Suspension Structures of Deionized Colloidal Silica Spheres As Studied by the Reflection and Transmission Spectroscopy

Abstract

Structural distributions of colloidal silica spheres are studied from the reflection and transmitted-light spectrum measurements. The crystal-like structures are suppressed when the sphere size is small and/or monodispersity index (standard deviation of the diameters divided by the mean diameter) is large; monodisperse (0.05 to 0.13 in monodispersity index) and large spheres (100 to 192 nm in diameter) give sharp peaks in the reflection and transmitted-light spectra, which supports the existence of the crystal-like structure in the suspensions. When the suspensions are diluted, phase transition toward liquid-like distribution occurs. On the other hand, the deionized suspensions of polydisperse (0.20 and 0.21 in monodispersity index) and large spheres (133 to 148 nm in diameter) display the amorphous solid-like distributions, and crystal-like structures are not observed at any concentrations examined. By dilution the structure is transformed to the liquid-like one. The crystal-like and amorphous solid-like distributions are melted out into the liquid-like structures by the addition of a small amount of sodium chloride. The intersphere distances (D) decrease in the course of the melting. The transmitted-light spectra for the suspensions of small spheres (45 nm in diameter) show the single broad peak, which supports liquid-like distribution of the spheres. The D-values of both crystal-like and amorphous solid-like structures agree with the mean distances calculated assuming that the spheres distribute homogeneously throughout the suspension. These results are consistent with the effective hard-sphere model.