Studies of Ordered Monodisperse Latexes. IV. Mechanism of Ordering

The order-disorder phase transition of aqueous suspensions of charged polystyrene spheres was studied both experimentally and theoretically. Phase diagrams of four kinds of samples with different surface charge numbers were determined from the observation of disappearance of the characteristic iridescence in the ordered state. The ordered phase appeared in the large volume fraction and low salt concentration regions. The ordered region became larger according to the surface charge number increase. On the basis of Lindemann's law, phase diagrams were also obtained theoretically. The theory reproduced quantitatively the experimental phase diagrams of all the samples used. The applicability of the Debye-Hückel approximation was discussed and the evaluation of the static rigidity reported in a previous paper [Jpn. J. Appl. Phys.: 19 (1980) 439] was improved.