Chemically surface-etched nanoparticles for tailored Pickering emulsions in enhanced oil recovery

Nanoparticles are widely used in Pickering emulsions, but their hydrophilic nature often limits interfacial effectiveness. This study explores a novel chemical etching method using the strong reductant sodium borohydride (NaBH₄) to enhance the interfacial properties of silica nanoparticles, improving oil recovery through more efficient Pickering emulsification. Characterization confirms surface etching through TEM and XPS, showing a rougher surface and a hydrodynamic size of 164.43 nm for etched silica, compared to 197.74 nm for bare silica. The etched silica nanoparticles exhibit increased hydrophobicity, as evidenced by FT-IR and contact angle measurements (θ = 75 ± 1° for etched silica vs. θ = 20 ± 1° for bare silica). At 500 ppm, modified silica nanoparticles facilitate Winsor emulsions I and II, with etched silica producing smaller, more stable droplets. Increasing the concentration to 2500 ppm reduces droplet size and tightens distributions, especially with etched silica, enhancing emulsion stability due to stronger interfacial layers, non-spherical shape, and lower bending resistance, as shown by desorption energy values of ⁓ 3.91 × 10⁻¹⁸ J for bare silica and ⁓ 3.49 × 10⁻¹⁶ J for etched silica. Pore-scale experiments demonstrate that surface-etched silica nanoparticles improve oil displacement and reduce residual trapping by promoting oil-in-water emulsions, outperforming bare silica due to stronger mechanical interactions, higher negative charge, and increased disjoining pressure (−4.81 × 10⁻⁴ Pa vs −8.38 × 10⁻⁴ Pa). This innovative modification approach, previously unexplored in oil recovery, offers a new pathway for enhanced oil mobilization and emulsion stability, with potential for broader applications in wastewater treatment, catalysis, and pharmaceuticals.