Effect and mechanism of sodium chloride on the formation of chitosan–cellulose sulfate–tripolyphosphate crosslinked beads

Chitosan obtained by deacetylation of chitin is the only pseudo-natural cationic polymer and has been widely used for a variety of applications. It can crosslink with small molecules or form polyelectrolyte complexes with polyanions, and these reactions are usually carried out in aqueous solutions under mild conditions without toxic additives or organic solvents, which is beneficial for its application in medical and pharmaceutical industries. However, these reaction processes can be affected by several factors such as charge density, ionic strength and pH. This study investigated the effect of sodium chloride (NaCl) on the formation of chitosan beads composed of chitosan, sodium cellulose sulfate (NaCS) and sodium tripolyphosphate (TPP). The results revealed that the bead size was influenced by NaCl and TPP which was mainly due to the osmotic pressure and "salting out" effects. The addition of NaCl in the three-component system led to the formation of chitosan beads without NaCS linked on the surface, which is probably because of the screening effect of NaCl in the solution. Detailed mechanisms were discussed and a model mechanism was proposed.