Synthesis And Self-Assembly Of Stimuli-Responsive Surfactant Based On Dynamic Imine

In recent years,aggregates formed by stimuli-responsive surfactants have attracted more and more attention from researchers.In addition to the incorporation of stimuli-responsive groups into surfactant molecules,dynamic covalent bonds are also one of the ideal tools for constructing stimuli-responsive aggregates.Imine bond is a typical dynamic covalent bond,which is usually obtained by mixing aldehyde-containing molecules and amine-containing molecules and widely used in the construction of stimuli-responsive aggregates.The formation of new molecules by imine bond does not only avoid the complicated synthesis steps,but also save the cost.To date,most of researches focus on stimuli-induced complete breakage and reformation of aggregates based on imine bonds.However,few studies focus on the controllable changes of aggregates.Therefore,in this project,we designed and synthesized a series of aldehyde-containing surfactants（AS）,and constructed stimuli-responsive aggregates with controllable characteristics making use of the Schiff base reaction between aldehyde groups and primary amines.The aggregation behaviors were investigated by means of ¹H NMR,surface tensiometer,dynamic light scattering,and rotational rheometer,etc..The main findings are as follows:（1）Three AS with different hydrophobic tail were synthesized and characteristized:N-dodecyl-N,N-dimethyl-N-（2-（4-formyl-phenoxy）ethyl）ammonium bromide chloride（DOBAB）,N-tetradecyl-N,N-dimethyl-N-（2-（4-formyl-phenoxy）ethyl）ammonium bromide（TOBAB）and N-hexadecyl-N,N-dimethyl-N-（2-（4-formyl-phenoxy）ethyl）ammonium bromide（HOBAB）.The results showed that the hydrophobicity of 2-（4-formyl-phenoxy）ethyl is approximately equivalent to 7 methylene groups,and its introduction causes the critical micelle concentration（cmc）of AS to drop by one order of magnitude.Increasing the hydrophobic chain is beneficial to the micellization of surfactants.The longer the alkyl chain,the smaller the cmc of AS,and the more negative the Gibbs free energy of micellization.The addition of Na Cl significantly reduced the cmc of HOBAB,but the addition of urea had little effect on the cmc of HOBAB.In addition,the antibacterial activity of TOBAB is significantly better than that of DOBAB and HOBAB,especially for Staphylococcus aureus,its minimum inhibitory concentration is only 1.96μmol/L.（2）A series of double-tailed surfactants were in situ prepared making use of the Schiff base reaction between alkyl amines（CH₃（CH₂）_nNH₂）and HOBAB.When n=5-7,double-tailed surfactants tend to self-assemble to form vesicles with a size of about 80-100nm.Under alkaline condition,double-tailed HOBAB/n-octylamine（OA）based on imine bonds can self-assemble into vesicles with a radius of about 80 nm;under acidic conditions,vesicles breakdown due to the disruption of double-tailed HOBAB/OA resulted from the breakage of imine bond,and thus spericla micelles were formed.（3）A pH and redox dual responsive vesicle was developed by the Schiff base reaction between HOBAB and 2-（benzylselanyl）ethan-1-amine（BSe EA）.At pH<7,HOBAB/BSe EA-based vesicles were destroyed and converted to spherical micelles.After adding H₂O₂,the selenide is oxidized to hydrophilic selenoxide,leading to a change of the hydrophilic-hydrophobic balance of the system.And thus the imine is exposed to the water environment which is unforable for imine bonds.Although oxidation cannot eliminate all imine bonds,the vesicles can be completely transformed into spherical micelles because of the change of the aggregate microenvironment Note that the hydrophobic Nile Red（NR）loaded in the vesicles can be released by the trigger of H₂O₂,but not the stimulation of pH.（4）A series of dynamic gemini surfactants（DGS）were in situ prepared by the Schiff base reaction between AS and alkyl diamines.DGS can self-assemble into typical worm-like micelles when the spacer was 1,3-propanediamine（Pr DA）,1,4-butanediamine（BDA）,1,5-pentanediamine（Pe DA）or 1,6-hexanediamine（HDA）.Among those,DGS based on HOBAB/HDA exhibited the strongest thickening ability.（5）Under alkaline conditions,DGS based on HOBAB/HDA aggregated into viscoelastic wormlike micelles owing to the formation o imine bond;under acidic conditions,viscoelastic fluid converted into water-like solution due to the damage of DGS originated from the beak of imine bonds,reflecting the emergence of spherical micelles.The zero-shear viscosity（η₀）of HOBAB/HAD-based DGS solution gradually increases with increasing concentration,and reached a maximum（51 Pa·s）at 75 mmol·L^-1.Furthermore,the viscosity drops by about one order of magnitude upon increasing temperature from 25 ℃ to 45 ℃.