Modulation of slippage at brine–oil interfaces by surfactants: The effects of surfactant density and tail length

Fluid transport involving brine–oil interfaces plays an important role in applications including enhanced oil recovery and oil–brine separation and can be affected markedly by the slippage at these interfaces. The slippage at brine–oil interfaces, however, is not well understood, especially in the presence of surfactants, which are ubiquitous in natural and engineering systems. Here, we report molecular dynamics studies of the slippage at brine–decane interfaces in the presence of two surfactants, nonylphenol and phenol. They share essentially the same head but nonylphenol has a nine-carbon alkyl tail and phenol has no clear tail. At zero surfactant density, a slip length of 1.2 nm exists at the brine–decane interface. As either surfactant is introduced to brine–decane interfaces, the slip length initially decreases linearly, with nonylphenol being more effective in reducing the slip length. As more surfactants are introduced, the decrease in slip length slows down and eventually, the slip length plateaus at −1.4 and −0.5 nm for interfaces populated with nonylphenol and phenol, respectively. The mechanisms of the observed slip length vs surfactant density relations and the effects of tail length on the interfacial slippage are elucidated by analyzing the molecular structure and transport of interfacial fluids and surfactants.