Molecular dynamics study of "quasi-gemini" surfactant at n-decane/water interface: The synergistic effect of hydrophilic headgroups and hydrophobic tails of surfactants on the interface properties

Chemical enhanced oil recovery (EOR), especially surfactant injection, has recently received a great deal of attention. Proper selection of the surfactant can lead to a high recovery from an oil reservoir by alternating the wettability and reducing interfacial tension. It is important to understand how the efficiency in reducing the interfacial tension is related to the structure of a surfactant. In this work, the aggregation behavior and the interfacial performance of a series of "quasi-gemini" nonylphenol-substituted dodecyl sulfonate surfactants denoted as n-C12S(NP) (n = 1, 2, 3, 4) at n-decane/water interface are explored in detail by employing the allatomic molecular dynamics simulation. The influence of the substitution position of benzene ring on the interfacial performance is investigated by analyzing the interaction between headgroup and water and the interaction between the alkyl tails and n-decane. The interaction between headgroup and water and the interaction between the alkyl tails and n-decane are investigated by analyzing the density profiles, the radial distribution function, coordination number, the number of hydrogen bonds and the interface thickness. Simulation results suggest that a subtle structural variation in the headgroup and tails results in significant effects on limiting interface tensions at the fixed coverage: the interface properties of 3-C-12 S(NP) and 4-C12S(NP) are significantly better than those of 1-C12S(NP) and 2-C12S(NP). 3-C12S(NP) leads to the lowest interface tension and the largest interface formation energy by having the strongest interaction with water and decane. The results are useful for designing high performance surfactants for oil displacement.