophobic groups in the polymer (2-4 mol %) and of the amount of salt in solution (0.025-0.1 M) the weight fraction of aggregates increases, but the aggregation number remains unchanged. This behavior was attributed to the core-shell structure of the aggregates, which provides a low surface energy and strong attraction of associating groups inside the core. At the same time, the effects of the content of hydrophobic groups in the polymer and the ionic strength of the solution on the radii of the aggregates are quite different. Increasing the content of hydrophobic groups induces growth of the gyration radii of the aggregates, but does not affect their hydrodynamic radii. These data suggest the expansion of the hydrophobic core of the aggregates and the contraction of their highly swollen shell. On the other hand, increasing the salt concentration leads to a decrease of both the gyration and hydrodynamic radii of the aggregates, which is due to partial screening of electrostatic repulsion between similarly charged units and lowering of the osmotic pressure of counterions confined inside the aggregates.