PH-responsive Surfactant Micellar System Research

pH-Responsive surfactant micelle is a special kind of stimuli-responsive micelle whose structures and properties can be controlled by adjusting pH values. Generally speaking, pH-responsive surfactant micelle has many advantages such as easy preparation, reversible and controllable properties, and economic environment. These fascinating properties of the pH-responsive surfactant micelles render them extremely useful in many aspects especially for oil and gas field development. Therefore, it is of theoretical significance and promising prospect to research pH-responsive surfactant micelle.In this article, three different pH-responsive surfactant micelles-single long chain, pseudogemini, and gemini-have been developed. Besides, their properties and pH-responsive mechanism have been researched respectively.A pH-responsive single long chain surfactant micelle system has been developed by using sodium oleate (NaOA) and NaCl. Transparent viscoelastic solution presents when pH>9.53. The solution in such pH ranges has shear-thinning behavior and follows Maxwell fluid model. When decreases pH to 9.43, the transparent visocoelastic solution transforms to slightly turbid water-like fluid. Moreover, phase separation appears while contimually adjust pH value to 9.33. The transition between spherical micelle and wormlike micelle for the surfactant system at different pH values could be further confirmed by cryo-TEM and DLS. At higher pH value, Na+ may compress the electric double layers of the interface and may screen the electrostatic repulsion between OA-, the charged head-groups, which results in more OA- going into micelles and thus wormlike micelles forms. At lower pH values, NaOA prefers nonionic species (-COOH) which cannot combine with NaCl, and forms spherical micelles, or even insoluble in water at an extremely lower pH value.A pH-responsive pseudogemini surfactant micelle system has been developed by using N, N-dimethyl oleoaminde-propylamine (DOAPA) and disodium oxalate with a molar ratio of 2:1. When the pH is smaller than 4.20, transparent solution with low viscosity is obtained. When the pH is located at 4.20-7.39, the solution is still transparent but has a high viscosity. Besides, the solution in such pH ranges has shear-thinning behavior and follows Maxwell fluid model. While when the pH is higher than 7.39. the viscoelastic solution switch back to the water-like fluid, which can be attributed to the separation of white precipitate. The transition between spherical micelle and wormlike micelle for the surfactant system at different pH values could be further confirmed by cryo-TEM and DLS. When pH<4.20, only single chain surfactant of DOAPAH+ and DOAPAH+-HC2O4- main formed and assembled into spherical micelles. When 4.20<pH<7.39. the wormlike micelles existed because vast C2O42- and DOAPAH+ are in this pH range and one C2O42- bound with two DOAPAH+ through the strong electrostatic attraction to establish pseudogemini surfactant. When pH>7.39, the wormlike micelles were severely destroyed because the DOAPA was almost insoluble in water and separated out from the system.A pH-responsive gemini surfactant DSTAPA has been synthesized by using tartaric acid, stearoyl chloride, and N, N-dimethyl-1,3-propylene amide. The molecular structure of the DSTAPA has been confirmed through FT-IR and 1HNMR methods. In addition to pH responsive ability, DSTAPA also has temperature responsive property. When pH<6.80, transparent solution with low viscosity is obtained. When the pH is located at 6.80-7.80, the solution is still transparent but has a high viscosity. Besides, the solution in such pH ranges has shear-thinning behavior and follows Maxwell fluid model. While when pH>7.80, the viscoelastic solution turns back to the water-like fluid due to the separation of white precipitate. The transition between spherical micelle and wormlike micelle for the surfactant system at different pH values could be further confirmed by cryo-TEM and DLS. At lower pH, the strong intra- and inter-molecular electrostatic repulsions between the entirely ionized DSTAPAH2+ make the surfactant molecules tend to exhibit a conical structure, which is prone to gain a small p value, and hence spherical micelles are formed. With the increased pH, the disappearance of electrostatic repulsion of the partial ionized DSTAPAH+ causes decrease of a, leading to the formation of wormlike micelles. While at extremely higher pH, the wormlike micelles were severely destroyed because the non-ionized DSTAPA was almost insoluble in water and separated out from the system.