Novel Smart Surfactants With Structure Switchable Between Amphiphilic And Highly Polar

Smart surfactants are of great significance for the construction of stimulus-responsive self-assembly/dispersion systems and the realization of the recycling of surfactants.At present,the main control method for smart surfactants is to convert hydrophilic groups into non-polar or weakly polar groups（hydrophobic groups）which results that the surfactants dissolve in organic phase after inactivation.However,in many applications,the deactivated surfactants are expected to return to water to reduce the contamination to the oil phase.For this purpose,in this thesis a series of new smart surfactants were synthesized by connecting a tertiary amine group on hydrophobic end of the conventional alkyl trimethyl ammonium bromides.In acidic condition,the tertiary amine group will be protonated to be strongly hydrophilic amine-salt,which makes the surfactant convert to Bola structure and lose surface activity and returned from interface to aqueous phase.Overall this study obtained following results and conclusions.（1）Using long-chain alkanedioic acid as raw material,through four step reaction the new surfactants N-C_n-N⁺（n=14,16）were synthesized and their structure was identified and characterized by using ESI-MS,FT-IR,¹H NMR and ¹³C NMR as well as chemicals analysis.The purity of the two surfactants were determined to be 95.4%（n=14）and 98.7%（n=16）respectively.（2）The surface activity parameters of N-C_n-N⁺（n=14,16）were determined by measuring the surface tension of their aqueous solutions at different concentration using Du Noüy method.In neutral aqueous media,both surfactants behave as normal surfactant with cmc being 7.5 mM（n=14）and 3.6 mM（n=16）,respectively,andγ_cmc being 33.1 m N/m and 35.4 m N/m,respectively.At saturated adsorption both surfactants give same cross section area,0.94 nm²,at air/water interface,but at acidic condition,the cross section area is almost doubled due to that the surfactants are turned to Bola structure which adsorb at air/water interface in a form of reversed U shape.Moreover,no micelle formation is revealed up to 10 mM.（3）The cationic surfactant N-C_n-N⁺（n=14,16）solely can stabilize conventional O/W emulsion with n-decane as oil phase.N-C₁₆-N⁺is superior to N-C₁₄-N⁺which needs lower concentration and gives more stable emulsions.The conventional O/W emulsions formed have good p H-responsiveness.In acidic condition（p H<3.0）,demulsification occurs and stable emulsion can be re-formed by replace the oil phase separated using fresh n-octane,adding Na OH to turn the p H back to original value,and followed by homogenization.Overall more than 4 cycling can be realized.（4）With n-octane as oil phase,the cationic surfactants N-C_n-N⁺（n=14,16）and the negatively charged Si O₂ nanoparticles can co-stabilize O/W Pickering emulsions,where the cationic surfactants adsorb on particle surface via electrical attraction to form a hydrophobic monolayer,so as to in situ surface-activate the particles endowing particle with surface activity.N-C₁₆-N⁺behaves better than N-C₁₄-N⁺by giving smaller droplet size and more stable emulsions.The Pickering emulsion are also p H-responsive,which are stable in neutral or weak alkaline condition,but demulsified at acidic condition,because the hydrophobic tertiary amine group is protonated to strongly hydrophilic group which makes the surfactant turn to Bola structure and thus lose surface activity.Similarly,the Pickering emulsion can be cycled for at least 5 times.（5）With n-octane as oil phase,the cationic surfactant N-C_n-N⁺（n=14,16）and the positively charged Al₂O₃ nanoparticles can co-stabilize novel oil-in-dispersion emulsions.Compared with the conventional and Pickering emulsions,the oil-in-dispersion emulsion need much lower surfactant concentration.And the emulsion also p H-responsive,which are stable in neutral or weak alkaline conditions,but quickly demulsified in acidic condition.Overall more than 4 cycling is possible.（6）For all three type emulsions the limited cycling times is due to the accumulation of the NaCl which as an electrolyte can screen the double layer repulsion between oil droplets,inhibit adsorption of surfactant on particle surface via electric attraction,and reduce double layer repulsion between charged nanoparticles.However,in limited cycling times the effect of the NaCl is not serious.（7）For all three type emulsions,the surfactant returned to aqueous phase after demulsification without contaminating to the oil phase.The aqueous phase with emulsifiers is therefore reusable.Thus,the new surfactants synthesized in this study have potential applications such as in transport of emulsified oil products,emulsion polymerization,detergency,and synthesis of new materials etc.Using these surfactants is beneficial and helpful for saving surfactants and reducing their emission to environment,which are both economically and environmentally benign.