Spotlight on reversible emulsification and demulsification of tetradecane-water mixtures using CO2/N2 switchable surfactants: Molecular dynamics (MD) simulation

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CO2/N2 switchable surfactants have recently attracted significant attention in enhanced oil recovery. Their environmental and economic benefits are the main driving forces for their application in the oil and gas industry. This study evaluated mixtures of sodium dodecyl sulfate (SDS), C18 naphthalene sulfonate (C18PS), and CO2-switchable acetamidine surfactant N′-dodecyl-N, N-dimethylacetamidine (C12DMAA) in water/dodecane systems by using molecular dynamics (MD) simulation. The effects of the molecular structure of anionic surfactant and surfactant concentration on switchable emulsification and demulsification in an oil/water/SDS or C18PS/C12DMAA system triggered by CO2/N2 injection were investigated. The molecule mobility, hydration behavior, density field analysis, diffusion coefficient, radial distribution functions (RDFs), and the density profiles of various systems were thoroughly examined. Based on MD simulation outputs, increasing surfactant concentration can intensify a demulsification process and create a clear phase separation between water and tetradecane. On the other hand, during an emulsification process, adding more surfactant monomers improved a hydration layer around oil droplets and increased hydrogen bonds; however, the contribution of hydrogen bonds between water and N′-dodecyl-N, N-dimethylacetamidinium (C12DMAAH+) compared to SDS and C18PS is limited. MD simulations proved the switchability of SDS/C12DMAA and C18PS/C12DMAA for emulsification and demulsification processes. The results of the current study will be beneficial for developing and optimizing the switchable surfactant structures for further use in the oil and gas industry through surfactant injection.