Dear Artem,

Most of chitosan -metal complex are water insoluble, so it is not the solution to your problem.

If chitosan film (dried one) was prepared (cast-dried) in salt form (e.g. acetate, hydrochloride, glutamate, lactate, etc) it will remain soluble in water easily . However, with weeks the film will continue losing water molecules and will be less hydrated and maybe more crystalline.

The same thing will happens if chitosan film was previously neutralized but at higher extension.

One way to keep its solubility performance is to keep chitosan films under constant humidity conditions instead of a dryer container. 

Best regards,

Gustavo

Actually, Chitosan is a polycation. So, having it react with anionic polyelectrolytes or with polyvalent anions might be a more promising approach to stabilize the films than  complexing them with other actions.

Dear Gustavo, thanks a lot for your answer! However, from the recebt data, we found out that the biggest problem was the mass also. 100kDa samples were insoluble in water right after the cast, while 20 kDa remained water soluble within a week at r.c. we will try humidity control as you've suggested to prolong film life.

Dear Nikolaus, could you be so kind to spread your thought a bit wider, my knowledge of chemistry are not so far good. Thanks in advance!

[1] I humbly submit  that the  transition metal complexes  with CA  should be MUCH LESS SOLUBLE THAN CA ITSELF  in DI H2O.

[2]The reason lies in an indirect proof that  quite a number of  complexes  of CA  act  as  sorbent/s for other  ions like  As(V), As(III ) and Pb(II) to remove them from H2O . If they, themselves,  had sufficient solubility then they would not have possessed this property of having good sorption.

[3]However, I give a table of solubility of CA at different concentrations( see attachment; zoom if needed).