CENTRIFUGAL COMPRESSION OF CRYSTAL-LIKE STRUCTURES OF DEIONIZED COLLOIDAL SPHERES

A new and convenient technique for the study of the elastic properties of the crystal-like structures of deionized colloidal spheres (85-173 nm in diameter) is developed by reflection spectrum measurements at centrifugal equilibrium. Beautiful iridescent color bands appear in the observation cell, and centrifugal compression occurs for the lattice spacings (D) of the crystal-like structures a way from center. The D values are explained by the effective hard-sphere model; a colloidal sphere is coated with an electrical double layer. The elastic moduli (G) for the ordered lattices are estimated to be between 50 and 800 Pa from the D values, as a function of the distance from the center of the cell. G increases as concentration increases and/or the size of the spheres decreases. The g factor, which is a parameter for the thermal fluctuation of a sphere in the colloidal crystals, is estimated to be between 0.01 and 0.1. The way D and G are influenced by sphere concentration shows that electrostatic intersphere repulsion and the elongated Debye screening length around the spheres are both essential to explain the appearance of the crystal-like structures. © 1990, American Chemical Society. All rights reserved.