Synergistic Effect of Salt and Anionic Surfactants on Interfacial Tension Reduction: Insights from Molecular Dynamics Simulations

Surfactants are commonly utilized in chemical flooding processes alongside salt to effectively decrease interfacial tension (IFT). However, the underlying microscopic mechanism for the synergistic effect of salt and surfactants on oil displacement remains ambiguous. Herein, the structure and properties of the interface between water and n-dodecane are studied by means of molecular dynamics simulations, considering three types of anionic surfactants and two types of salts. As the salt concentration (? (salt)) increases, the IFT first decreases to a minimum value, followed by a subsequent increase to higher values. The salt ions reduce the IFT only at low ? (salt) due to the salt screening effect and ion bridging effect, both of which contribute to a decrease in the nearest head-to-head distance of surfactants. By incorporating salt doping, the IFTs can be reduced by at most 5%. Notably, the IFTs of different surfactants are mainly determined by the hydrogen bond interactions between oxygen atoms in the headgroup and water molecules. The presence of a greater number of oxygen atoms corresponds to lower IFT values. Specifically, for alkyl ethoxylate sulfate, the ethoxy groups play a crucial role in reducing the IFTs. This study provides valuable insights into formulating anionic surfactants that are applicable to oil recovery processes in petroleum reservoirs using saline water.

Automated summary

Surfactants and salt are commonly used together in chemical flooding processes to reduce interfacial tension (IFT). However, the microscopic mechanism behind their synergistic effect on oil displacement is unclear. Molecular dynamics simulations were used to study the structure and properties of the water-n-dodecane interface, considering different anionic surfactants and salts. The study found that the addition of salt reduces the IFT initially, but later increases it due to the salt screening and ion bridging effects. The study also showed that the number of oxygen atoms in the surfactant headgroup determines the IFT values.