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 leveling off at bulk degree-of-substitution of 0.4. 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 were anisotropically swollen in the helicoidal direction, revealing regular alternating birefringence under optical microscope. Further mechanical treatment with high-pressure homogenizer lead to slender nanofibrils, whose dimaters 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 AFM and TEM measurement showed presence further smaller fragments with diameter of 3-4 nm and length of ~300 nm. The current approach presents a rapid and efficient modification to chitin, and a strategy for the preparation of stable nanochitins suspension.