Rhamnolipid Biosurfactants for Oil Recovery: Salt Effects on the Structural Properties Investigated by Mesoscale...

Rhamnolipids (RLs) are biosurfactants produced by Pseudomonas. The biodegradability and the variety of their functionality make them suitable for environmental remediation and oil recovery. We use dissipative particle dynamics simulations to investigate the aggregation behaviors of ionic RL congeners with nonane in various operating conditions. Under zero-salinity conditions, all RL congeners studied here form small ellipsoidal clusters with detectable free surfactants. When salt ions are present, the electrostatic repulsion between the ionized heads is overcome, resulting in the formation of larger aggregates of unique structures. RLs with C10-alkyl tails tend to form elongated wormlike micelles, while RLs with C16-alkyl tails tend to form clusters in spherical symmetry, including vesicles. Di-rhamnolipids (dRLs) require stronger solvation than monorhamnolipids (mRLs) to form clusters, and the resulting size of micelles is decreased. The morphology of the mixed dRL/mRL/oil systems is controlled based on the type of the congeners and the oil contents. In addition, the divalent calcium ions are found to be influential to the structure of the micelles through different mechanisms. For 5 wt % salinity, the ionic RLs can form oil-swollen micelles up to a 1:1 surfactant-to-oil ratio, suggesting that ionic RLs are superb to act as cleaning agents for petroleum hydrocarbons in the marine area. These key findings may guide the design for RL-based washing techniques in enhanced oil recovery.