Elastic modulus of crystal-like structures of deionized colloidal spheres in aqueous organic solvent mixtures

Abstract

Elastic moduli (G) of deionized polystyrene spheres (diameter = 109 ± 3 nm; 2.14 and 2.32 vol %) have been determined by the reflection spectrum measurements in aqueous organic solvent mixtures and in sedimentation equilibrium. The organic solvents used were methanol, ethanol, propanol, 1,4-dioxane and ethylene glycol. The sedimentation equilibrium was attained ca. 20 days after the suspension is placed in the cell. The peak wavelengths of the reflection spectra and the intersphere distances (D) increased with the suspension height, and the magnitudes of the increase were large in the presence of the organic solvents. The G values first increased and then began to decrease as the fraction of the organic solvents in the mixtures increased. The way in which G is influenced by the organic solvent is explained reasonably by the change in the effectively dissociated charge numbers of the spheres in aqueous organic solvent mixtures, and suggests that electrostatic intersphere repulsion and the very long Debye-screening length around the spheres are both essential for the appearance of crystal-like structures.