Study On The Synthesis And Properties Of Bola Type Smart Surfactants

In recent years,smart emulsions stabilized by stimulus responsive surfactants or colloidal particles have received keen attention from researchers.Under the stimulation of external environment（such as p H,CO₂/N₂,oxidizing reducing agent,temperature,light,magnetic field,etc.）,smart emulsions can realize the stability/demulsification on demand,which solves the problems of difficult demulsification in some application fields（such as two-phase catalysis,pipeline transportation,material synthesis,etc.）.For example,smart（stimulus responsive）Pickering emulsions not only retain the characteristics of high stability and excellent rheology of traditional Pickering emulsion,but also can demulsification and separation by external stimulation.However,due to the high viscosity of Pickering emulsions,slow demulsification and incomplete demulsification often occur.In 2018,a new O/W emulsion stabilized by similarly charged surfactants and nanoparticles（named Oil-in-dispersion emulsion）was found to have excellent mobility,rapid response and thorough demulsification,but low long-term stability.Therefore,it remains to be further explored and studied how to prepare the emulsions with high stability and rapid demulsification,how to further realize the recovery and reuse of emulsifier after demulsification,and how to accurately regulate other properties of emulsion（such as droplet size and viscosity,etc.）by using stimulus response mechanism.In this paper,the self-assembly behaviors of various Bola type smart surfactants and nanoparticles at oil-water interface was studied.The smart Pickering emulsions and the smart Oil-in-dispersion emulsions were constructed respectively,and the stability,droplet size,viscosity and type of emulsion were controlled by simple external stimuli.The main research results are as follows:（1）CO₂-responsive emulsions stabilized by tertiary amine anionic surfactant and nanoparticles.Firstly,an anionic surfactant NCOONa was studied,with sodium carboxylate and tertiary amine groups located at both ends of its hydrophobic chain.Under the stimulation of CO₂/N₂,its structure can be reversibly converted between single-head single-tail type and Bola type（N⁺COONa）.A smart Pickering emulsion with both ultra-high stability（>5 months）and good CO₂/N₂ responsiveness was co-stabilized by NCOONa and hydrophilic Al₂O₃ nanoparticles at low concentrations（0.3 m M,0.1 wt.%）.Alternating bubbling CO₂ and N₂,the Pickering emulsion could be rapidly demulsified and re-stabilized,and all emulsifiers（N⁺COONa plus Al₂O₃）entered the aqueous phase after demulsification to ensure that there is no residual emulsifier in the oil phase.NCOONa can also stabilize a smart Oil-in-dispersion emulsion with hydrophilic Si O₂ particles.Under CO₂/N₂,not only can the emulsion be demulsified/stabilized on demand,but also the aqueous phase of emulsifiers（surfactants and particles）can be recycled.And the reversible transition between the Oil-in-dispersion emulsion and the Pickering emulsion can be achieved.The viscosity,droplet size,and stability of the emulsions can be controlled intelligently by bubbling CO₂/N₂.（2）p H-responsive emulsions stabilized by tertiary amino cationic Bola surfactant and nanoparticlesSecondly,a Bola type cationic surfactant(di-NC₁₀TAB)containing tertiary amine group was synthesized.Due to di-NC₁₀TAB has strong hydrophilicity and weak ability to reduce oil/water interfacial energy,making it unable to stabilize the emulsion alone.After introducing nanoparticles,di-NC₁₀TAB can prepare a p H-responsive Pickering emulsion by in situ hydrophobic modification of the hydrophilic Si O₂ nanoparticles.Through p H control,the surfactant and modified particles are competitively adsorbed at the oil/water interface,which makes droplet size and viscosity of the Pickering emulsion present an unconventional change rule of"droplet size increases,viscosity increases;droplet size decreases,viscosity decreases".Using this Pickering emulsion as template,polymer microspheres with controllable size and roughness were obtained by interfacial curing.In addition,di-NC₁₀TAB and positively charged Al₂O₃ particles can synergistically stabilize an Oil-in-dispersion emulsion.Due to electrostatic repulsion and intermolecular hydrogen bonds,di-NC₁₀TAB forms a dense adsorbed molecular layer at the interface,enhances the viscoelasticity of the interfacial film,which significantly improves the stability of the Oil-in-dispersion emulsion（>1 year）.Through p H regulation,the conversion of Oil-in dispersion to Pickering emulsion can also be achieved.（3）Redox-responsive emulsions stabilized by ferrocene cationic Bola surfactant and nanoparticles.Finally,a Bola type cationic surfactant di-Fc C₁₁TAB containing ferrocene group was designed and synthesized.Due to the enhanced hydrophobicity of the intermediate long chain,the interfacial activity of di-Fc C₁₁TAB increased greatly,and only 0.05 m M di-Fc C₁₁TAB was required to stabilize the conventional emulsion.When the intermediate ferrocene group is oxidized and charged,the adsorption of di-Fc⁺C₁₁TAB at the interface is very loose due to the electrostatic repulsion and steric hindrance between the charged groups.Therefore,di-Fc⁺C₁₁TAB with low interfacial activity can return to aqueous phase after demulsification.In synergy with nanoparticles,di-Fc C₁₁TAB can stabilize emulsions with very low concentration.For example,the Pickering emulsion with high stability can be prepared with only 0.003 m M di-Fc C₁₁TAB（0.1 wt.%Si O₂）;the Oil-in-dispersion emulsion can be prepared at an ultra-low di-Fc C₁₁TAB concentration of 0.00001 m M（0.1 wt.%Al₂O₃）.In addition,both emulsions showed good redox response,and droplet size and stability of the emulsions can be controlled under the action of oxidant and reducing agent.