Chitosan hydrochloride/carboxymethyl starch complex nanogels as novel Pickering stabilizers: Physical stability and rheological properties

The Pickering emulsions stabilized by chitosan hydrochloride - carboxymethyl starch (CHC-CMS) nanogels prepared through a facile covalent cross-linking method using 1-ethyl-3-(3-dimethyl-aminopropyl-1-carbodiimide) were investigated. The smallest mean size of CHC-CMS nanogels (378.2 nm) was obtained when the volume ratio of CHC: CMS was 2:1. The three-phase contract angle of CHC-CMS nanogels was 89.3°, which exhibited that the CHC-CMS nanogels could be used as effective Pickering emulsifiers. The effects of CHC-CMS nanogels concentration, oil phase fraction and environment factors viz, pH, ionic strength on the stability of emulsions were evaluated. An increase in CHC-CMS nanogels concentration led to a formation of smaller droplets, and the droplet size of Pickering emulsions was increased with the rise of oil phase fraction. The Pickering emulsions were highly stable at pH 6 and above. Increasing NaCl concentration was found to generate the aggregation of droplets. Confocal laser scanning microscopy revealed that the CHC-CMS nanogels could be adsorbed on the oil-water interface and form a densely packed layer at the surface of spherical oil droplets, which exhibited long-term stability for 3 months storage. Rheological results illustrated that all emulsions showed the typical pseudoplastic fluid characteristics and satisfied the Herschel-Bulkley model. The fact that the elastic modulus (G') was higher than the loss modulus (G″) in all the samples was indicative of the formation of an elastic gel-like structure. These results provided a potential way for Pickering emulsions preparations, which could be used as an effective delivery carrier of bioactives.