Design of Amphiphilic Carbon Dots for Producing Stable CO2 Foams as Eco-Friendly Fluids for Enhanced Oil Recovery

In recent years, amphiphilic nanoparticles with low toxicity, low cost, and good biocompatibility have received extensive attention in foam stabilization, specifically formulated for enhanced oil recovery or CO₂ geological storage. To address this issue, we designed novel amphiphilic carbon dots (ACDs) by utilizing citric acid as a carbon source via the hydrothermal method. The as-prepared ACDs (ACD-C₁₄, ACD-C₁₆, and ACD-C₁₈) exhibit spherical particles of 1–2 nm in size and hydrophobic character, which could move to the interface as surfactants and cause a sharp decrease in surface tension, showing diffusion-limited kinetics and a rapid interfacial rearrangement. By bubbling the CO₂ in ACD suspension, ACDs could assemble, accumulate, and pack at CO₂-water interfaces, allowing them to aggregate from nanoscale particles to microscale platelets and then generate armored bubbles, which enables a repulsive energy as well as a detach behavior between CO₂ bubbles to inhibit the drainage, coarsening, and coalescence and thus significantly boost the foam stability. ACDs could promote bubble formation and stabilize the flow pattern in a multibranched microchannel. In general, combined with these significantly superiors, i.e., simple pretreatment, nontoxicity, rapid interfacial adsorption, and special self-assembly, ACDs can produce highly stable CO₂ foams as an eco-friendly alternative to enhance oil recovery.