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 μ 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.