Abstract
Chitosans having degrees of N-acetylation, F-A, ranging from 0 to
0.6, were randomly degraded to different molecular weights and studied
by multi angle static laser light scattering (LLS). Under the given
experimental conditions, negative second virial coefficients of the
solutions, A'(2), revealed the presence of concentration dependent
aggregates. Attempts to remove the aggregates, or to influence the
aggregation behavior, were made by ultracentrifugation and extensive
filtering of the solutions. Modification of the solvent conditions
such as pH, ionic strength and temperature were carried out, and
chitosan solutions were digested with an acidic proteinase. Non-degraded
samples and chitosans prepared by both heterogeneous and homogeneous
N-deacetylation of chitin were also studied. In all cases, the negative
A'(2) remained. However, it was observed that ultracentrifugation
and filtering of the solutions decreased the measured molecular weights
and radii of gyration, indicating that some of the material of high
molecular weight and size could be removed by ultracentrifugation
and filtration. The chemical nature of the physical basis of the
molecular association was not revealed. Nevertheless, by the use
of gel permeation chromatography coupled to an on-line low angle
laser light scattering instrument and a differential refractive index
concentration detector (HPSEC-LALLS-RI), a bimodal molecular weight
distribution was observed in which about 5% of the sample had a very
high molecular weight. These results coupled with the positive virial
coefficients obtained earlier from osmotic pressure measurements
suggest that a small fraction of the chitosan is aggregated to high
molecular weight material, probably following a closed association
model. Electron microscopy revealed the presence of some supramolecular
structures. The positive second virial coefficients obtained earlier
from osmometry are in harmony with these findings. The results demonstrate
the occurrence of reversible aggregation in chitosan solutions. Static
laser light scattering therefore cannot readily be used to determine
molecular weights and sizes of chitosans under these conditions.
It was not possible to correlate the extent of aggregation with the
chemical composition of the chitosans.
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