Binary colloidal nanoparticles with a large size ratio in analytical ultracentrifugation

Abstract

Sedimenting colloidal particles may feel surprisingly strong buoyancy in a mixture with other particles of a considerably larger size. In this paper we investigated the buoyancy of colloidal particles in a concentrated binary suspension in situ in a centrifugal field. By dispersing two different fluorescence labeled silica nanoparticles with a large size ratio (90 nm and 30 nm, size ratio = 3) in a refractive index matching solvent, we used a multi-wavelength analytical ultracentrifuge to in-situ measure the concentration gradients of both particles. The concentration of 90 nm silica nanoparticles was used to calculate the effective solvent density for the 30 nm silica nanoparticles. Then the Boltzmann equation for a sedimentation equilibrium concentration gradient with locally varying effective solvent density was used to fit the concentration gradient of 30 nm silica nanoparticles, which describes the experimental result well. This finding proves the validity of effective buoyancy in colloidal mixtures and provides a good model to study sedimenting polydisperse colloids.