Stability of Superparamagnetic Iron Oxide Nanoparticles At Different pH Values: Molecular Theory and Experiment

The development of nanosensors for enhanced reservoir characterization in conventional oil reservoirs must take into consideration the small pore size of the reservoir rock (0.03 um to 10 um) and the harsh environment (75 to 125°C and 1,000 to 6,000 psi).  This study aims to develop stable superparamagnetic iron oxide (Fe₃O₄) nanoparticles which are stable and easily detectable under these conditions. We synthesized citric acid coated iron oxide nanoparticles and the surface of the nanoparticles was functionalized with different molecular weight of poly(eth ylene glycol) (PEG).  The effect of pH on stability of the iron oxide nanoparticles was studied in pH 5, 7, 9, and 11 solutions at low ionic strength (10 mM) because basic conditions around pH 12 are required for hydraulic fraction, although the pH range in the reservoir is from pH 6.5 to 8.5. The effective diameter of the nanoparticles measured by dynamic light scattering (DLS) increased by 100% at pH 5, but did not increase at pH 7 and 9 after 30 days. Thermal gravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy indicated that the citric acid coating desorbed from the surface at pH 11, which may have caused aggregation. The relaxivity (1/T₂) of the nanoparticles with nuclear magnetic resonance (NMR) at 2 MHz agreed well with the stability results from DLS, indicating that aggregation behavior of the nanoparticles can be easily detected by NMR quantitatively. The interaction between two nanoparticles was predicted by modeling the effects of pH, salt concentration, and PEG molecular weight (MW) using a molecular theory. The surface potential of the individual citric acid coated iron oxide nanoparticle was also calculated by the theory. The predicted value for the critical surface coverage required to produce a steric barrier of 5 kT to prevent aggregation for PEG 2K is 0.072 nm^-2, which is less than experimental value, 0.143 nm^-2, implying the nanoparticles are stable in neutral pH. This study shows that the inexpensive and stable nanoparticles as enhanced contrast agents for NMR or MRI could further elucidate reservoir characterization.