Preparation Of Oleic Acid Micelles And Study On Their Stimulatory Response Behavior

Stimuli responsive wormlike micelles have attracted many interests owing to their superior viscoelastic properties and unique micellar structures in recent years.Thereinto,the pH and CO2-responsive wormlike micelles based on low molecular weight compounds have become competitive candidates for enhanced oil recovery or fracture in oil fields due to simple,inexpensive,biocompatible,nontoxic and environmental characteristic.Therefore,we prepared CO2 and pH responsive wormlike micelles based on oleic acids derivatives.Besides,the stimuli responsive mechanism and property change of two systems were studied in detail.At first,we utilized the long chain amine oxide surfactant OAPAO synthesized to prepare a pH and temperature responsive wormlike micelles.The structure of OAPAO was characterized by 1H NMR and FT-IR.The micellization and aggregation properties of OAPAO surfactant in aqueous solution at different pH and temperature were studied.The OAPAO solution exhibited pH-regulated surface activity at the air/water interface by assessing relevant parameter such as cmc,γcmc,Γmax,Amin at pH = 2.00,6.80 and 12,respectively.The viscosity and rheological measurements demonstrated the pH-sensitive flowing behavior and property,which was attributed to the interactions transition between OAPAO head groups.Moreover,the system could be promptly switched between low viscous fluid and highly viscoelastic fluid.Combined the distribution state of OAPAO,a mechanism of the different intermolecular force at different pH region was proposed.Furthermore,the OAPAO solution exhibited evident temperature-responsiveness at pH=6.8,11.88 in a certain temperature range resulting from the effect of temperature on the hydrophobic effect and intermicellar branching junctions.Secondly,we prepared two CO2-responsive anionic surfactant wormlike micellar systems based on anionic surfactant sodium oleate and hydrophobic tertiary amine N,N-dimethylcyclohexylamine(DMCHA),N,N-dimethylbenzylamine(DMBA).The CO2-responsive viscoelastic fluids were easily prepared by simply introducing CO2-responsive hydrophobic tertiary amine into anionic surfactant solution.The effects of concentration and molar ratio on viscosity,rheological property,CO2-responsive characteristics and mechanism of mixed system were studied in detail.The viscositiy of the aqueous system strongly depended on the concentration and molar ratio.It was obvious that the viscosity of NaOA-DMBA aqueous system was evidently superior to NaOA-DMCHA aqueous system,which was ascribed to the difference of hydrophobic chain.The mixed aqueous system underwent two macroscopic appearance states from waterlike fluids to high-viscoelastic solution by the inlet of CO2.The conductivity,pH and rheological measurements demonstrated the CO2-sensitive flowing behavior and property,which was attributed to the spherical-wormlike micelles transition verified by cryogenic transmission electron microscopy(Cryo-TEM)and dynamic laser light scattering(DLS)measurements.Moreover,the transition can be easily cycled more than three times without deterioration of viscosity between a viscoelastic state and a waterlike state.Combined with the species distribution curve and 1H-NMR spectra,a mechanism of the intermolecular electrostatic interaction and hydrophobic effects between DMCHA or DMCHA·H+ and OA-was proposed.DMCHA could be protonated into DMCHA·H+ after bubbling CO2,which behaved like hydrotrope that can strongly bind to the OA-head group noncovalently through electrostatic attraction and penetrate into hydrocarbon chains of OA-through hydrophobic effects,synergistically resulting in the spherical-wormlike micelles transition.After CO2 was removed,the ammonium groups are deprotonated back to tertiary amines,leading to the transition between wormlike micelles and spherical micelles.