Chitin nanocrystal dispersions: rheological and microstructural properties

Dispersions of colloidal particles in aqueous media are widely investigated because of their use in many industrial applications (foods, pharmaceuticals, cosmetics, paints, etc.). Acid-hydrolyzed chitin has been reported to spontaneously disperse into colloidal rod-like crystalline particles, which can form rigid structures. Such colloidal dispersions of acid-degraded chitin can undergo an isotropic–anisotropic nematic transition when their concentration is increased, leading to the formation of a nematic gel. In the present study the gel forming ability of aqueous chitin nanocrystal dispersions and the factors that influence the microstructural and viscoelastic properties of such gels were examined. Chitin nanocrystal aqueous dispersions were prepared by acid hydrolysis of crude chitin from crab shells. The resulting dispersions were studied with small deformation oscillatory rheological testing and polarized optical microscopy under different conditions of nanocrystal concentration, ionic strength, pH and temperature. The effect of soluble biopolymer, like whey proteins, addition was also examined. The samples with increased chitin nanocrystal concentration presented an elastic behaviour. Additionally, the chitin nanocrystal dispersions shifted from viscous to an elastic behaviour with the addition of NaCl and the increase of pH (from 3.0 to 6.0). Heating also caused an increase to the elactic modulus (G ́), and the addition of whey proteins or guar gum in dilute chitin nanocrystal dispersions, also resulted in gel formation. All the above gel-like materials exhibited birefringence and nematic-like ordering, when observed under polarized optical microscope.