The Effect of Size on Nanoparticle Separation and Recovery Using Tunable Solvents

Gold nanoparticles (NPs) are of recent interest for use in the production of specialty chemicals and products in the pharmaceutical and agricultural sectors due to their high catalytic activity. Normally these particles are attached to support structures for the purpose of simple recovery. However, the supported catalysts have lower rates of reaction and less selectivity than those that can be dispersed in solution due to mass transport limitations and a decrease in active sites. The dispersibility of the NPs is facilitated by the interactions between the solvent and the stabilizing ligand attached on the NP surface. A suitable method of recovery is highly desired due to the high cost of materials for dispersed catalysts.  Organic-aqueous tunable solvents (OATS) are a facile method to separate homogeneous catalysts from products. OATS combines a homogeneous mixture of water and a water-soluble organic, coupled with a heterogeneous separation. The organic solvent is selected specifically for its ability to absorb large amounts of CO₂. When CO₂ pressure is applied to the homogeneous mixture, a heterogeneous separation results because of the inability of water to absorb CO₂ relative to the organic solvent inducing a phase separation between the water phase and the CO₂-expanded organic. In this study we utilize OATS to perform selective extractions in tunable solvents (SETS) for dispersed aqueous phase gold nanoparticle catalysts. Gold nanoparticles were synthesized using poly(vinylpyrrolidone) (PVP) as the stabilizing ligand on the surface as it is polar and water soluble. We evaluated the effect of nanoparticle size on their stability to remain dispersed in the heterogeneous mixture over time. The maximum recovery for small NPs (~3.5 nm) was 68% after 24 hours compared to 99% recovery for NPs of size 9.4 nm. Recovery for the small NPs was likely caused by the highly curved surface resulting in the PVP having a decreased ability to stabilize the NPs. Our hypothesis going forward is that larger NPs (~25nm) will be less stable than the 9.4 nm NPs because of the stronger Van der Waals forces resulting in NP flocculation and precipitation.