Surface sulfation of crab chitin for anisotropic swelling and nanodispersion

Crab chitin was sulfated under heterogeneous conditions using sulfamic acid in N’N-dimethylformamide to selectively sulfate the microfibril surface. The degree of sulfation followed a first order kinetics assuming a limited available reaction sites on the surface, and leveled off at a bulk degree-of-substitution of 0.4, corresponding to 2 mol/kg of sulfate groups. The reaction rate was proportional to the square of sulfamic acid concentration, suggesting involvement of two sulfamic acid molecules in a reaction. When washed with water after sulfation, the crab shell chitin fragments swelled anisotropically in the helicoidal axis direction, revealing regular alternating birefringence under optical microscope. Further mechanical treatment with high-pressure homogenizer led to slender nanofibrils, whose diameters were of the order of 6 nm according to turbidimetric analysis, in agreement with the Scherrer size estimated from X-ray diffraction line broadening. Both atomic force microscopy and transmission electron microscopy measurement showed presence of further smaller fragments with diameter of 3–4 nm and contour length of 0.5–1 \(\upmu \hbox {m}\) with a few kinks. The current approach presents a rapid and efficient modification to chitin, and a strategy for the preparation of stable nanochitin suspension.