Effect of pH on the mechanical, interfacial, and emulsification properties of chitosan microgels

Abstract

Food microgels have attracted much attention recently especially in the use of emulsion stabilization due to their colloidal nature, such as nano-to micro-size, large amount of dangling chains and deformability. However, the stabilization mechanism for pH-sensitive microgels is still not well understood. In this paper, the polysaccharide chitosan (CS) was used to prepare pH-responsive chitosan microgels (CSMs) by physically cross-linking with sodium tripolyphosphate (TPP) and the subsequent ultrasound treatment. The particle size and morphology of CSMs, the mechanical properties of CSM at different pH, and the adsorption and interfacial rheology of CSMs at oil-water interface were characterized by nanoparticle size analyzer, confocal laser scanning microscope, atomic force microscopy, and pendant drop tensiometer, respectively. The results showed that the optimum condition for CSM formation occurred at CS/TPP mass ratio of 1:1 under mechanical stirring at 1000 rpm, and sonication for 9 min. The size of the CSM particles strongly depended on pH with larger size under weak acid conditions and smaller size under neutral condition. CSMs obviously have better interfacial activity than pure CS polymers under the same conditions, and their interfacial properties strongly depended on pH and concentration. It was found that although the swollen CSM particles diffuse more slowly than the smaller ones, they formed interface with stronger viscoelasticity, and displayed strain hardening during extension process in the dilatational measurement under relatively large amplitude. Combined with the mechanical strength results, these may be due to the greater deformability of the swollen CSMs at weak acid condition. However, over swelling may not facilitate the decrease of surface tension due to the steric and electrostatic repulsion of the dangling chains. The interfacial behaviors of the CSMs were successfully linked to their emulsification ability and confirmed by the emulsion structure and stability tests. It was found that the interfacial viscoelasticity played a vital role in the emulsification ability of the CS microgels.