Endowing a cationic hydrophobic associating polyacrylamide solution with CO2 switchable properties using N,N-dimethylolamidopropylamine via the assembly transition between vesicles and spherical micelles by CO2

Abstract

Carbon dioxide triggers have attracted much attention due to their low cost, non-toxicity, simplicity and environmental benignity. N,N-dimethylolamidopropylamine (DOAPA) is a CO₂ switchable surfactant, the properties of which can be regulated by CO₂ easily. A mixture of a cationic hydrophobic associating polymer (PAD) and the CO₂ switchable DOAPA was prepared, which endowed the non-switchable PAD solution with CO₂ switchable properties. Then the viscosity and rheology behaviors of the solution were investigated under different pH conditions, with or without CO₂. The PAD–DOAPA mixture can be stimulated by CO₂ to alter the viscosity of the solution dramatically, with DOAPA assembled as vesicles at a pH of 6.8, and with a water-like viscosity at a pH of 6.0. The reversible transitions of micelle structure and viscosity can be recycled several times through the bubbling and removal of CO₂. Most important of all, the viscosity of the PAD–DOAPA system increased over time, which is different from the usual hydrophobic associating polymer–surfactant system, in which the viscosity has a maximum which decreases with further surfactant addition, due to the assembly of DOAPA into vesicles. Finally, a reasonable mechanism is proposed, based on the molecular structures, micelle assembly and intermolecular interactions. Upon the removal of CO₂, the assembled form of DOAPA changed from vesicles to spherical micelles, which shielded the hydrophobes and dissociated the gel into a sol. PAD–DOAPA aqueous systems with CO₂-reversible properties have wide applications in the fields of oil and gas production.