Asymmetric Propylene Oxide Extension of Monorhamnolipid: Synthesis, Interfacial Behavior, and Enhanced Oil Recovery Performance

Abstract Rhamnolipids are attractive biobased surfactants for enhanced oil recovery (EOR), but their interfacial activity and salt tolerance are limited by a compact molecular architecture. Here we graft short propylene oxide (PO) chains onto a monorhamnolipid (Rha-C10), obtaining three extended derivatives (RL4-POp, p = 3, 6, 9) via a selective protection–alkoxylation–deprotection sequence. Their interfacial performance varies nonmonotonically with the length of the PO spacer: the p = 6 derivative yields the lowest oil–water interfacial tension (3 × 10–3 mN·m–1 against crude oil at 45 °C, 0.3 wt % surfactant, 0.8–1.2 wt % added NaCl) and reduces the contact angle on naturally oil-wet core slices from 121.3° to 70.7°, while the p = 9 derivative exhibits inferior performance in the interfacial, emulsification, wettability, and displacement tests relative to RL4-PO6. In core flooding experiments, the RL4-PO6/HPAM formulation delivers 28.5 percentage points of incremental oil recovery after waterflooding, compared with 22.5 for RL4-PO9, 20.0 for RL4-PO3, 15.9 for unmodified RL4, and 12.4 for HPAM alone. Instead of acting merely as an added hydrophobic moiety, the PO spacer modulates molecular conformation. Spacer length is a tunable design parameter for rhamnolipid-based EOR surfactants.