Abstract
Coating magnetic nanoparticles (MNPs) with sodium oleate (SO) is known to be an excellent method to create biocompatible, stable colloids with a narrow size distribution. However, the mechanism of oleate adsorption on the MNP surface in aqueous systems, as well as its influence on colloidal stability, is not yet fully understood. In this context, we present here a physico-chemical study to provide a deeper understanding of surfactant interaction mechanisms with nanoparticles. We examined the effect of temperature and the SO/MNP ratio (w/w) on the adsorption process in water and observed the existence of a maximum for the adsorbed oleate amount at lower temperatures, whereas at higher temperatures, the isotherm can be adapted to the Langmuir model with constant capacity after saturation. The oleate load on the MNP surface was quantified using reversed-phase high-performance liquid chromatography measurements of samples in solution. The thermogravimetric analyses of the solid residues together with infrared spectroscopy analyses indicate a bilayer-similar structure at the MNP/water interface even for low oleate loads. The oleate interacts with the iron oxide surface through a bidentate coordination of the carboxyl group. Zeta potential measurements demonstrate the high stability of the coated system. The maximal oleate load per unit mass of MNPs reaches approximately 0.35 goleate g −1MNP .
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Acknowledgments
The authors would like to gratefully thank Prof. Dr. T. Nilges (Technical University of Munich) for his support with powder XRD and Stefan Heissler (Karlsruhe Institute of Technology, Institute of Functional Interfaces, Germany) for help with FT-IR measurements. Special gratitude is expressed to Martina Haack (Technical University of Munich, Industrial Biocatalysis Group, Germany) for performing gas chromatographic analysis. Furthermore, we would like to express our appreciation to Jonathan Ritscher for conducting part of the work in the context of student research projects. Additionally, we want to acknowledge the support of the TUM Graduate School, Technical University of Munich. We are particularly grateful for the financial support of this work by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology (grant number 1340/68351/3/11), the Ministry of Education and Research (Grant number 031A173A) and Clariant.
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Roth, HC., Schwaminger, S., Fraga García, P. et al. Oleate coating of iron oxide nanoparticles in aqueous systems: the role of temperature and surfactant concentration. J Nanopart Res 18, 99 (2016). https://doi.org/10.1007/s11051-016-3405-2
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DOI: https://doi.org/10.1007/s11051-016-3405-2