Multistimuli-Responsive Pickering Emulsion Stabilized\nby Se-Containing Surfactant-Modified Chitosan

Abstract

Particle-stabilized\nemulsions that can respond to external stimuli\nhave attracted significant concerns due to their intelligent-controlled\nstability, whereas particle-stabilized Pickering emulsions responding\nto multistimuli but based on biomass have been rarely reported. Here,\na multistimuli-responsive Pickering emulsion was developed using the\nmodified chitosan as stabilizer. Due to electrostatic attraction,\nSe-containing anionic surfactant, sodium 11-(butylselenyl)­undecylsulfate\n(C₄SeC₁₁S), can bind with CS at an acidic pH\nand form CS–C₄SeC₁₁S complexes which\ncan further self-associate to form micrometer-sized particles with\nthe character of partially hydrophobicity. Therefore, at pH < p<i>K</i>_a, an oil-in-water Pickering emulsion can be\nformed using CS–C₄SeC₁₁S particles as\nstabilizers and can spontaneously respond to redox, ion, and pH. First,\nwith the addition of oxidation, the hydrophilicity of C₄SeC₁₁S was enhanced, and thus, hydrophobic association\nof CS–C₄SeC₁₁S decreased, leading to\nthe disruption of CS–C₄SeC₁₁S particles.\nHence, the emulsion destabilized. The demulsification process is closely\nrelated with the dosage of oxidant and the oxidation time. Second,\n introduction of a competitive ion (e.g., CTAB) could break the binding\nbetween C₄SeC₁₁S and CS, leading to the disruption\nof particle emulsifier. Thereby, demulsification occurred. Third,\nwith sequentially increasing/decreasing pH, the emulsion can be switched\nfrom stable to unstable and then to stable again accordingly. Such\na unique pH-responsive behavior has never been discovered in other\npH-responsive Pickering emulsions. All of the stimuli-responsive behaviors\nwere reversible. Upon alternately adding oxidant/reductant, CTAB/C₄SeC₁₁S, or base/acid, the current emulsion can\nbe reversibly switched off (destabilization) and on (stabilization).\nSuch a Pickering emulsion may be a good candidate as a vehicle of\nfunctional ingredient.