Fabrication of chitosan membranes via aqueous phase separation: Comparing the use of acidic and alkaline dope solutions

Various approaches have been developed to prepare aqueous solutions of chitosan, which enable fabrication of chitosan membranes via immersion precipitation in a completely aqueous environment. In addition to the most commonly used method that is based on the protonation of chitosan in acidic conditions, it was recently established that chitosan can be efficiently dissolved in an alkali/urea aqueous solution. Along with the concept of aqueous phase separation (APS), both the acidic and alkaline dope solutions were employed to fabricate chitosan membranes in an effort to elucidate the effects of the different dissolution mechanisms on the film-formation kinetics, the film characteristics, and the filtration performance. When combined with conventional methods, optical coherence tomography (OCT) played a dual role in this study; that is, the OCT-based characterization was explored to not only resolve the kinetic behavior of the chitosan gelation, but also identify the mechanisms accounting for the rejection of particles by the chitosan membranes. It was established by comparing the characterization results that the morphological evolution of the gelling layer could be substantially varied by changing the way of eliminating the gelation inhibitors; the variation in porous morphology of the resulting chitosan membrane would have significant impacts on the water permeability and the rejection of colloidal particles. This comparative study highlights the importance of understanding the structure-performance relationship on the basis of interpreting mechanisms underlying different APS processes and will provide valuable insights into the development of strategies for improving chitosan membranes.