## 5 Solubility of Chitosan  
Pure chitosan is a white or gray transparent flake or solid powder, tasteless, odorless, non-toxic and stable at room temperature. There is a strong hydrogen bond between chitin molecules, forming a highly crystalline structure, so chitin is highly insoluble in dilute alkali, dilute acid, water and most organic solvents, only soluble in concentrated acid and some solvents. The solubility of chitosan is better than that of chitin because the active group of chitosan molecules is an amino group rather than the acetyl group. Chitosan can be soluble in dilute acid, formic acid and acetic acid, but not in water and most organic solvents. It is easy to react with acid to generate salt due to the existence of an amino group in the structural unit of chitosan, forming a positive cationic group.  
Chitosan solubility is related to the degree of deacetylation, relative molecular weight and viscosity. The higher degree of deacetylation leads to reducing relative molecular weight and makes it easier to be soluble in water, whereas the lower degree of deacetylation leads to an increase in relative molecular weight and high viscosity. Chitosan is soluble in dilute acid and presents a viscous shape, with strong adsorption capacity. Chitosan contains hydroxyl, amino and other polar groups, hygroscopicity is very strong and can be used as a humectant. Chitosan, a high-performance metal ion collector, can chelate the heavy metal ions in vivo because the ortho of the free amino group of chitosan is hydroxyl, which can chelate the divalent metal ions.  
Chitosan, as a solution, needs to be stored and used in an acidic environment, but because of the presence of acetal structure, the glucose ring is opened and degraded when it is placed in an acidic solution for a long time, the \(\beta\)-1,4-glycoside bond of chitosan will be hydrolyzed slowly to form low molecular weight chitosan. Chitosan can be developed as the immobilized carrier of antigen, antibody, enzyme and other physiologically active substances due to the presence of the free amino group, and has a broad application prospect because of its good physical, chemical and biological properties, excellent stability to organic solvents and convenience for secondary processing [36].  
The active adsorption center of chitosan is a surface free amino group. Many inorganic acids, organic acids and acid compounds, even amphoteric compounds, can be adsorbed by chitosan. The adsorption rate decreases with the decrease of the dielectric constant of the adsorption medium. The surface energy of the chitosan solution first decreases with the increase of the solubility parameter and then increases rapidly. The dissociation constant pKa of chitosan is not significantly related to the change of deacetylation degree, but the ionic strength and species in the solution. A cationic polyelectrolyte (pKa 6.5) produced by amino groups of chitosan. The dissolution of chitosan in the acidic-aqueous solution produces a proton, which led the soluble polysaccharide to become positively charged, especially a more positive charge on amino groups. Chitosan aggregates and chelates with polyanionic compounds and heavy metal ions, respectively. Both the aggregation with polyanionsand solubility in acidic solution produce chitosan with admirable gelling properties [37]. Under the same hydrolysis time, the reciprocal of the relative molecular weight of the hydrolysate is directly proportional to the temperature. Chitosan emerges as an obvious self-polymerization phenomenon in 0.1 mmol/L acetic acid solution. With the increase of chitosan concentration, the molecular chain of chitosan changes from self-polymerization of the stretch chain structure to the single-chain coil group structure, and the single-chain coil group structure was further transformed into an intertwined coil group structure.