Arteleo Chemistry Journal

Structural Fragility of Colloidal Crystals

It is now clear that, at low salt concentrations, colloidal particles form ordered (crystalline) structures though in liquid media. Noteworthy is the difference from real atomic and molecular crystals. As stated in the Introductory Remarks, the lattice constant of colloidal crystals depends on the particle concen-

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Fig. 4.20. Time evolution of radial distribution function determined by micrographs. Time after the onset of crystallization:the same as in Fig. 4.18. Taken from [44] with the permission of the Royal Society of Chemistry

Fig. 4.20. Time evolution of radial distribution function determined by micrographs. Time after the onset of crystallization:the same as in Fig. 4.18. Taken from [44] with the permission of the Royal Society of Chemistry

f (h)

Fig. 4.21. Time evolution of the number of nearest neighbor particles during crystallization. The experimental condition is the same as in Fig. 4.18 except that the volume fraction; 0 = 5 x 10~3 (□), 10~2(A), 2x10~2 (o). The division into four stages refers to 0 = 10~2. Taken from [44] with the permission of the Royal Society of Chemistry

Fig. 4.22. Time change of the fractions of the particles forming the clusters of three (o), five (A), and 10-20 (□) and more than 21 particles (•). The experimental condition is the same as that in Fig. 4.18. Taken from [44] with the permission of the Royal Society of Chemistry t (h)

Fig. 4.22. Time change of the fractions of the particles forming the clusters of three (o), five (A), and 10-20 (□) and more than 21 particles (•). The experimental condition is the same as that in Fig. 4.18. Taken from [44] with the permission of the Royal Society of Chemistry tration, salt concentration, charge density, the dielectric constant of medium, and so on. These dependences are not observed for atomic/molecular crystals. The difference comes from the interparticle interaction being essentially different from that in the atomic/molecular systems. Another characteristic of colloidal crystals is their fragility. When dispersion vessels are shaken violently, the colloidal crystals inside are easily destroyed and particles show Brownian motion (shear-melting). When strong light is irradiated on colloidal crystals for a prolonged period, the colloidal crystals are broken. When these external factors are removed, colloidal crystals are re-formed sooner or later. These facts indicate that the minimum of the interparticle potential is very shallow. The fact that the lattice constant varies with the experimental conditions indicates that the attractive component responsible for the potential minimum is sensitive toward the conditions. We have been proposing the existence of a counterion-mediated attraction between particles, and we believe that its physical basis is given, at least qualitatively, by the theory of Sogami (Chap. 6).

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