Functional Janus-SiO2 nanoparticles prepared by a novel "cut the Gordian knot" method and their potential application for enhanced oil recovery.

Currently available methods (e.g. interfacial protection, phase separation, etc.) for preparing Janus nanoparticles are often complex and expensive. Furthermore, the preparation of Janus nanoparticles with a particle size of below 10 nm is challenging. In this work, we combine in-situ surface modification route with chemical etching route to establish a novel "cut the Gordian knot" method for the preparation of functional Janus-SiO2 nanoparticles. Hydrophobic SiO2 nanoparticles with three-dimensional (3D) network structure prepared via in-situ surface-modification route were dispersed in NaOH solution containing surfactant or ethanol to enable corrosion close to the modifier-nanoparticle interface with a relatively low content of surface modifiers. Thus amphipathic Janus-SiO2 nanoparticles with a hydrophilic surface containing Si-OH species and a hydrophobic surface containing -CH3 fragments were generated. The as-prepared Janus-SiO2 nanoparticles with a size of 4 ~ 9 nm and a specific surface area of up to 612.9 m2/g can be easily dispersed in water, and they also can transfer from water phase to oil phase by tuning the surface polarity. Moreover, they can be tuned to achieve bidirectional regulation of surface wettability plus a reduction of the oil/water interface tension. Hence a significant reduction (by 33% ~ 50%) of water injection pressure and an enhanced oil recovery (EOR) (by 21.1% ~26.6%) can be achieved. Apart from that, Janus-SiO2 nanoparticles are able to increase the viscosity of partially hydrolyzed polyacrylamide by 282.9% and significantly decrease its viscosity loss ratio in brine, causing an EOR of about 36.6%. With simple, low-cost and scalable procedures, the following approach could be well applicable to fabricating Janus-SiO2 nanoparticles with high potential for augmented water-injection as well as EOR of low-permeability reservoirs.