Colloidal crystal formation

X 10 cmr/s. Thus it appears that colloidal crystal formation bears further investigation as a source of extraordinarily slow diffusion coefficients in low salt polyelectrolyte solutions. 

The study of colloidal crystals was initiated as part of research into the determination of phase diagrams for colloids, which itself was perceived as a means to model phase behaviour in molecular systems [22]. Extensive literature is available on the dynamics of colloidal crystal formation, as a function of several parameters, such as the nature of the solvent, surface charge, particle size and concentration. The results described here refer to the formation of colloidal crystals from dispersions of silica-coated gold nanoparticles in ethanol, after silica surface functionalization with 3-(trimethoxysilyl)propyl methacrylate (TPM). Earlier studies by Philipse and Vrij [23] showed that TPM adsorption leads to a reduction in surface charge, so that the particles are stable in organic solvents with low polarity, such as ethanol, toluene or DMF. This means that the particle be-... 

Fig. 2 Schematic representation of various methods for colloidal crystal formation...

Thermophoresis in liquids has found wide applications in microfluidic separation and trapping of macromolecules [1], colloidal crystal formation, and protein functionality characterization. Thermophoresis in liquids depends on numerous parameters such as particle size, particle concentration, particle material, salt ions and concentration, solvent type, viscosity and temperature, and so on. As the dependence of thermophoresis on these parameters is complicated, thermophoresis in liquids cannot be fully described by the existing theories. Therefore, experimental investigation of thermophoresis in liquids plays an important role in thermophoresis studies. 

The porous membrane templates described above do exhibit three-dimensionality, but with limited interconnectedness between the discrete tubelike structures. Porous structures with more integrated pore—solid architectures can be designed using templates assembled from discrete solid objects or su-pramolecular structures. One class of such structures are three-dimensionally ordered macroporous (or 3-DOM) solids, which are a class of inverse opal structures. The design of 3-DOM structures is based on the initial formation of a colloidal crystal composed of monodisperse polymer or silica spheres assembled in a close-packed arrangement. The interconnected void spaces of the template, 26 vol % for a face-centered-cubic array, are subsequently infiltrated with the desired material. 

Any study of colloidal crystals requires the preparation of monodisperse colloidal particles that are uniform in size, shape, composition, and surface properties. Monodisperse spherical colloids of various sizes, composition, and surface properties have been prepared via numerous synthetic strategies [67]. However, the direct preparation of crystal phases from spherical particles usually leads to a rather limited set of close-packed structures (hexagonal close packed, face-centered cubic, or body-centered cubic structures). Relatively few studies exist on the preparation of monodisperse nonspherical colloids. In general, direct synthetic methods are restricted to particles with simple shapes such as rods, spheroids, or plates [68]. An alternative route for the preparation of uniform particles with a more complex structure might consist of the formation of discrete uniform aggregates of self-organized spherical particles. The use of colloidal clusters with a given number of particles, with controlled shape and dimension, could lead to colloidal crystals with unusual symmetries [69]. 

Owing to the simphcity and versatility of surface-initiated ATRP, the above-mentioned AuNP work may be extended to other particles for their two- or three-dimensionally ordered assemblies with a wide controllabiUty of lattice parameters. In fact, a dispersion of monodisperse SiPs coated with high-density PMMA brushes showed an iridescent color, in organic solvents (e.g., toluene), suggesting the formation of a colloidal crystal [108]. To clarify this phenomenon, the direct observation of the concentrated dispersion of a rhodamine-labeled SiP coated with a high-density polymer brush was carried out by confocal laser scanning microscopy. As shown in Fig. 23, the experiment revealed that the hybrid particles formed a wide range of three-dimensional array with a periodic structure. This will open up a new route to the fabrication of colloidal crystals. 

A general three-step procedure for the formation of macroporous materials by colloidal crystal templating is illustrated in Figure 6. In the first step, monodispersed colloidal spheres assemble into ordered 3D or sometimes 2D arrays to serve as templates. Secondly, the voids of colloidal crystals are filled by precursors that subsequently solidify to form composites. Finally, the original spheres are removed, creating a solid framework with interconnected voids, which faithfully replicate the template arrays. 

The above features of a sheared colloidal crystal appear to be similar in both BCC and FCC structures. However, there are differences in details, and perhaps even within a given symmetry the flow behavior might vary with particle concentration or charge density. For example, Chen et al. (1994) have shown that between the strained crystal and sliding-layer microstructures there can be a polycrystalline structure, the formation of which produces a discontinuous drop in shear stress (see Fig. 6-33). Ackerson and coworkers gave a detailed description of the fascinating shear-induced microstructures of these systems (Ackerson and Clark 1984 Ackerson et al. 1986 Chen et al. 1992, 1994). 

For very low particle concentrations, the shearing of colloidal crystals produces only a weak stress above that of the solvent. However, in more concentrated suspensions, the shear viscosity and normal stress differences have been found to have quite unusual behavior, which can, in part, be explained by (a) the formation of sliding layers and (b)... 

Interestingly, the powder calcined at 600°C showed the formation self-assem-bled inverse opals of LiNb03 (Figure 6.7b-d). Usually the reported methods for the preparation of LiNb03 inverse opals use colloidal crystals as templates, but... 

The extent of mixed-ci-ystal contamination is governed by the law of mass action and increases as the ratio of contaminant to analyte concentration increases. Mixed-crystal fomiation is a particularly troublesome type of coprecipitation because little can be done about it when certain combinations of ions are present in a sample matrix. This problem is encountered with both colloidal suspensions and crystalline precipitates. When mixed-crystal formation occurs, the interfering ion may have to be separated before the final precipitation step. Alternatively, a different precipitating reagent that does not give mixed crystals with the ions in question may be used. 

M Mixed-crystal formation may occur in both colloidal and crystalline precipitates, whereas occlusion and mechanical entrapment are confined to crj s-talline precipitates. 

Scheme 2. Formation of colloidal crystals and their use as templates. A colloidal dispersion containing monodisperse particles undergoes controlled filtration, centrifugation, dip coating, sedimentation, or physical confinement, which results in ordered packing of the particles with void spaces between them. By infiltrating these spaces with precm-sor solution or preformed nemoparticles the hybrid material is formed. Removal of the polymer template (using solvent (toluene) orheatingtechniques) gives an inverse replica with air-fiUed, interconnected voids of monodisperse size, which is dependent on the initial particle size...

Preformed metal oxide nanoparticles have also been used in the formation of inverse colloidal crystals [24-26]. Slurries of titania nanocrystals and the PS spheres are dropped onto a glass substrate and dried slowly (over 24 h) [24]. After pressing (cold isostatic press) the film is slowly heated to 520 °C to remove the PS and produce a titania matrix of >10 pmxlO mmx2-3 mm with ordered... 

Zhong et al. [64] have shown that the cell designed for fabricating colloidal crystals and their metal oxide inverse can also be used for the formation of... 

Asher and co-workers have recently reported [25] the formation of colloidal crystals made from silica particles doped with a random distribution of Ag nanoparticles. These authors found that the plasmon band of the Ag nanoparticles varied during the transition from a disordered to an ordered state, which suggests the existence of some sort of coupling between the two optical responses of the system. 

In summary, the formation of colloid crystal structure and the corresponding positive structural component of the disjoining pressure in-... 

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