Chitosan is a biopolymer sourced from crustacean exoskeletons, such as shrimp, crabs, etc., and fungal sources, such as mushrooms. It is a versatile biopolymer as it’s the only biopolymer with a positive cationic charge. It’s used in a wide range of applications including, pharmaceuticals, medical, foods, dietary supplements cosmetics, agriculture, water treatment, etc. However, chitosan’s inherent insolubility in water and conventional solvents can pose a dissolution challenge. In this Chitosan FlashFocus edition, two easy-to-use methods for chitosan dissolution are provided: an acidic and an ionic approach. In addition, key factors that affect chitosan solubility will be summarized, i.e., its N-acetyl-D-glucosamine units, pH, molecular weight, and Salt effect/ionic strength, and temperature.

Chitosans exhibit different levels of acetylation. Their crystalline structure with strong hydrogen bonds forms a highly aggregated three-dimensional network, making them insoluble in typical solvents. Pure chitin contains approximately 90% N-acetyl groups but undergoes deacetylation during extraction. Chitin consists of two monomer units: N-acetyl-D-glucosamine, which is insoluble due to strong hydrogen bonds, and N-amino-D-glucosamine, which is hydrophilic and positively charged in acidic conditions. The degree of deacetylation (DDA%) determines solubility, with chitosan forming when DDA% is 60-98% and chitin being insoluble below 60%. Deacetylation processes result in different acetyl group distributions: micro-block and random. High DDA or low DD leads to chitosan aggregation, while low DDA results in hydrophobic bonds and crystalline domains in chitin. The ratio of N-acetyl-D-glucosamine units significantly influences solubility and solution properties.

The aggregation behavior of chitosan is significantly influenced by the pH of the solvent. Chitosan molecules are ionized up to pH 6.0, with ionization increasing at lower pH values. At pH < 6.0, amino groups on chitosan chains (particularly those with low DA) capture H+ ions, resulting in a positive surface charge measurable by zeta potential. This charge hinders chitosan aggregation. However, as the DA value increases, aggregation begins to dominate over the coulombic repulsion forces of charged groups. The pH at which the net charge of a chitosan solution prevents aggregation is termed the critical pH. There are two distinct types of aggregation based on pH: closed type at very low pH, where chitosan remains stable and fully protonated, and open type at higher pH, where aggregation occurs due to association forces and hydrogen bonds involving neutralized NH2 groups. Additionally, hydrolytic cleavage can occur when chitin is treated with strong acids like concentrated acetic acid or HCl, involving glycosidic bonds.

In addition to factors like degree of deacetylation (DA) and pH, the molecular weight of chitosan plays a significant role in its solubility. Chitosan becomes more soluble as its molecular weight decreases. The process of chitosan solubilization involves various chemical and physical interactions, including hydrogen bonds and hydrophobic interactions. When the DA is less than 50%, protonated amino groups dominate and electrostatic repulsions between chitin chains weaken, allowing chitin with low DA to become soluble at acidic pH. Chitin with low DA is fully ionized at pH 3.0 but precipitates at pH 6.0. Thus, the transition between dissolved and undissolved chitin is primarily governed by pH and the degree of deacetylation. High molecular weight chitosan (300 kDa) exhibits an α-chitin crystalline structure upon aggregation, driven by entropy loss and compensated by water release. However, low molecular weight chitosan (2.43 kDa) resists aggregation due to shorter chains with fewer intermolecular hydrogen bonds, shifting the transition to kDa).

Ionic strength, representing the concentration of ions in a solution, has a notable impact on chitosan behavior. Charged particles exhibit electrostatic effects within a certain distance, known as the Debye screening length (k−1). This electric field influences the movement of charged colloidal particles and gives rise to a double layer in chitosan solutions due to the charged polysaccharide particles and counterions. Debye screening length (k−1) is inversely related to ionic strength and follows a square root relationship. Chitosan acts as a polyelectrolyte in solution, resulting in electrostatic repulsion. However, increasing the ionic strength, often by adding salt, can shift interactions from repulsive to attractive, reducing k−1 and promoting flocculation or precipitation of chitosan. Higher ionic strength also decreases the volume occupied by chitosan chains, leading to a drop in intrinsic viscosity. Additionally, temperature affects the formation and dissolution of hydrogen bonds between acetyl and hydroxyl groups, with a dissolution temperature dependent on ionic strength, increasing with higher ionic strength beyond a critical threshold.

 Chitosan solubility can present challenges, but implementing either of these methods offers easy-to-achieve solutions.

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We also would like to acknowledge the following as they provided the inputs to the factors that affect chitosan solubility: Roy, Jagadish C., et al. ‘Solubility of Chitin: Solvents, Solution Behaviors and Their Related Mechanisms’. Solubility of Polysaccharides, InTech, 29 Nov. 2017.Crossref, doi:10.5772/intechopen.71385.