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Superparamagneticiron oxide (SPIO) nanoparticles have the potentialto be used in the characterization of porous rock formations in oilfields as a contrast agent for NMR logging because they are smallenough to traverse through nanopores and enhance contrast by shorteningNMR T2relaxation time. However, successfuldevelopment and application require detailed knowledge of particlestability and mobility in reservoir rocks. Because nanoparticle adsorptionto sand (SiO2) and rock (often CaCO3) affectstheir mobility, we investigated the thermodynamic equilibrium adsorptionbehavior of citric acid-coated SPIO nanoparticles (CA SPIO NPs) andpoly(ethylene glycol)-grafted SPIO nanoparticles (PEG SPIO NPs) onSiO2(silica) and CaCO3(calcium carbonate).Adsorption behavior was determined at various pH and salt conditionsvia chemical analysis and NMR, and the results were compared withmolecular theory predictions. Most of the NPs were recovered fromsilica, whereas far fewer NPs were recovered from calcium carbonatebecause of differences in the mineral surface properties. NP adsorptionincreased with increasing salt concentration: this trend was qualitativelyexplained by molecular theory, as was the role of the PEG graftingin preventing NPs adsorption. Quantitative disagreement between thetheoretical predictions and the data was due to NP aggregation, especiallyat high salt concentration and in the presence of calcium carbonate.Upon aggregation, NP concentrations as determined by NMR T2were initially overestimated and subsequently correctedusing the relaxation rate 1/T2, whichis a function of aggregate size and fractal dimension of the aggregate.Our experimental validation of the theoretical predictions of NP adsorptionto minerals in the absence of aggregation at various pH and salt conditionsdemonstrates that molecular theory can be used to determine interactionsbetween NPs and relevant reservoir surfaces. Importantly, this integratedexperimental and theoretical approach can be used to gain insightinto NP mobility in the reservoir. [ABSTRACT FROM AUTHOR]