Versatile protonic acid mediated preparation of partially deacetylated chitin nanofibers/nanowhiskers and their assembling of nano-structured hydro- and aero-gels

The partially deacetylated α-chitin nanofiber/nanowhisker (DEChN) aqueous dispersions were prepared by alkali deacetylation. The following mechanical treatment under acidic conditions was mediated by five different organic protonic acids, including acetic acid, gluconic acid, itaconic acid, citric acid and ascorbic acid. Thereafter, these acidic DEChN dispersions were transformed into hydrogels under an alkali gas phase coagulation bath. After solvent exchange and freeze drying, the DEChN-based aerogels were prepared. Previous research showed that the kinds of acid used for the adjustment of the pH value during the fibrillation process influenced the nanofibrillation efficiency. In this study, it was discovered that the types of protonic acid also influenced the properties of the DEChN hydrogels and aerogels under the same mass concentration (0.6%) of DEChNs, including the appearance, strength of hydrogels, the pore size and specific surface area of aerogels, as well. Nevertheless, all of the hydrogels showed good mechanical strength and all of the aerogels possessed a porous nanostructure sustained by nanofibrillar networks. The highly porous chitin aerogels showed a broad size mainly distribution from 2 to 100 nm, and the specific surface area of chitin aerogels ranged from 90 to 170 m²g⁻¹. Although it was not clear how the types of protonic acids influenced the properties of the hydrogel and aerogel, we were able to obtain DEChN dispersions and gels by mechanical treatment of partially deacetylated chitin at acidic conditions mediated by versatile acids. In the meantime, the observed differences among the gels provided the possibility to use the acid medium to select the gels with certain mechanical strength, porosity or specific surface area for the prospective desired applications.