Towards a less biased dissolution of chitosan

• Clear solutions of chitosan do not ensure complete solubility.
• Aggregation of chitosan is observed in size-exclusion chromatography conditions.
• HCl and DCl cause deacetylation of chitosan during dissolution in water.
• The extent of chitosan dissolution is more limited in aqueous acetic acid than in aqueous HCl.
• The dissolution of chitosan in aqueous HCl is relatively quick; however, it may not be complete.

The dissolution of polysaccharides is notoriously challenging, especially when one needs a “true” solution. Factors influencing chitosan's solubility include composition, also known as degree of acetylation (DA). The dissolution of chitosan was investigated by visual observation, size-exclusion chromatography (SEC), pressure mobilization (PM), free-solution capillary electrophoresis (CE) and real-time solution-state NMR spectroscopy. Aqueous HCl dissolves around 15% more chitosan than the commonly used aqueous acetic acid (AcOH), however aggregates were detected in SEC suggesting incomplete dissolution. Significant deacetylation of chitosan over the period needed for dissolution at high temperature was observed by NMR spectroscopy in DCl by about 20% of the initial DA value. Accurate DA determination by NMR spectroscopy may thus be possible only in the solid state (with a precision within 1% on the DA % scale above a DA of 10%). Overall a compromise between maximum solubilization and minimum degradation is required in attempting to obtain a “true” solution of chitosan. The completeness of the dissolution may be more influenced by the average DA than by molar mass.

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