Fractionation of Charged Colloidal Particles by Charge-induced Phase Separation

Abstract

We report a fractionation of colloidal silica particles by using charge-induced phase separation in the binary mixtures of high and low charge colloidal silica particles. Charged colloidal silica dispersions are stabilized by a strong electrostatic interparticle interaction, and undergo liquid to solid (crystal) phase transition with increasing the magnitude of the interaction. We examined the binary mixtures of two kinds of charged silica having nearly the same diameters (110 and 120 nm) and different surface charge densities (0.06 and 0.23 μC/cm²). In one-component dispersion, the less charged sample took liquid state, while the high charge sample formed the colloidal crystal. The binary mixtures were prepared at various mixing ratios (R, a ratio of the low-charge particle concentration to the total concentration), with keeping the total particle concentration constant at 3vol%. Since the lattice spacings of the colloidal crystals were of the order of visible light wavelength (0.3micron), the formation of the crystal phase could be identified by observing iridescent color due to Bragg diffraction of visible light. At R<0.05, the samples were macroscopically uniform and in the crystal state. At 0.05<R<0.5, the solid-liquid phase-separated structure with a well-defined phase boundary was observed. At 0.5 < R, the binary mixture took a liquid one-phase. The particle size distribution in the crystal region under the phase separated condition was determined from TEM images and by applying dynamic light scattering method. The high-charge sample had a bimodal particle size distribution, while the low-charge one was unimodal. The particles size distribution in the crystal phase was much narrower than that for the original one, and nearly unimodal. The present finding suggests a spontaneous fractionation mechanism accompanying with the phase separation process, which resembles to molecular-weight fractionation in polydisperse linear polymer and re-crystallization purification of atomic or molecular materials.