Interfacially active carbon dots regulate oil–water–solid interactions for enhanced capillary-driven imbibition in high-salinity porous media

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: Capillary-driven imbibition is an important oil recovery mechanism in low-permeability reservoirs, particularly during post-fracturing shut-in, but is often hindered by unfavorable wettability, strong oil–solid adhesion, and high oil–water interfacial resistance under high-salinity conditions. Here, ultra-small amphiphilic carboxylate-modified carbon dots (C-CDs) were synthesized through hydrothermal treatment followed by stepwise alkyl and carboxylate functionalization and evaluated as aqueous interfacial modifiers. The C-CDs had an average size of 3.5 ± 0.1 nm, a zeta potential of −40.8 mV, and excellent stability under high salinity and elevated temperature. They reduced oil–water interfacial tension to 2.05 mN/m in 2.0 M NaCl and 1.42 mN/m in 2.0 M CaCl 2 . In crude oil–brine containing divalent ions, the interfacial tension decreased to 1.09 mN/m at 80 °C. Atomic force microscopy showed that C-CDs reduced hydrophobic oil–solid adhesion by more than two orders of magnitude. Quartz crystal microbalance and optical photothermal infrared analyses further demonstrated suppressed crude oil retention, inhibited re-adhesion, and a transition toward an oil-repellent surface state. Core-scale imbibition increased oil recovery from 10.2% with brine to 30.9% with C-CDs, while nuclear magnetic resonance confirmed enhanced oil mobilization from micropores and mesopores. A single exploratory core flooding experiment demonstrated the potential of C-CDs to enhance oil recovery under flow conditions, yielding a final oil recovery of 46.3%. These results reveal an interface-controlled imbibition mechanism involving wettability alteration, weakened oil–solid adhesion, reduced interfacial resistance, and facilitated droplet deformation, highlighting C-CDs as salt-tolerant, thermally stable agents for improving oil recovery in low-permeability, high-salinity porous media.