Magnetic nanoparticles that can be transported in subsurface reservoirs at high salinities and temperatures are expected to have a major impact on enhanced oil recovery, carbon dioxide sequestration, and electromagnetic imaging. Herein we report a rare example of steric stabilization of iron oxide (IO) nanoparticles (NPs) grafted with poly(2-acrylamido-2-methylpropanesulfonate-co-acrylic acid) (poly(AMPS-co-AA)) that not only display colloidal stability in standard American Petroleum Institute (API) brine (8% NaCl + 2% CaCl2 by weight) at 90 °C for 1 month but also resist undesirable adsorption on silica surfaces (0.4% monolayer NPs). Because the AMPS groups interacted weakly with Ca(2+), they were sufficiently well solvated to provide steric stabilization. The PAA groups, in contrast, enabled covalent grafting of the poly(AMPS-co-AA) chains to amine-functionalized IO NPs via formation of amide bonds and prevented polymer desorption even after a 40,000-fold dilution. The aforementioned methodology may be readily adapted to stabilize a variety of other functional inorganic and organic NPs at high salinities and temperatures.

The transport of engineered nanoparticles in porous media is of interest in numerous applications including electromagnetic imaging of subsurface reservoirs, enhanced oil recovery, and CO 2 sequestration. A series of poly(2-acrylamido-2methyl-1-propanesulfonic acid-co-acrylic acid) (poly(AMPS-co-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 %CaCl 2 , anhydrous basis). A combinatorial approach, which employed grafting poly(AMPS-co-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 (Ca 2+ ) 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.

A series of sulfonated random and block copolymers were adsorbed on the surface of ~100 nm iron oxide (IO) nanoparticles (NPs) to provide colloidal stability in extremely concentrated brine composed of 8% wt NaCl + 2% wt CaCl2 (API brine; 1.4 M NaCl + 0.2 M CaCl2) at 90 °C. A combinatorial materials chemistry approach, which employed Ca(2+)-mediated adsorption of anionic acrylic acid-containing sulfonated polymers to preformed citrate-stabilized IO nanoclusters, enabled the investigation of a large number of polymer coatings. Initially a series of poly(2-methyl-2-acrylamidopropanesulfonate-co-acrylic acid) (poly(AMPS-co-AA)) (1:8 to 1:1 mol:mol), poly(styrenesulfonate-block-acrylic acid) (2.4:1 mol:mol), and poly(styrenesulfonate-alt-maleic acid) (3:1 mol:mol) copolymers were screened for solubility in API brine at 90 °C. The ratio of AMPS to AA groups was varied to balance the requirement of colloid dispersibility at high salinity (provided by AMPS) against the need for anchoring of the polymers to the iron oxide surface (via the AA). Steric stabilization of IO NPs coated with poly(AMPS-co-AA) (1:1 mol:mol) provided colloidal stability in API brine at room temperature and 90 °C for up to 1 month. The particles were characterized before and after coating at ambient and elevated temperatures by a variety of techniques including colloidal stability experiments, dynamic light scattering, zeta potential, and thermogravimetric analysis.

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