1. Molecular structure: The “binding” of a strong hydrogen bond network
In the molecular structure of each glucosamine unit, there are two strongly polar groups, namely the hydroxyl group (-OH) and the amino group (-NH₂).
These polar groups form a large number of hydrogen bonds within the molecule (between different units of the same chitosan molecule) and between molecules (between different chitosan molecules): The hydrogen atom (δ⁺) of the hydroxyl group will be closely combined with the oxygen atom (δ⁻) or nitrogen atom (δ⁻) in another molecule, constructing a dense and stable hydrogen bond network.
This strong hydrogen bond interaction far exceeds the intermolecular force (hydration) between water molecules and chitosan molecules, resulting in water molecules being unable to effectively “break apart” the molecular aggregation structure of chitosan, and ultimately making it difficult to dissolve.
2. Molecular aggregation state: The compact crystalline region hinders dissolution
In the crystalline region, the molecular chains are arranged regularly and closely, and the hydrogen bond interaction is stronger and more concentrated, and water molecules can hardly penetrate into it;
Although the amorphous region has a relatively loose structure, its proportion is usually low, and it is still “restrained” by the surrounding crystalline regions, which is not enough to allow the entire molecule to be solvated by water. This aggregation state dominated by “crystalline regions” further enhances the insolubility of chitosan in water.
3. Solution pH value: The “switching effect” of amino protonation
Neutral/alkaline environment (pH > 6.5): The amino group (-NH₂) in the chitosan molecule is in a non-protonated state (without charge). At this time, the hydrogen bond and hydrophobic interaction between molecules dominate, and the molecules aggregate closely and cannot dissolve in water;
Acidic environment (pH < 6.0): The amino group will combine with hydrogen ions (H⁺) to form a positively charged amino cation (-NH₃⁺). The repulsive force between the positive charges will break the hydrogen bond network between molecules, causing the chitosan molecules to disperse and form a hydration layer with water molecules, thereby dissolving (such as dissolving in dilute hydrochloric acid, acetic acid, etc.).
