Preparation And Stabilization Mechanism Of PH Controllable Lignin-based Pickering Emulsion

Lignin is a natural three-dimensional network polymer with many intrinsic characteristics,such as UV absorption,anti-oxidation,etc.It is one of the most abundant green resources that can be utilized.Alkali lignin from the alkaline pulping industry not only has hydrophobic skeleton composed of phenylpropane monomers,but also contains hydrophilic functional groups such as carboxyl groups and phenolic hydroxyl groups,showing the structural characteristics of anionic polymer surfactants.Thus,lignin has been developed as an emulsifier to stabilize Pickering emulsions.However,alkali lignin only contains a few weak hydrophilic functional groups,which have poor water solubility and tend to agglomerate easily.These alkali lignin-based Pickering emulsions exhibited poor p H response and low salt tolerance,limiting its applications,for instance,in the drug controlled release,oil recovery and emulsion polymerization.Inorganic nanoparticles have the advantages of rigidity,dimensional stability,and thermal stability,which are often used as emulsifiers to stabilize Pickering emulsions.However,most of the applied inorganic nanoparticles tend to agglomerate easily due to the large number of hydroxyl groups on the surface,making it difficult to form stable and sensitive emulsions.Thus,measurements should be taken to hydrophobize the surface of the particles.In this study,in order to improve the adsorption performance and p H response of lignin,alkali lignin was first modified by quaternization to synthesize amphoteric lignin?AML?with controllable isoelectric point.Based on electrostatic interaction,AML was compounded with small molecule surfactant sodium dodecylbenzene sulfonate?SDBS?,inorganic oxides Si O₂,Ti O₂,respectively.The formed compounds were then ultilized as emulsifiers to prepare Pickering emulsions which had higher stability,controllable p H stability range and more sensitive responsiveness.The interaction between AML and guiding agent was studied by ¹H NMR,FT-IR and zeta potential.Through microscope,rheometer and Turbiscan Lab Expert stability index?TSI?,the optimum conditions for emulsion formation and the stability of the emulsions were systematically studied.In order to reveal the p H response stability mechanism of Pickering emulsion,adsorption kinetics,Zeta potential,quartz crystal microbalance with dissipation?QCM-D?,atomic force microscopy?AFM?and three-phase contact angle were used to explore the adsorption between AML and the guiding agent,as well as the interfacial activity of the composite.The main conclusions in this paper are as follows:?1?Amphoteric lignin?AML₁?with an isoelectric point of 4.2 was selected and combined with a small amount of anionic small molecule surfactant SDBS through electrostatic interaction to prepare Pickering emulsions which was stable under acidic conditions.When p H<3,SDBS and AML₁ were combined by electrostatic interaction,and the cluster-like aggregated AML₁ was peeled into a small-sized sheet structure,which increased the interfacial activity of the formed AML₁@SDBS compounds and produced stable O/W Pickering emulsions.When p H>3,there was electrostatic repulsion between AML₁ and SDBS.Only a little amount of SDBS was adsorbed on the surface of AML₁ molecules.The independent activity of AML₁ and SDBS at the oil/water interface was too low to form stable emulsions.By alternately adding acid or alkali,the emulsion could be circulated at least 4 times between emulsification and demulsification.When the inorganic salt concentration increased to 0.4 M,the emulsions remained stable,indicating that the emulsion had good salt-tolerant stability.?2?AML₁ was used as an in-situ hydrophobic modifier for Si O₂ nanoparticles,and the compounds formed by electrostatic interaction could produce stable O/W Pickering emulsions in the range of p H 3-4.When 3<p H<4,AML was absorbed on Si O₂ to realize the in-situ hydrophobization of Si O₂,forming stable Pickering emulsions.When p H was above 4,only a little amount of AML was absorbed on Si O₂ due to the strong electrostatic repulsion between them,resulting in unstable emulsions.By alternately adding acid or alkali,the emulsions could be circulated at least 5 times between emulsification and demulsification.When the inorganic salt concentration in the system was as high as 1M,the emulsion remained stable,indicating that the emulsions had excellent salt-tolerant stability.?3?AML₂?isoelectric point 7.5?was selected as an in-situ hydrophobic modifier for amphoteric oxide Ti O₂ by electrostatic interaction and prepared double p H responsive Pickering emulsion.The emulsions were stable at p H 6-7,but unstable under acidic and basic conditions.In neutral aqueous media,AML₂ and Ti O₂ were combined,and the surface activity of Ti O₂ was increased to form stable emulsions.Under acidic and alkaline conditions,there was a strong electrostatic repulsion between AML₂ and Ti O₂.The individual Ti O₂ was too hydrophilic to form stable emulsions.By alternately adding acid or alkali,the emulsion could be cycled between emulsification and demulsification at least 7 times.When the inorganic salt concentration in the system was as high as 1 M,the emulsions remained stable,indicating that the emulsions had excellent salt-tolerant stability.