Study On The Syntheses And Properties Of Stimuli-responsive Itaconate-based Surfactants

In this paper,a series of pH-stimulated responsive Gemini-like surfactants C₁₄-N_m-C₁₄(C₁₄-N₂-C₁₄?C₁₄-N₄-C₁₄?C₁₄-N₆-C₁₄4 and C₁₄-N₈-C₁₄)with varing link lengths were synthesized using the itaconic acid tetradecyl ester(C₁₄)and diamines?ethylenediamine,butanediamine,hexamethylenediamine,and diaminooctane?as the raw materials.Furthermore,itaconic acid monosodium salt surfactant(C₁₄-ONa)was prepared correspondingly by acid-base neutralization.The physicochemical properties of C₁₄-N_m-C₁₄,such as the Krafft points,surface tensions,HLB values,calcium soap dispersing abilities,foam properties,and emulsifying abilities,were characterized and the effects of spacer lengths,pH,and inorganic salts on critical micelle concentration?CMC?and surface tensions were studied.The properties of C₁₄-N_m-C₁₄4 and C₁₄-ONa surfactants were also been compared,indicating that Gemini-like surfactants do have advantages over mono-surfactants in increasing surface activity.In addition,the aggregation morphologies of C₁₄-N_m-C₁₄4 were observed by TEM at low concentrations,and the particle size distributions obtained by DLS were combined to confirm that the vesicles were easy to be formed at low C₁₄-N_m-C₁₄4 surfactants concentration.The sizes and morphologies of the vesicles transformed with pH changing,and the formation mechanisms of the aggregates were analyzed based on the protonation distribution diagrams found by the pH titration method.The effects of pH and spacer lengths on the rheological behavior of C₁₄-N_m-C₁₄were investigated.Steady rheological results indicated that the sample exhibited Newtonian fluid behavior in the original solution,while it showed a shear thinning behavior by adding NaOH solution,which was typical of the presence of wormlike micelles.As the pH further increased,the viscoelasticity gradually decreased and the wormlike micelles ruptured.Cole-Cole plots further confirmed the presence of worm micelles in the dynamic rheological results.The pH of the solution was adjusted by cyclic dropwise addition of a HCl solution or a NaOH solution to reversibly change the rheological properties,which was manifested by state transitions between the gels and aqueous solutions.Among the C₁₄-N_m-C₁₄4 systems,C₁₄-N₄-C₁₄4 has the best elasticity and the widest elastic range,while C₁₄-N₆-C₁₄4 has the widest pH stimuli-responsive range,and the viscoelasticity behavior of C₁₄-N₆-C₁₄4 was slightly better than C₁₄-N₈-C₁₄.The viscoelastic behavior of C₁₄-N₂-C₁₄4 was the least obvious and its pH adaptation range was the shortest.The pH-tunable mechanism of the C₁₄-N_m-C₁₄4 system was proposed by combining the protonation curves and ¹H NMR,and confirmed by DPD simulations.The experimental results indicated that the pH-tunable properties could be ascribed to different protonation degrees and noncovalent interactions of the diamine to carboxyl groups of C₁₄4 at different pH values.The Gemini-like surfactant C₁₄-N₆-C₁₄could be changed into C₁₄-ONa when the pH was high enough.The formation of wormlike micelles was a result from the synergy between�Gemini�and�unimer�.When the Gemini-like surfactants were completely converted into mono-surfactants,the wormlike micelles were destroyed.P-index theory showed that not only the alkaline environment,but also the number of carbon atoms in the spacer length played an important role in the self-assembly of wormlike micelles.The DPD simulations and experimental results showed a good consistency,revealing the formation and destruction processes of wormlike micelles in the long chain acids and short amines mixed system.