Study On Recyclable Surfactants And Various Smart Emulsions

In recent years,smart surfactants have become a research hotspot in surfactant and colloid fields.Smart surfactants,also known as switchable or stimuli-responsive surfactants,contain at least one stimuli-responsive group in their molecules that can be reversibly transformed between polar and non-polar/weak polar.Accordingly,the surfactant can be reversibly converted between surface-active and surface-inactive,and then can be used to prepare smart dispersion systems,such as smart emulsions,smart foam systems and so on.So far lots of triggering mechanisms have been reported,including p H,CO₂/N₂,temperature,redox,light,and ion-pair formation etc.,but the present smart surfactants mostly take the hydrophilic group as stimuli-responsive group,which makes the smart surfactants oil-soluble after de-activation and transfers to the oil phase in emulsion systems after demulsification.This not only contaminates the oil phase in the case that the oil phase is the target product,but also makes it inconvenient to recycle or re-use the surfactants.In response to this problem this thesis proposed a new molecular structure design for the smart surfactants,that is attaching a responsive group to the end of the hydrophobic chain of a conventional surfactant.When the responsive group is in non-polar or weakly polar form,the whole surfactant behaves as amphiphilic and surface-active,while when it is in polar model,the whole surfactant molecule turns to Bola type and thus becomes surface-inactive or less surface-active.Thanks to strong hydrophilicity,the surfactant molecules in Bola form transfer to the aqueous phase alone or together with charged particles after demulsification,making the whole aqueous phase recyclable and re-usable.This thesis studied the molecular designing,synthesis,characterization,and stimuli-responsiveness of a series of Bola type smart surfactants including cationic-cationic,nonionic-anionic,and anionic-anionic types,and the feasibility of using them as recyclable or re-usable emulsifiers alone or together with charged nanoparticles to construct stimuli-responsive emulsions,including conventional emulsions,Pickering emulsions and oil-in-dispersion（OID）emulsions.The main conclusions obtained are as follows:（1）A cationic-cationic Bola type surfactant,abbreviated as N⁺-8P8-N（P=phenyl）,was synthesized by connecting a dimethylamine group at the end of the alkyl chain of a quaternary ammonium salt surfactant.In neutral and alkaline media,the dimethylamine group behaves as a non-polar or weakly polar group,and the whole surfactant is amphiphilic.With n-decane as oil phase,N⁺-8P8-N can stabilize O/W type conventional emulsions solely,as well as Pickering emulsions and OID emulsions in combination with negatively/positively charged nanoparticles,respectively.However,in acidic media,the tertiary amine group turns to strongly hydrophilic amine salt,and the whole surfactant molecule converts to Bola type（N⁺-8P8-NH⁺）,resulting in rapid demulsification of all three kinds of emulsions.These emulsions are thus p H-responsive and CO₂/N₂-responsive.Specifically,after demulsification,the surfactant N⁺-8P8-NH⁺returns to the aqueous phase,and the whole aqueous phase separated can be recycled for more than 5times.（2）A nonionic-anionic Bola type surfactant,abbreviated as CH₃O（EO）₇-R₁₁-COOH（p Ka=9.74）,was synthesized by connecting a nonionic hydrophilic group（EO chain）at the end of the alkyl chain of a carboxylate anionic surfactant,which can stabilize O/W conventional emulsions alone and the emulsion is p H-responsive and temperature-responsive.In acidic and neutral media,the EO and COOH groups in CH₃O（EO）₇-R₁₁-COOH can adsorb to surfaces of positively charged alumina nanoparticles,which are then in situ hydrophobized to be surface-active and can stabilize O/W Pickering emulsions with the oil droplet size sensitive to p H.However,in alkaline media,the alumina nanoparticles which are converted to negatively charged（PI=9.5）cannot co-stabilize OID emulsions with the CH₃O（EO）₇-R₁₁-COONa due to high ionic strength.The Pickering emulsions are thus p H-responsive and the surfactant CH₃O（EO）₇-R₁₁-COONa returns to the aqueous phase together with alumina nanoparticles after demulsification.The whole aqueous phase is therefore recyclable and re-usable.（3）A nonionic homologue,CH₃O（EO）₅-R₁₁-COOH,was synthesized by using methyl polyoxyethylene（5）ether,which has a low p Ka（6.56）and behaves differently from CH₃O（EO）₇-R₁₁-COOH.In acidic media（p H<6）,CH₃O（EO）₅-R₁₁-COOH cannot stabilize conventional emulsions alone,but in neutral and alkaline media（p H>6）,CH₃O（EO）₅-R₁₁-COONa can stabilize conventional emulsions alone,although they are not p H-responsive because of significant effect of Na Cl.CH₃O（EO）₅-R₁₁-COOH can co-stabilize O/W emulsions with alumina nanoparticles,which are Pickering emulsions in acidic and neutral media but turn to OID emulsions in alkaline media（p H>10）（and vice versa）triggered by p H.The Pickering emulsions are partly sensitive to p H and temperature,but the OID emulsions are not sensitive to temperature.CH₃O（EO）₅-R₁₁-COOH can also co-stabilize O/W emulsions with silica nanoparticles,which are Pickering emulsions sensitive to temperature in acidic media and OID emulsions which are not sensitive to temperature in alkaline media.（4）An anionic-anionic Bola type surfactant,abbreviated as FA-AA,was constructed by formation of the C=N dynamic covalent bond using 4-formylbenzoic acid（FA）and 11-aminoundecanoic acid（AA）.In acidic media,the C=N bond is broken and the H⁺AA released behaves as a cationic surfactant,which can stabilize conventional emulsions alone and co-stabilize Pickering emulsions/OID emulsions with negatively charged silica nanoparticles/positively charged alumina nanoparticles,respectively.In alkaline media,however,the FA-AA shows strong hydrophilicity resulting in demulsification of all three types of emulsions.Accordingly,the three types of emulsions are all p H-responsive,and FA-AA returns to the aqueous phase after demulsification,making the whole aqueous phase recyclable and re-usable.Noticeably the use of dynamic covalent bond avoids complicated synthesis.In summary,this thesis proposed a new protocol for designing novel smart surfactants and has successfully synthesized a series of products,which are proved to have expected performances and are recyclable and re-usable.Particularly constructing a novel smart surfactant by means of dynamic covalent bond between two relevant compounds available can avoid complicated synthesis,which is in good accordance with the green chemistry.The relevant smart emulsion systems may have potential applications in wide fields such as transport of emulsified oil products,emulsion polymerization,new material synthesis,heterogeneous catalysis and so on.