Nanoparticle concentration and solvent exchange via organic solvent ultrafiltration

Downstream processing of nanoparticles after synthesis frequently involves purification, concentration and solvent exchange steps. While these are typically done via centrifugation in lab scale, ultrafiltration is a practical and economical alternative in large scale continuous production. Treating suspensions in organic solvents via ultrafiltration requires solvent-stable membranes and in this study we present the performance of cellulose ultrafiltration membranes in the concentration and solvent exchange of hydrophilic and hydrophobized silica nanoparticles using isopropanol, water, dimethyl formamide and ethanol as solvents. Hydrophobized particles formed less permeable cake layers than hydrophilic particles during concentration. All fouling was reversible upon ultrasonication while physical cleaning via stirring could only recover the complete membrane permeance fouled with hydrophilic particles in dimethyl formamide and hydrophobic particles in isopropanol and water. Quality of nanoparticle recovery was assessed by the amount recovered in retentate and cleaning suspensions as well as the particle size after these steps. Highest recovery of hydrophilic particles was obtained in dimethyl formamide and that of hydrophobic particles in isopropanol. No agglomeration of recovered hydrophilic particles was observed, while hydrophobic particles recovered via physical cleaning partly agglomerated in isopropanol and dimethyl formamide. Finally, continuous solvent exchange from isopropanol to ethanol, water and dimethyl formamide was achieved with no performance loss throughout the process.