ALLOY STRUCTURES IN BINARY-MIXTURES OF HIGHLY DEIONIZED COLLOIDS AT SEDIMENTATION EQUILIBRIUM

Alloy structures for binary mixtures of different sizes of monodisperse polystyrene spheres at sedimentation equilibrium and in deionized suspensions are studied by the reflection spectrum method and ultramicroscopic observation. Substitutional solid solution (sss)-like alloy structures form in mixtures of D1C25(85 nm in diameter) and D1C27 (91 nm). When D1C25 and D1B76 (109 nm) are mixed, the structures are eutectic mixtures of sss-type lattices and the MgCu2-type super lattices. The alloy structures of D1C25 and D1B72 (137 nm) spheres are the eutectic mixtures of (1) sss-like lattices, (2) MgCu2-type super lattices, (3) fcc crystals of D1B72 spheres, and (4) crystals of D1C25 spheres. D1C25 and D1B41 (173 nm) suspensions are also eutectic mixtures of (1) NaZn13-type super lattices, (2) D1C25 lattices, and (3) lattices of D1B41 spheres. These dramatic changes in the alloy structures depending on the combination of the spheres are explained reasonably by the changes in the effective size ratio (effective size of small spheres containing the electrical double layer thickness against that of large spheres) and the segregation effect. The intersphere distances in the alloy structures agree with the effective sizes of spheres including the Debye-screening length. The elastic moduli of the alloy structures are between 18 and 108 Pa. These results are consistent with the significant role played by the electrical double layers under the influence of purely electrostatic repulsion in the effective hard-sphere model. © 1990 American Institute of Physics.