Effect of acidity on the physicochemical properties of α- and β-chitin nanofibers.

We have investigated whether acidity can be used to control the physicochemical properties of chitin nanofibers (ChNFs). In this study, we define acidity as the molar ratio of dissociated protons from the acid to the amino groups in the raw chitin powder. The effect of acidity on the physicochemical properties of α- and β-ChNFs was compared. The transmittance and viscosity of the β-ChNFs drastically and continuously increased with increasing acidity, while those of the α-ChNFs were not affected by acidity. These differences are because of the higher ability for cationization based on the more flexible crystal structure of β-chitin than α-chitin. In addition, the effect of the acid species on the transmittance of β-ChNFs was investigated. The transmittance of β-ChNFs can be expressed by the acidity regardless of the acid species, such as hydrochloric acid, phosphoric acid, and acetic acid. These results indicate that the acidity defined in this work is an effective parameter to define and control the physicochemical properties of ChNFs.