Unveiling the congener-dependent performance of mono-rhamnolipids and di-rhamnolipids in high-salinity brine: From micellization and interfacial behavior to molecular origin revealed by DFT.

Rhamnolipids are promising biosurfactants for enhanced oil recovery (EOR); however, the structure-performance relationships of distinct rhamnolipid congeners under reservoir-relevant conditions remain insufficiently elucidated. In this study, mono-rhamnolipids and di-rhamnolipids with well-defined, homogeneous molecular structures were biosynthesized using genetically engineered Pseudomonas strains. High-purity (>98%) isolates of both congeners were obtained to enable rigorous comparative characterization of their physicochemical properties. Systematic physicochemical measurements revealed significant differences between the two congeners in critical micelle concentration, hydrodynamic diameters, and zeta potential, indicating distinct micellar assembly and interfacial behaviors. When evaluated in simulated formation water containing divalent cations (Ca2⁺ and Mg2⁺), di-rhamnolipids demonstrated markedly superior functional performance over mono-rhamnolipids, including higher solubility retention (90% vs. 25%), more stable emulsification, improved wettability preservation (contact angle increase of 20.99% vs. 66.10%), and greater oil-washing efficiency (55.7% vs. 21.8%). Mechanistic investigations revealed that the additional rhamnose moiety in di-rhamnolipids induces reorientation of the fatty acyl chains, increases steric bulk, and expands the spatial extent of positive electrostatic potential-collectively enhancing hydration and salt tolerance. This work establishes a mechanistic framework linking congener architecture to functional resilience, providing rational criteria for screening biosurfactant variants for EOR applications.