Effects of Hydrophobic Substituents and Salt on Core−Shell Aggregates of Hydrophobically Modified Chitosan: Light Scattering Study

摘要

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