Carbon Dioxide-in-Oil (C/O) Emulsions Stabilized by Silica Nanoparticles Functionalized with Oleophilic and CO2‑philic Ligands

High pressure CO2-in-mineral oil (C/O) emulsions with apparent bulk viscosities up to 14 mPa s (at 25 °C and 17.34 MPa) were generated with surface-modified nanoparticles (NPs). The silica NPs were designed with binary grafted ligands to be soluble in mineral oil and to adsorb at the CO2–oil (C–O) interface. The NPs were readily dispersible in mineral oil primarily from the oleophilic normal hexadecane (C16) ligands. Subsequently, a secondary CO2-philic moiety (either ethyl acetate, methyl acrylate, or propyl trimethoxysilane) was grafted to raise the NPs’ interfacial activity. The most viscous emulsions were obtained with NPs functionalized with ethyl acetate and C16, which lowered the C–O interfacial tension (IFT) the most, from 4.7 mN/m to as low as 3.1 mN/m, at a NP concentration of 2 wt % in oil. C/O emulsions were generated at shear rates as low as 370 s–1 with CO2 droplets sizes down to 10 μm. C/O emulsions were stable for at least 72 h, during which time an excess oil phase appeared after the cessation of mixing and grew slowly thereafter.