Solid‐phase modification of chitosan hydrogel membranes and permeability properties of modified chitosan membranes

A novel preparation method for modified chitosan membranes was developed. Chitosan hydrogel membranes were prepared by immersing an aqueous acetic acid solution of chitosan in KOH solution and modified with 3,3′‐dithiodipropionic acid (DTPA), which has a functional group that causes the thiol ⟷ disulfide transition through a redox reaction. It was smoothly modified with DTPA even in a solid‐phase modification when carbodiimides were used as the condensing reagents. The chitosan membrane modified with DTPA was reduced with tri‐ n ‐butyl phosphine (M SH membrane) and then oxidized with iodine (M SS membrane). Permeabilities of KCl, sucrose, and urea through these modified chitosan membranes were investigated. The permeability of urea was quite high and followed by KCl and sucrose in that order. The permeabilities of KCl and sucrose through the M SS membrane showed a decrease compared to those through the M SH membrane. The thiol ⟷ disulfide transition was responsible for changes in the permeabilities of KCl and sucrose. The permeability of urea was almost the same for both membrane systems, i.e., permeation of urea was not affected by the thiol ⟷ disulfide transition, probably due to the break of hydrogen bonding in the membranes.