Small-Angle Neutron Scattering Investigation of Gold Nanoparticle Clustering and Ligand Structure Under Antisolvent Conditions

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Solution-based nanoparticle synthesis offers many benefits, primarily control over nanoparticle size, shape, and surface chemistry, as appropriate for different applications. Often, excess stabilizing ligand or surfactant is required during synthesis and remains in solution. This can be deleterious to end applications, and thus postsynthesis purification must be employed. Ethanol is a common liquid antisolvent used to purify and size-selectively fractionate hydrophobically modified metallic nanoparticles dispersed in organic solvents, particularly toluene or hexane. We have employed small-angle neutron scattering (SANS) to investigate the nanoparticle ligand response to antisolvent conditions for gold nanoparticles (GNPs) in toluene-d8 and n-hexane-d14 at varying ethanol-d6 antisolvent compositions. These conditions are common in postsynthesis nanoparticle purification, fractionation, and surface deposition. The ligand lengths and ligand solvation for octadecanethiol and dodecanethiol modified GNPs were found to decrease from 16 to 8 Å and 13 to 7 Å, respectively, with increasing ethanol-d6 concentrations, directly impacting their dispersibility in solution. Calculated Flory–Huggins interaction parameters were found to support the trends determined by SANS. This research has led to a greater understanding of ligand structure and solvation during the nanoparticle precipitation process, providing critical results to model the ligand repulsion contributions to the interparticle interaction energy, which governs the nanoparticle behavior in solution. In addition, nanoparticle clustering was observed at dilute concentrations, and a fractal cluster model was used to interpret the SANS data.