Rate-Controlled Washing of Surface-Modified Nanoparticles Using Rationally Designed Supercritical CO₂ Media

In practical applications of surface-modified nanoparticles (NPs), the washing stage has a number of challenges, such as insufficient washing, long treatment time, and various waste liquors. Cosolvent-enhanced supercritical CO₂ (scCO₂) is an appealing solvent system for complete, rapid, and eco-friendly washing owing to its high diffusivity and recyclability. In this paper, we report a rapid washing guideline for surface-modified NPs using ethanol-enhanced scCO₂. Kinetic analysis was performed on the washing behavior of oleic acid-modified NPs mixed with various modifiers (C10 to C18 fatty acids) at 40 °C and 20.0 MPa while designing scCO₂ media based on rationally estimated modifier solubilities. Notably, the scCO₂ medium showed superior washing rates to that of ethanol for various modifiers with a wide range of solubilities in scCO₂. The washing rate was dependent on solubility and could be organized into two regions, with a threshold value of 0.016 mol kg^-1: solubility/diffusivity-controlled and diffusivity-controlled washing. These findings provide valuable guidelines for designing cosolvent-enhanced scCO₂ media for the rapid washing of surface-modified NPs.