Stimuli-responsive Pickering Emulsions Based On Modified Chitosan Particles

In recent years,Pickering emulsions based on solid particles have gradually become a new hotspot due to their advantages such as low toxicity and high stability.The irreversible adsorption of the solid particles on the oil/water interface provides good thermosdynamic stability for the emulsion,but that also brings certain difficulties to the emulsion breaking-up.The emergence of stimuli-responsive Pickering emulsifier provides a perfect solution for achieving the balance between long-term stability and rapid demulsification of the emulsion.Under external stimulation,the emulsifier could adjust its own structure and properties,resulting in the disintegration or desorption of the particle emulsifier,thereby realizing the intelligent regulation of the emulsion.Given the safety of emulsion,stimuli-responsive Pickering emulsions stabilized by food-grade biomass solid particles with good biodegradability,biocompatibility and reproducibility?such as polysaccharides,proteins,etc.?have attracted significant attention.Chitosan?CTS?is the only basic polysaccharide present in nature,and the abundance of it is just below cellulose;however,it would be protonated under acidic conditions and then become a water-soluble poly-cation electrolyte,which limits the application of CTS.Therefore,construction of CTS-stabilized Pickering emulsion under acidic conditions is highly desirable for the comprehensive utilization of CTS.Herein,based on the electrostatic interaction,hydrophilic CTS is hydrophobically modified by sodium dodecylsulfate?SDS?and sodium butylselanylundecyl sulfate?SBSeUS?under acidic conditions,respectively.As a result,two types of stimuli-responsive CTS particle emulsifiers are prepared.Thus stable CTS-based Pickering emulsions are formed and exhibit stimuli-responsive behaviors under acidic conditions.The macroscopic appearance,micro-morphology and the stbale or stimuli-responsive mechanism of the emulsion are systematically studied through a variety of technical means.The main findings are summarized as following:1.Under acidic conditions,the trace amounts of SDS can functionalize the protonated CTS hydrophobically,producing the partially hydrophilic composite particles?SDS-h-CTS?which are Pickering stabilizer and pseudo-oligomeric surfactants?SDS-l-CTS?which are molecular emusifier.Under the synergistic stabilization of these two emulsifiers,stable oil-in-water Pickering emulsions are prepared under acidic conditions.The current scheme of emulsion preparation shows good acid resistance and universality,and high internal phase emulsion can be formed by adjusting the ratio of oil and water,showing broad application prospects in the field of food and cosmetics.More importantly,both the SDS-CTS complex and the SDS-CTS based Pickering emulsion show typical ion-responsive behaviors:with the addition of cationic surfactant cetyltrimethylammonium bromide?CTAB?,SDS dissociates from the SDS-CTS complex and then combines with CTAB,the particle emulsifiers and the pseudo-oligomeric surfactants disintegrate at the same time,and the emulsion breaks subsequentaly.When SDS is added again,the new particle emulsifiers and pseudo-oligomeric surfactants are reformed,and a stable emulsion reappears.In short,with alternately addiing SDS and CTAB,granular emulsifiers and traditional surfactant emulsifiers can be formed and disintegrated,and the switch of the emulsion can be realized with that.2.A redox-responsive selenium-containing surfactant SBSeUS is designed and synthesized.Upon the addition of small amount of SBSeUS,the hydrophilicity of CTS is decreased and the hydrophobicity is increased through electrostatic action under acidic conditions.Therefore,SBSeUS-CTS composite particles with appropriate wettability are formed in situ,which can be used to construct multistimuli-responsive Pickering emulsions.Firstly,the competitive adsorption between ions imparts Pickering emulsion with ion-responsive performance:with the addition of CTAB,SBSeUS desorbs from the complex particle and then combines with CTAB to form an oil-soluble SBSeUS-CTS composite,particle emulsifier disintegrates and emulsion destabilizes;upon adding SBSeUS again,the SBSeUS-CTS composite is reformed and adsorbs at the oil-water interface to provide steric hindrance to the emulsion droplets,achieving the stability of the emulsion.Secondly,SBSeUS provides redox-responsive performance for the Pickering emulsion:when H₂O₂ is added,SBSeUS is oxidized to SBSeUS-Ox with double hydrophilic head groups,making SBSeUS-Ox-CTS particles show strong hydrophilicity,and thus these particles transfer from the oil-water interface to the water phase,causing the breakup of the emulsion;when the equivalent amount of Na₂SO₃ is added,SBSeUS-Ox was reduced to SBSeUS again,and the hydrophobicity of the modified CTS particles increases again and re-adsorbs to the oil-water interface,a stable emulsion can be re-obtained after homogenization.In addition,the changes in pH will affect the electrostatic interaction between the molecules,thereby influencing the stability of the Pickering emulsion:under acidic conditions,the SBSeUS-CTS complex self-associats to form micron-sized particles with partial hydrophobicity,which participated in the stability of the emulsion;when the pH increases to neutral,the hydrophobic interaction between the SBSeUS-CTS particles and the hydrogen bonding between the CTS molecules are simultaneously destroyed,causing the particle emulsifiers to disintegrate and the emulsion to break as well;due to the hydrogen bonding at pH>7.8,CTS can self-associate to form micro-particles or micro-gels,acting as a particle emulsifier to participate in emulsion stabilization.Such a system can be used as a carrier for embedding and targeted/sustained releasing of the oil-soluble cargoes?like curcumin?under multiple stimulations,showing lots of promising applications in fields including food,cosmetics and medicine.