Cellular structure in an N-acetyl-chitosan membrane regulate water permeability

A novel model of the water permeation mechanism in an N-acetyl-chitosan membrane with a cellular structure is proposed. Although the entire membrane structure has a hydrophilic character, the cellular structure incorporates junction zones that practically prevent water permeation. Chitosan membranes with a controlled degree of deacetylation (DD) were prepared using a casting method. Changes in the water flux and total water content of the membrane were observed with a change in DD. The membrane properties were analyzed and evaluated using water permeability measurements, scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). SEM observations indicated that the membrane structure was an individual cellular structure and that this cellular structure grew with decreasing DD. XRD measurements indicated the crystal structure of the membrane was amorphous regardless of the DD in the experimental range. The free water content (W_f), the freezable bound water (W_fb), and the bound water not able to freeze (W_b) were evaluated by DSC. The free water mainly contained inside the cellular structure, and resulted in swelling the chitosan membrane. Water flux was measured using ultrafiltration apparatus; it was dependent on the operational pressure, membrane thickness, and the feed solution viscosity, and obeyed the Hagen-Poiseuille flow. At a higher DD, water permeation proceeds due to degradation of the cellular structure; the amount of water in permeation channels was greater than that for lower DD membranes even though the total water content in the membrane was less. The water flux of the chitosan membrane was determined by the water content constructing channels through the membrane and not on the total water content in the membrane.