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A series of water-soluble sulfoethylated chitosans (SEC) with degrees of substitution (DS) up to 130% were obtained using a heterogeneous reaction of chitosan with sodium 2-chloroethanesulfonate in 85% isopropanol in the presence of NaOH. NMR and FTIR spectroscopy confirmed that sulfoethylation of chitosan preferentially happens at hydroxyl groups and to some extent at amino groups, giving mixed substituted O,N -SEC. Chitosan shows positive birefringence, whereas SEC shows negative values, indicating self-organization in dilute solution. Dynamic light scattering studies revealed the presence of aggregates in dilute solutions of chitosan and SEC. The sizes of the SEC aggregates are sensitive to the DS and the nature of the solvent. X-ray diffraction of SEC films revealed that the introduction of sulfoethyl groups into chitosan leads to amorphization, which is more pronounced at higher DS. During the storage of SEC films, the samples loose solubility due to the formation of ionic crosslinks upon dehydration. [ABSTRACT FROM AUTHOR]