Drying strategies for magnetite nanocrystals: Unveiling the effects of air- and freeze-drying on fatty acid coatings

The accurate characterization of magnetic nanoparticles suspended in liquid media, such as those obtained through wet-chemical synthesis methods or from magnetic colloids, often requires the removal of the liquid phase. To obtain a nanoparticle powder, different drying strategies are frequently employed, such as freeze-drying and air-drying. These procedures can impact particle aggregation, surface oxidation, morphology, and magnetic properties; however, systematic comparisons remain limited. This study provides a detailed evaluation of these two drying methods, highlighting their effects on the crystal structure, crystallite size, surface chemistry, and magnetic behavior of nanocrystals. Our findings reveal that both methods yield comparable results for crystallite size, but subtle variations in strain and surface properties, with air-dried samples exhibiting slightly higher strain, likely due to oxidation effects during the drying process. Furthermore, a significant discrepancy in saturation magnetization was noted between liquid and powder samples, underscoring challenges related to mass normalization. This paper addresses a critical gap in our understanding of the role of drying techniques in nanoparticle preparation and lays the groundwork for extending these findings to other nanomaterials. • Often neglected, drying methods may influence surface and magnetic properties of nanoparticles. • Crystallite size appears consistent across freeze- and air-dried samples. • Air-drying is practical for routine use while freeze-drying suits sensitive systems.