Monodisperse particles structure

Recently it has been reported that even colloidal particle suspensions themselves, without added polymers, can form dissipative structures. Periodic stripes of colloidal particles (monodisperse particles of diameter 30 nm and 100 nm, respectively) and polystyrene particles (monodisperse diameters from 0.5 to 3 pm) can be formed from dilute aqueous suspensions. The stripes are parallel to the receding direction of the edge of the suspension droplet and thus indicate that a fingering instability... 

A significant increase in catalytic activity as compared to the limiting values, shown in Figure 8.1, can be achieved by the use of bidisperse porous structures. Such catalyst pellets are formed by compressing, extruding or in some other manner compacting finely powdered mkroporous material into a pellet. Ideally the micropores are due to the porosity in the individual microparticles of catalyst. The macropores result from voids between the microparticles, after pelletization or extrusion. In such catalysts, most of the catalytic surface is contained in the micropores, since S llre. The bidisperse structure is illustrated in Figure 8.2 compared to monodisperse particle. 

An inkjet printing of colloidal crystals was proposed by Frese et describing inkjet printing processes of monodispersed particles which are able to form two- or three-dimensional photonic crystals on the substrate surface by arranging in a closely packed lattice structure on the surface. The particle size was selected so that it will diffract light in the visible spectral region, i.e., particle size of 200-500 nanometers. In this work drop-on-demand inkjet printing techniques are utilized. 

FIGURE 11.17 Energy per point defect in an ordered structure of monodisperse particles. 

Monodisperse particles may also be produced with a cross-linked structure, and monodisperse porous particles may be obtained (Ugelstad et aL, 1980a) by applying methods known from suspension polymerization. Particles with functional surface groups have been prepared by chemical modification of the surface of cross-linked monodisperse particles of styrene-divinylbenzene or by copolymerization with monomers containing the desired functional groups. 

Various compounds have been prepared by solvothermal reactions metals, metal oxides, chalcogenides, - ° nitrides, - -" phosphides, open-framework structures, - oxometalate clusters, - organic-inorganic hybrid materials, - - and even carbon nanotnbes. - Most of the solvothermal products are nano- or microparticles with well-defined morphologies. The distribution of the particle size of the prodnct is nsnally qnite narrow, and formation of monodispersed particles is freqnently reported. - When the solvent molecules or additives are preferentially adsorbed on (or have a specific interaction with) a certain surface of the products, growth of the surface is prohibited and therefore products with unique morphologies may be formed by the solvothermal reaction. - - Thus nanorods, wires, tnbes, and sheets of various types of products have been obtained solvothermally. 

Rheological properties of ceramic dispersions and green structures are related to the interaction forces between the particles and with the microstructure of the material. Unfortunately, detailed quantitative interpretation in terms of microstructure is only possible for model systems of monodisperse particles of which the interaction energies are precisely known [14]. 

With solid-in-liquid dispersions, such a highly ordered structure - which is close to the maximum packing fraction (q> = 0.74 for hexagonally closed packed array of monodisperse particles) - is referred to as a soHd suspension. In such a system, any particle in the system interacts with many neighbours and the vibrational amplitude is small relative to particle size thus, the properties of the system are essentially time-independent [30-32]. In between the random arrangement of particles in dilute suspensions and the highly ordered structure of solid suspensions, concentrated suspensions may be easily defined. In this case, the particle interactions occur by many body collisions and the translational motion of the particles is restricted. However, this reduced translational motion is less than with solid suspensions - that is, the vibrational motion of the particles is large compared to the particle size. Consequently, a time-dependent system arises in which there will be both spatial and temporal correlation. 

N ew opportunities and future directions in the area of microchannel emulsification are most likely in the areas of scale-up [140,141], encapsulation/polymeriza-tion [123, 125, 158, 164—169], rapid quenching of droplets [135], and the use of emulsions as templates for uniform macroporous particle structure formation [172]. MicroChannel emulsification is also likely to open up new opportunities with systems that are highly shear-sensitive [120, 135, 173]. The ability to scale up the process will spur new markets that require high production rates and the production of monodisperse capsules and polymer particles. Such developments will be useful in drug delivery applications and will contribute to the further quantification of micro-particle properties. Additionally, the use of monodisperse emulsions as particle templates is likely to enhance the utility of highly functional nanoparticles in need of a deployment mechanism [172]. 

Using special synthetic techniques (6), polymeric and silica particles can be prepared with a uniform particle size. Despite occasional unsubstantiated statements to the contrary in the literature, no intrinsic advantages can be found for monodisperse particles. Neither do they form a closest packing structure (which would actually be a detriment since the pressure drop would rise about 6-fold), nor do they yield an increased plate count, nor is there any theoretical reason why they should do such things. 

The anionic colloidal silica used in this study was either monodisperse colloidal silica with a particle size of about 4 nm or so-called structured colloidal silica, consisting of linear aggregates of about 4 nm particles. Structured colloidal silica is, like monodisperse colloidal silica, characterized by its specific surface area and charge density, which decreases with pH but can be maintained high even at pH as low as 3 and 4 by aluminizing the... 

Xia and coworkers demonstrated the assembly of colloids into well-defined clusters by dewetting of aqueous dispersions of monodisperse particles across surfaces patterned with two-dimensional arrays of templates or relief structures [136]. 

FIGURE 10.16. In many colloidal systems, the interaction energy curve will have a small minimum, the secondary minimum, M , that allows the particles to undergo a lose, reversible flocculation. In some systems of relatively large, monodisperse particles, the secondary minimmn may lead to an optical phenomenon called opalescence in which a very regular structure is developed (similar to a crystal structure) that produces beautiful and interesting patterns with incident light. 

Figure 13.3 Electron micrographs of core-shell UHPLC particles on the left, a cross section showing the core-shell structure and, on the right, particle surfaces (note relative smooth, spherical surface of monodisperse particles). The superficially porous structure contributes to the flatter right-hand portion of the Van Deemter plot tor UHPLC, and the particles uniformity allows them to paok efficiently and reduce the A term contribution. (With permission from Phenomenex, Inc., Torrance, CA, www.phenomenex.com.)...

McDonald et al found that the modification of an emulsion polymerization with a water-miscible alcohol and a hydrocarbon nonsolvent for the polymer can influence the morphology and enables the formation of monodisperse particles with a hollow structure or difiuse microvoids [58]. Both kinetic and thermodynamic aspects of the polymerization dictate particle morphology. Complete encapsulation of the hydrocarbon occurs, provided that a low molecular-weight polymer is formed initially in the process. Monodisp>erse hollow particles with diameters ranging from 0.2 to 1 pm were obtainable, and void fractions as high as 50% are feasible. 

De-smeared scattering curves of monodisperse particle systems are well structured (cf. Fig. 2.14) and allow for an accurate determination of size and shape. For polydisperse systems, smooth spectmms are obtained. The resolution of size distributions is rather low (peak distance for bidisperse distribution approx. 25 %,... 

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