Effect of Grafted Copolymer Composition on Iron Oxide Nanoparticle Stability and Transport in Porous Media at High Salinity

The transport of engineered nanoparticles in porous media is of interest in numerous applications including electromagnetic imaging of subsurface reservoirs, enhanced oil recovery, and CO₂ sequestration. A series of poly­(2-acrylamido-2-methyl-1-propanesulfonic acid-<i>co</i>-acrylic acid) (poly­(AMPS-<i>co</i>-AA)) random copolymers were grafted onto iron oxide (IO) nanoparticles (NPs) to provide colloidal stability in American Petroleum Institute (API) standard brine (8 wt/wt % NaCl and 2 wt/wt %CaCl₂, anhydrous basis). A combinatorial approach, which employed grafting poly­(AMPS-<i>co</i>-AA) with wide ranges of compositions onto platform amine-functionalized IO NPs via a 1-ethyl-3-(3-(dimethylamino)­propyl)­carbondiimidecarbondiimide (EDC) catalyzed amidation, was used to screen a large number of polymeric coatings. The ratio of AMPS/AA was varied from 1:1 to 20:1 to balance the requirements of particle stabilization, low adsorption/retention (provided by 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS)), and permanent attachment of stabilizer (provided by acrylic acid (AA)). The resulting nanoparticles remained stable in aqueous suspension despite the extremely high salinity conditions and exhibited low adsorption on silica microspheres. Greater than 91% of applied IO-NP mass was transported through columns packed quartz sand, and the mobility of IO NP increased by ca. 6% when the AMPS to AA ratio was increased from 1:1 to 3:1, consistent with batch adsorption data. In both static batch reactor and dynamic column tests, the observed attachment of IO NPs was attributed to divalent cation (Ca²⁺) mediated bridging and hydrophobic interactions. Collectively, the rapid, high throughput combinatorial approach of grafting and screening (via batch adsorption) provides for the development of high mobility NPs for delivery in various porous media under high salinity conditions