Influence of Solvent Steric Effects On CO2 Induced Nanoparticle Precipitation

Nanoparticles have unique size-dependent chemical and physical properties that need to be harnessed for the proper application of these materials. However, the current methods used to produce monodisperse nanoparticles have several operational drawbacks such as the use of expensive equipment and large quantities of solvents or having low throughputs. A technique to size-selectively precipitate and fractionate organic phase gold and silver nanoparticle dispersions has been developed which utilizes the pressure tunable physico-chemical 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. These changes in solvent strength affect the subtle balance between the osmotic repulsive forces (due to the solvation of the stabilizing nanoparticle ligand tails) and the van der Waals forces of attraction between differently sized nanoparticles necessary to maintain a stable dispersion. Through modest changes in CO₂ pressure, increasingly smaller sized nanoparticles can be controllably precipitated from the dispersion.

The aim of this particular study is to investigate how the steric nature of a solvent affects the ligand-solvent interaction and thus the size dependent precipitation of nanoparticles using this GXL technique. Nanoparticles with a broad size distribution were synthesized by the arrested precipitation method using HAuCl₄·3H₂O as the precursor, TOAB as the phase transfer catalyst, NaBH₄ as the reducing agent and dodecanethiol as the stabilizing ligand. In order to investigate the steric effects of the solvent, these nanoparticles were dispersed in solvents with varying degrees of branching and then precipitated using the GXL technique.  The pressures necessary to induce precipitation and the pressure range over which the nanoparticles precipitate in each of these solvents provide new physicochemical insights that help further our fundamental understanding of the phenomenon.

n-Hexane was chosen as the control solvent and the effect of several of its isomers on the precipitation and fractionation process will be discussed. The precipitation characteristics of dodecanethiol stabilized gold nanoparticles were quantified by measuring the intensity of the surface plasmon resonance band for the nanoparticle dispersion using UV-vis spectroscopy at various levels of CO₂ pressurization. The characterization of these nanoparticles was performed using transmission electron microscopy (TEM) to analyze their size distribution and hence judge the efficacy of the size-selective fractionation.