Partitioning behavior of silica-coated nanoparticles in aqueous micellar two-phase systems: evidence for an adsorption-driven mechanism from QCM-D and ATR-FTIR measurements.

Quartz crystal microbalance with dissipation (QCM-D), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and total organic carbon detection (TOC) are employed to examine the cause of the differences in the partitioning of silica-coated nanoparticles in an aqueous micellar two-phase system based on nonionic surfactant Eumulgin ES. The particles partition into the micelle-rich phase at pH 3 and into the micelle-poor phase at pH 7. Our results clearly show that the nonionic surfactants are adsorbed to the silica surface at pH 3. Above the critical temperature, a stable surfactant bilayer forms on the silica surface. At pH 7, the surfactants do not adsorb to the particle surface; a surfactant-loaded particle is therefore drawn to the micelle-rich phase but otherwise repelled from it. These results suggest that the partitioning in aqueous micellar two-phase systems is mainly driven by hydrogen bonds formed between the surfactants and the component to be partitioned.