(393v) Tunable Steric Stabilization Effects On Iron Oxide Nanoparticle Dispersability in Gas Expanded Liquid Systems

Magnetic nanoparticles have properties which are extremely important in applications like catalysis, biomedicine, environmental remediation and data storage. The most widely researched magnetic nanoparticles are iron-oxide and cobalt nanoparticles, which are useful because of their relatively high magnetic susceptibility and easy synthesis procedures. Since the magnetic properties of materials are heavily size dependent, the sizes of the nanoparticles are of critical importance to the application for which they are being used. While synthesis procedures to create monodisperse nanoparticles are available in the literature, most of these methods require expensive reagents, high temperatures and are useful only in lab scale preparations. These simple synthesis procedures usually generate particles having a large size distribution and hence nanoparticles obtained from these procedures require some post-processing before use in demanding applications. While magnetic separation might initially seem straightforward, it is not recommended at these small length scales due to particles of comparable size having very similar magnetic susceptibility values which make it difficult to get efficient separation without using extremely accurate and expensive instrumentation. Our lab has developed a technique to size-selectively fractionate nanoparticles from organic phase dispersions which utilize the pressure tunable properties of CO₂ Gas-eXpanded Liquids (GXLs). This size-selective fractionation technique is based on the controlled reduction of the solvent strength through increases in the concentration of CO₂(a known nonsolvent for aliphatic stabilizing ligands) via pressurization. The advantages of this process are the facts that it is reversible and it allows facile recycle of solvent and antisolvent. These subtle changes in solvent strength affect the balance between the osmotic repulsive forces and the Van der Waals forces of attraction between differently sized nanoparticles necessary to maintain a stable dispersion, thereby allowing fractionation of the nanoparticles into multiple narrowly sized populations.

The aim of this particular study is to investigate how the steric nature of a solvent or ligand affects the ligand-solvent interaction and the size dependent precipitation of iron oxide nanoparticles. Iron oxide nanoparticles were synthesized by the co-precipitation method using Fe²⁺ and Fe³⁺ salts and ammonium hydroxide. These particles were then coated with fatty acids which acted as the lipophilic ligands and were dispersed in various solvents of our choice. Characterizations of these nanoparticles involve the use of transmission electron microscopy (TEM) and dynamic light scattering (DLS) to analyze their size distribution and hence judge the efficacy of the size-selective fractionation. The pressure of incipient precipitation and the pressure range over which the particles continue to precipitate is investigated. The effectiveness of the different ligands and solvents on the monodispersity of particles obtained after fractionation is also scrutinized with the belief that understanding the ligand-solvent interactions will help further our fundamental understanding of the phenomenon of nanoparticle precipitation from GXLs. The applicability of these relatively monodisperse particles in areas like catalysis and emulsion stabilization will also be discussed.