pH responsive Pickering emulsion stabilized by amphiphilic silicon quantum dots for enhancing oil recovery.

Recently, the silicon quantum dots (SiQDs) were widely utilized in enhanced oil recovery (EOR) due to their ultrasmall size, high specific surface area and excellent dispersion stability. Herein, the amphiphilic pH responsive silicon quantum dots (AR-SiQDs) were synthesized via the mild method to stabilize smart Pickering emulsion. Both the carboxyl and cinnamyl aldehyde were successfully grafted on the AR-SiQDs surface as hydrophilic and hydrophobic responsive groups, respectively. The structure of AR-SiQDs was characterized by Fourier transform infrared spectrometer and X-ray photoelectron spectroscopy, and the response mechanism was demonstrated via the zeta potential and interfacial tension (IFT) analyses. The results revealed that the diffusion and adsorption behaviors of AR-SiQDs relied on the protonation/deprotonation of carboxyl and reversible dynamic covalent bonding of imines. Specifically, the AR-SiQDs reduced the oil/water IFT from 51.00 mN/m to 7.58 mN/m at pH of 10.09, whereas the IFT only decreased to 24.12 mN/m at pH of 4.26. The AR-SiQDs effectively emulsified oil at alkaline condition, while the demulsification was rapidly achieved in 30 min upon the addition of hydrochloric acid. The emulsion exhibited excellent recyclability to maintain small average droplets size (from 1.42 μm to 2.12 μm) after five emulsification/ demulsification cycles. The core flooding experiments indicated that the AR-SiQDs stabilized emulsion evidently enhanced oil recovery by 14.65% at pH of 10.09, highlighting the remarkable synergistic effect with alkali. The work provided the novel strategy for designing smart SiQDs and recyclable emulsions for EOR.