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Melting temperature ( T) of colloidal crystals of monodispersed polystyrene and silica spheres has been measured for the completely deionized suspensions as a function of sphere concentration. More than 3 weeks are needed before achievement of the completely deionized state. Tincreases substantially as the deionization process of the suspension proceeds. The most reliable values of Tobserved for the completely deionized suspensions are successfully analyzed again with the theory of Williams et al. The new Tvalues are compared also with the theory of Robbins et al., which treats the repulsive Yukawa potential between colloidal spheres. [ABSTRACT FROM AUTHOR]

Close-up color photographs are taken for crystallites (single crystals surrounded by the grain boundaries) in the colloidal crystals of monodisperse silica spheres (diameter: 110 nm±4.5 nm (standard deviation)). Very large crystallites (3∼4 mm) are observed with the naked eye (for the first time) for the completely deionized and diluted suspensions. Deionization is carefully made with the mixed beds of ion-exchange resins more than 2 weeks old. Size of the crystallites increases sharply as the concentration of spheres decreases, and becomes small at the concentrations slightly higher than the critical concentration of melting toward liquid-like structure. Shape of the crystallites, i.e., mixture of triangle, cubic, pentagonal, hexagonal, cone-like, etc., is recognized in the photographs. [ABSTRACT FROM AUTHOR]

Unsymmetrical crystal-like ordering of anisotropic-shaped particles are observed directly by a metallurgical microscope in sedimentation equilibrium and in deionized suspensions. The anisotropic colloids are polydispersed gold sols, rectangular tungstic acid, ellipsoidal poly(tetrafluoroethylene)(PTFE), aggregated (dimer, trimer, ...) polystyrene spheres, and bowl-shaped polymers. The distribution of the particles is analyzed by the two-dimensional distance distribution function. The interparticle distance continues to decrease as the initial concentration of the particles increases. Young's elastic modulus for the crystal-like structures is estimated to be 3.1 Pa for PTFE particles. The transformation from the unsymmetrical ordering to the symmetrical (hexagonal) array takes place as the concentration of diffusible ions decreases, which shows that the electrostatic interparticle repulsion and the elongated Debye-screening length around the particles play a key role. Furthermore, a bowl-shaped particle is observed to include a sphere in its cavity. This inclusional association is influenced greatly by the delicate changes in the effective sizes of the host and guest molecules containing the electrical double layers. These results show that electrostatic interparticle repulsion and the elongated Debye-screening length around the particles are essential for the ordering. [ABSTRACT FROM AUTHOR]

Compressed crystal-like structures of very large polystyrene spheres (specific gravity=1.05, diameter=2-66 μm) are observed directly by a metallurgical microscope in sedimentation equilibrium and in deionized aquueous and 20 vol % methanol aqueous suspensions. Spheres of 2-42 μm in diameter float above the cover glass apart from each other. The center-to-center intersphere distance decreases as the initial concentration of spheres increases, from which Young's elastic moduli ( G) for the crystal-like structures are estimated to be between 1 to 10 dyn/cm, irrespective of the spheres size. G's in aqueous suspension are smaller than those in methanol aqueous mixtures. The crystal-like structures are compressed substantially by the gravitational field. These results are consistent with the significant role of the electrical double layers under the influence of purely electrostatic repulsive forces in the effective hard-sphere model. [ABSTRACT FROM AUTHOR]

The ordered structures are observed directly by the metallurgical microscope for the heavy (specific gravity=1.50) and monodisperse polystyrene type spheres in sedimentation equilibrium and in the presence of neutral polymers, polyvinylalcohol (PVA), polyethyleneglycol (PEG), polyvinyl-pyrrolidone (PVP), hydroxypropylcellulose (HPC), and polyacrylamide (PAA). The intersphere distances in the ordered lattices decrease by the addition of neutral polymer in the order, PVA < PEG < PVP < HPC < PAA. These effects of neutral polymers are reasonably well-explained by the intersphere repulsion from the electrical double layer interaction, which is influenced by the polymer adsorption on the latex surface by the hydrophobic and/or dipole-dipole interactions. [ABSTRACT FROM AUTHOR]

The ordered structures are observed by a metallurgical microscope for heavy gold particles (specific gravity=19.3) in sedimentation equilibrium. The hexagonal order is observed not only in the deionized state but also in the presence of NaCl, and the centre-to-centre distance between particles, 2 D, decreases with salt concentration. 2 D decreases with increasing initial concentrations of gold partilces, and the lattice structures are compressed under a gravitational field. The Young's modulus of the lattice is estimated as 1.3 dyn · cm. The results confirm that the electrostatic interparticle repulsion and/ or thick electrical double layers around the particles are essential for the ordering of the colloids. [ABSTRACT FROM AUTHOR]