The fate and behavior of environmental nanomaterials can be strongly affected by organic molecules, including surfactants. Quaternary ammonium compounds (QACs) are cationic surfactants of increasing concern in the environment due to their massive everyday use and continuous release into water and soil. To examine QAC-NP interactions, ~38 nm hematite (α-Fe2O3) nanoparticle (NP) suspensions were mixed end-over-end with cetyltrimethylammonium chloride (CTAC), a commonly-used QAC, for 22 hours (23 °C) at environmentally-relevant concentrations of 0, 10, 100, and 1000 μg/L. Iron concentrations were measured to determine how many NPs remained in suspensions after mixing, along with characterization of the hydrodynamic diameter (Z-average) and zeta potential. Without CTAC, the NPs were destabilized by contact with air-water interfaces (AWIs) with [Fe] decreasing from 17.65 ± 3.26 mg/L to an [Fe]final of 6.33 ± 2.90 mg/L. However, at 100 and 1000 μg CTAC/L, the NPs were stabilized and NP loss reduced with [Fe]final values of 9.50 ± 1.20 mg/L and 11.69 ± 1.25 mg/L, respectively. Measurements of surface tension and contact angles on CTAC-exposed glass were performed to explore possible stabilization mechanisms. Results suggest that in systems with air-water interfaces such as unsaturated pores or turbulent waters, elevated QAC content might alter the environmental fate and transport of iron oxide and other positively-charged NPs.

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