CO2-responsive surfactant for oil-in-water emulsification and demulsification from molecular perspectives

To economically and environmentally recover oil from reservoirs and promote CO2 utilization (CU) project, CO2 responsive surfactants have been developed to undertake multiple tasks including emulsification and demulsification during different production stages. Understanding the switching mechanisms from molecular perspectives is of great importance to the choice and design of high-performance CO2-responsive surfactants. In this work, we performed molecular dynamics (MD) simulations to study the emulsification and demulsification processes of a heptane/water mixture in the presence of a typical CO2-responsive surfactant-lauric acid (LA). Before injecting CO2, the deprotonated lauric acids (DLA) can stabilize O/W emulsions in an aqueous solution due to strong electrostatic repulsions and high interfacial activity of DLA, whereas the protonation of lauric acid (PLA) arising from CO2 injection would result in the coalescence of emulsion droplets thanks to the greatly reduced hydrophilicity of the polar groups of lauric acids and surface charge neutralization, which is unfavorable for emulsion stabilization. The potential mean force (PMF) results show a high energy barrier preventing the fusion process when two emulsions approach each other in the absence of CO2, indicating high stability of the emulsions. However, when DLA turns to be PLA, the energy barrier disappears and an attraction force emerges due to the entropic effect if two emulsions are close enough. Our study provides important insights into the structural properties of emulsions before and after CO2-triggered switching and sheds light on the switching mechanisms which may assist in selecting and designing efficient CO2-responsive surfactants.