Long-term colloidal stability of polymer-grafted silica nanoparticles in concentrated brine at elevated temperatures
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Random copolymer poly(AA-co-AMPS) chains were grafted covalently onto amine coated individual ~20 nm silica nanoparticles (NPs) without forming aggregates. The hydrodynamic diameter of the grafted silica NPs remained constant at ~25 nm for over six weeks up to 90°C in 100% API brine indicating electrosteric stabilization. The grafting efficiency was enhanced by catalysis with EDC and N-hydroxysuccinimide to increase the half-life of the intermediates on the AA groups to form negatively charged intermediates on the AA groups to attract the protonated amines on the NPs. The higher reaction rate for the activated AA groups, relative to the collision rate of the NPs favored multiple grafting of a bound polymer to the same nanoparticle versus bridging of the polymer chain to a second nanoparticle. After extreme dilution, the particles were still stable in brine indicating the chains were permanently grafted. The random copolymer poly(AA-co-AMPS) was found to be able to stay swelled and elongated in a wide range of pH (pH 4 to 12) and provide colloidal stability for the silica NPs due to the high solvation of the AMPS groups.