Synthesis And Properties Of Ionic Dynamic Imine Surfactants

In recent years,the construction of stimulus-responsive surfactants and their self-assembly systems have attracted extensive attention.Among them,the dynamic imine bond（-CH=N-）can be used to construct stimulus-responsive dynamic imine surfactants（DIS）.At present,the following problems of DIS and its self-assembly systems need to be further studied:（1）DISs are often confronted with the challenge of the accumulation of inorganic salts in the system as a pH-switchable surfactant,while the amphoteric DISs with good salt tolerance are rarely reported;（2）different from traditional surfactants,DIS is composed of two non-surface-active precursors（aldehyde and amine）.At present,the DISs used to prepare pH-switchable emulsions are mostly constructed from hydrophobic aldehyde/hydrophilic amine,but there are few reports of hydrophilic aldehyde/hydrophobic amine combined DIS to stabilize emulsion;（3）there are few research reports on the construction of pH-responsive viscoelastic fluid by the combination of DIS and other surfactants.Based on the above problems,an amphoteric DIS and an anionic DIS were designed and synthesized,and the pH response and oil-induced pH response behavior of DIS and its self-assembled system were studied.Within the scope of this article,the main research results and conclusions are summarized as follows:（1）Synthesis and performance of amphoteric DISsA benzene aldehyde containing sulfobetaine group,4-（2-sulfobetaine-ethoxy）-benzaldehyde（SBBA）,was designed and synthesized,and ¹H NMR,¹³C NMR,ESI-MS,and FTIR were used to characterize the molecular structure of SBBA.SBBA and C_nH_2n+1NH₂（n=4,BA;n=6,HA;n=8,OA;n=10,DA）were mixed in the aqueous solution with a molar ratio of 1:1 to obtain SBBA-DIS（SBBA-BA,SBBA-HA,SBBA-OA and SBBA-DA,respectively）.All SBBA-DIS have excellent surface activity.For the 50 mmol·L^-1 SBBA-HA solution,when the pH was lowered from 12 to 6,SBBA-HA was decomposed into non-surface-active SBBA and HA;at 25°C,the surface tension（γ）increased from 36.7 to 70.6 m N·m^-1,and the interfacial tension（IFT）increased from 8.3 to 25.4m N·m^-1,respectively;the micelles formed by SBBA-HA decomposed;macroscopically,the foaming and emulsifying properties of SBBA-HA almost disappeared.When the pH of the solution was restored to 12,the microstructure and surface activity of SBBA-HA returned to the original state.Due to the good salt tolerance of betaine surfactant,theγ,IFT,foaming properties and emulsion particle size of SBBA-HA did not change significantly after 5 cycles.（2）Oil-induced switchable DISFor SBBA-DIS composed of hydrophilic aldehyde/hydrophobic amine,the water solubility of the aliphatic amine precursor significantly affects the emulsification performance based on SBBA-DIS.SBBA-HA and SBBA-BA can form stable emulsions,while SBBA-OA and SBBA-DA were not suitable as emulsifiers to stabilize emulsions.OA and DA have poor water solubility and tend to dissolve in the oil-phase in an oil-water mixed system.Then part of OA or DA transfers from the water-phase to the oil-phase,resulting in a decrease in the pH of the water phase and a decrease in the concentration of aliphatic amines.Finally,the reaction equilibrium shifted towards the cleavage of SBBA-OA or SBBA-DA molecules.For DIS,in addition to pH,CO₂/N_2,and temperature,oil（pentane）can also be used as a trigger.For the 10 mmol·L^-1 SBBA-OA solution,theγand ITF reversibly changed between32.1 and 66.1 m N·m^-1,and 3.8 and 21.2 m N·m^-1 at 25°C by alternately adding and removing pentane.Further,alternate addition and removal of pentane can realize the reversible formation and cleavage of worm-like micelle based on SBBA-OA-SDS.Unlike using acid or alkali to adjust pH,using pentane as a trigged does not have any accumulation of inorganic salts during the cycle.（3）The pH-responsive behavior of anionic DIS and its self-assembly systemA simple precursor 3-（4-formylphenoxy）propyl sulfonate sodium（SBA）was designed and synthesized,and SBA was characterized by ¹H NMR,¹³C NMR,ESI-MS,and FTIR.SBA and octylamine（OA）were mixed with an equal molar ratio in an aqueous solution to form SBA-OA.SBA-OA shows a good ability to reduceγ,IFT,and form micelles.The cmc of SBA-OA is 3.55 mmol·L^-1,and theγ_cmc is 27.60 m N·m^-1 at 25°C.Theγand micellization of SBA-OA showed excellent pH responsiveness.By adjusting the pH of the SBA-OA solution to change between 6 and 12,SBA-OA can be reversibly converted between surface-active molecules and non-surface-active precursors.The pH-responsive viscoelastic fluid was prepared by compositing SBA-OA and N,N,N-trimethyl-N-tetradecylammonium bromide（TTAB）.The most suitable molar ratio of SBA-OA to TTAB is 1:3.By alternately adjusting the pH of the system to change between 10.6 and 2,SBA-OA-TTAB solution(SBA-OA,200 mmol·L^-1;TTAB,600 mmol·L^-1)can be reversibly converted between the viscoelastic fluid and the similar aqueous solution,and its zero shear viscosity was reversibly changed between 2420.0 m P·s and 7.7 m P·s at 25°C.This process can be cycled at least three times,which mainly depends on the reversible formation and breaking of imine bonds.