Striation Pattern of Impurity Particles in Charged Colloidal Crystals Formed by Stepwise Thermally Induced Crystallization

Abstract

Charged colloids form "crystal" structures, wherein the colloidal particles are regularly arranged as a result of strong electrostatic interparticle interactions. We have reported that silica colloids containing pyridine (Py) form colloidal crystals upon heating. Herein, we report that in Py + silica colloids including small numbers of fluorescent polystyrene (PS) particles, the PS particles are arranged in stripe patterns upon stepwise unidirectional crystallization when the temperature is increased by 1°C every 10 and 15 min. We think that the present stripe patterns are closely analogous to the thermally induced striations found in a number of crystalline materials.Charged colloidal particles self-assemble to form "crystal" structures when the electrostatic interactions among the particles are sufficiently strong. In the crystal states, the colloidal particles are regularly arranged in body-centered or face-centered cubic lattices. 14We have reported that colloidal silica containing pyridine (Py) exhibits thermally induced crystallization. 5,6 Silica particles have weakly acidic silanol groups (SiOH) on their surfaces, which partly dissociate to provide surface charges. On addition of Py, the dissociation of silanol groups is enhanced, resulting in an increase in the particle charge number Z, that is, the electrostatic interaction becomes stronger. Since dissociation of Py increases with temperature (T), the silica + Py colloids crystallize upon heating, under appropriate conditions. Furthermore, we reported that impurity particles added to the silica crystal are excluded from the crystal grains and accumulate in the crystal grain boundaries. 7 In particular, stripe patterns were observed in charged silica + polystyrene (PS) binary colloids after unidirectional crystallization driven by diffusion of Py. 812 This pattern formation was explainable in terms of a combination of (1) development of thin, lamellashaped crystal grains and (2) exclusion of the PS particles from the grains to the crystal grain boundaries.In the present study, we report the formation of another type of stripe pattern in silica + PS binary colloids, which is observed after unidirectional crystallization when the temperature is varied in a stepwise manner. We think that the stripe patterns observed here are closely analogous to the "striation" patterns observed in crystalline materials that results from the short-range composition fluctuations found in nearly all crystals. 13Aqueous dispersions of colloidal silica particles (KE-W10, Japan Catalyst Co., Ltd., Tokyo, Japan) were used after they were first purified by dialysis for four months and then purified using an ion-exchange method. The particle diameter, determined by the dynamic light-scattering method, was 105 nm. An aqueous dispersion of fluorescent PS particles (G300, Thermo Scientific Inc., Waltham, MA, USA, diameter: 333 nm) was purified using an ion-exchange method. In the binary colloids used for the experiments, the volume fractions of silica (º silica ) and PS particles (º...

Exclusion of impurity particles in charged colloidal crystals

“…When the heating temperature was varied in a stepwise manner, the impurities assembled in a striped pattern. 20 This was analogous to the striation patterns observed in most of the crystalline materials. 21 Colloidal silica exhibits unidirectional crystallization also due to diffusion of Py from the Py reservoir through semipermeable membranes.…”

Recrystallization and Zone Melting of Charged Colloids by Thermally Induced Crystallization

“…Refining Due to Recrystallization and Zone Melting. We recently reported that impurity particles added to colloidal crystals were excluded from the crystals during crystal growth 33,34 and grain growth 34 Figure 9a shows micrographs that reveal the variation in the distribution of impurity particles with time. The crystal region was formed within t < 10 min and grew from left to right, as shown in Figure 9a.…”

“…We recently reported that impurity particles added to colloidal crystals were excluded from the crystals during crystal growth , and grain growth processes. Here, we examined the exclusion of impurity particles during recrystallization and zone melting.…”

“…These particles act as impurities during the crystallization. Very recently, we reported that impurity particles added to colloidal silica crystals were spontaneously excluded from the crystallized regions upon crystallization , and during crystal grain growth processes. Nozawa et al recently found that the distribution of impurity particles in the crystal and melt regions of the colloidal crystallization process was well explained in terms of the theoretical framework of crystal growth.…”