Preparation And Modification Of Chitin Nanocrystals Towards Stabilizing Pickering Emulsions

In recent years,biomolecules and biobased polymers have received increasing attention from researchers due to their excellent biocompatibility and biodegradability as people pay more attention to environmental technology.Chitin,as the second largest natural polysaccharide,is abundantly available,and the chitin nanocrystal(ChNC)obtained by acid digestion of its amorphous region has high strength and high specific surface area,which can be used to prepare efficient drug delivery systems,high strength biomedical materials,high transparency optical materials and efficient catalysts.In recent years,ChNC has also been used as a solid emulsifier to stabilize Pickering emulsions,but literature reports are still limited and the mechanism of stabilization of oil-water emulsions by this type of polysaccharide nanoparticles has not been fully revealed.In this paper,ChNC was selected as a solid emulsifier and its hydrophobicity was chemically modified to obtain highly stable emulsions.The relationship between the emulsification ability of ChNC and the degree of surface modification was further evaluated from the perspective of emulsion morphology and rheology,and the ChNC-stabilized Pickering emulsion was used as a template to prepare composites with good biocompatibility.The specific studies and results are as follows:(1)The stabilization effect of surface unmodified ChNC and cellulose nanocrystal(CNC)on oil-in-water Pickering emulsion was compared:the mechanism of ChNC stabilizing the oil-water interface was explored by comparing the difference of two nanocrystals with the same topology stabilizing the same system.The results show that ChNC has better emulsification ability for the oil phase because its surface hydrophilicity is inferior to that of CNC,and the droplet size decreases with the increase of ChNC concentration and shows a single-peak distribution.When ChNC emulsifies a high oil phase,the emulsion obtained in the oil/water(O/W)system is still oil-in-water type and has a more complex droplet morphology.In shear flow,ChNC-stabilized emulsions exhibit higher strain sensitivity and stronger thixotropy;therefore,the sudden change of emulsion viscoelasticity due to the change of oil percentage can be used as a probe to reveal the phase behavior of emulsions.(2)The effect of surface acetylation modification on ChNC as an emulsifier to stabilize oil-in-water Pickering emulsions was investigated:ChNC particles with different substitution ratios were prepared to stabilize emulsions with different oil-to-water ratios,and the relationship between emulsion morphology and viscoelasticity was explored by varying the particle content,substitution ratio,and other factors.The results showed that the acetylated ChNC showed better emulsification ability and was able to stabilize emulsions with better morphology and stability at lower particle loadings,reducing the breakage caused by droplet interactions.The smaller and single-peaked distribution of emulsion droplets characterizes various emulsion systems that show gelation with frequency-independent energy storage and loss moduli.The emulsion viscosity increased with the increase in oil percentage and the number of droplets proliferated,forming a randomly tightly packed structure.The concentration and degree of substitution of ChNC affect the extent of strain overshoot in emulsions during large amplitude oscillation shear(LAOS),so the overshoot amplitude can be used as a probe to determine the effect of the concentration and degree of substitution of ChNC on its emulsification ability.(3)ChNC@PLA microspheres with acetylated ChNC as an emulsifier in Pickering emulsion as a template were prepared:acetylated ChNC-stabilized Pickering emulsions were prepared using aqueous mannitol solution as the aqueous phase and dichloromethane solution of poly(lactic acid)(PLA)as the organic phase.The results showed that the droplet size distribution in the acetylated ChNC-stabilized emulsion was narrower,the particle size increased with the increase of the organic phase,and the flow stability was good.acetylation of the ChNC surface effectively inhibited the emulsion break that age occurred by droplet aggregation.Using this Pickering emulsion as a template,the diameter of the ChNC@PLA hollow microspheres obtained was consistent with the emulsion droplet size,but some of the microspheres were agglomerated due to ChNC bridging,and the stability was enhanced with the increase of ChNC acetylation.Therefore.PLA microspheres with controllable size and good biocompatibility can be prepared by regulating the concentration and degree of substitution of ChNC.