Trans and Saturated Alkyl Impurities in Technical-Grade Oleylamine: Limited Miscibility and Impacts on Nanocrystal Growth

Oleylamine (OLAm) is commonly used in nanocrystal synthesis as a 70% purity technical-grade reagent containing large amounts of trans and saturated alkylamine impurities. Large differences in the melting temperatures of these impurities point to the possibility that differences in their physical properties may impact nanocrystal synthesis and/or assembly. Although there is significant literature on the phase transitions and mixing behavior of fatty acids, much less is known about structurally equivalent amines, such as those in OLAm blends. Here, we synthesize isomerically pure OLAm and its trans isomer elaidylamine (ELAm), preparing controlled blends with the saturated analogue octadecylamine (ODAm). Phase diagrams of binary mixtures of OLAm/ELAm, OLAm/ODAm, and ELAm/ODAm reveal limited pairwise miscibility. Differential scanning calorimetry (DSC) of ternary mixtures illustrates distinct peaks for the trans and saturated impurities. We also show that ELAm and ODAm have two competing impacts on gold nanowire (AuNW) synthesis. Both impurities decrease modal (most common) AuNW length when representing as little as 5% of the ligand blend; ODAm decreases wire length more than ELAm. However, ELAm addition is also associated with increased populations of wire lengths similar to 2X and 3X the modal wire length; conversely, blends with ODAm essentially eliminate these longer wires. We also find that ELAm and ODAm increase AuNW stability when present as part of the ligand blend.

Automated summary

Oleylamine (OLAm) is commonly used in nanocrystal synthesis, but its impurities can affect the synthesis process and the length of gold nanowires (AuNW). By studying the isomeric pure OLAm and its blends with saturated analogues, we found that these impurities have significant impacts on nanocrystal synthesis and AuNW length.