Separation of different-sized silica nanoparticles using asymmetric flow field-flow fractionation by control of the Debye length of the particles with the addition of electrolyte molecules

Abstract

Asymmetric flow field-flow fractionation (AF4) is widely used in nanotechnology to fractionate objective size from samples with wide size distributions. Essentially, the size separation of nanoparticles by AF4 is based on the diffusivity/size of the objects; however; we found unexpected results when using AF4 for the separation of different-sized silica nanoparticles. Using pure water as the carrier liquid in the AF4 assessment, silica nanoparticles of 50 and 100nm were eluted out at almost the same retention time because of the cooperative diffusion derived by larger amounts of particles. After reducing the total concentration of silica particles, since the interaction between silica particles decreased with decreasing number of particles, better separation of the two different-sized silica nanoparticles was obtained. Furthermore, in order to reduce the electrostatic interaction between silica particles, electrolyte molecules (sodium dodecyl sulfate: SDS) were added to the carrier liquid, resulted that the excellent separation of 50 and 100nm sized silica particles was achieved using 0.005mg/mL of SDS aqueous solution as the carrier liquid. However, it was surprisingly observed that the zeta-potential of particles and membrane were not changed at all after addition of SDS into aqueous carrier. A theoretical estimation using the zeta-potential assessment and DLVO (Derjaguin-Landau-Verwey-Overbeek) theory indicated the Debye length of silica particles is the most important factor to induce appropriate separation of silica particles using AF4. Namely, the cooperative diffusion of different sizes of silica nanoparticles were suppressed by the change of the effective distance of the electrostatic interaction between particles (Debye length of particles). Although there are many studies focused on membrane-particle interactions or loading sample concentration effect in AF4 separation supported by the obvious change of zeta potential of particles by adding electrolyte, this study clearly demonstrates that not only diffusivity/size and zeta potentials of particle/membrane, but also Debye length of particles as well as that of the membrane significantly contribute to determining the appropriate separation conditions for AF4 assessment of nanomaterials.