Microscopic observation of ordered colloids in sedimentation equilibrium and important role of Debye-screening length. I. Heavy and monodispersed polystyrene type spheres (specific gravity=1.50) in aqueous and aqueous methanol suspensions

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Abstract
A metallurgical microscope is used to directly observe the ordered structures of heavy and monodispersed polystyrene type spheres (specific gravity= 1.50) in sedimentation equilibrium. The interparticle distances (2Dexp) increase with time after the latex sample is set into the cell, and reach an equilibrium value after seven days. 2Dexp continues to decrease as the amount of spheres fed increases and from these data Young’s modulus of the ordered lattices is estimated to be ∼100 dyn cm−2. The hexagonal ordering is observable not only in the deionized state but also in the presence of sodium hydroxide (10−6–10−3 M), and 2Dexp decreases with increasing alkali concentration. The results show that electrostatic interparticle repulsion and the elongated Debye-screening length around the spheres are essential for the ordering.