Microparticles surface area

Sodium carbonate is soluble in ambient water but is insoluble in supercritical water the microparticles are generated as a result of the antisolvent effect of supercritical water. Because the surface area per unit mass for sodium carbonate microparticles is high, only a small amount of sodium carbonate needs to be injected in the reactor. For example, only 0.5wt% sodium carbonate is able to provide a microparticle surface area about 100-fold that of the reactor wall. 

It is clearly visible from Figure 28.24 that initial moduli of nanocomposites are higher than those of microcomposites at any loading, given that the surface activity and surface area between nanoparticles and microparticles are different. 

The pore size and distribution in the porous particles play essential roles in NPS synthesis. For example, only hollow capsules are obtained when MS spheres with only small mesopores (<3 nm) are used as the templates [69]. This suggests that the PE has difficulty infiltrating mesopores in this size range, and is primarily restricted to the surface of the spheres. The density and homogeneity of the pores in the sacrificial particles is also important to prepare intact NPSs. In a separate study, employing CaC03 microparticles with radial channel-like pore structures (surface area 8.8 m2 g 1) as sacrificial templates resulted in PE microcapsules that collapse when dried, which is in stark contrast to the free-standing NPSs described above [64]. 

Different architectures, such as block copolymers, crosslinked microparticles, hyperbranched polymers and dendrimers, have emerged (Fig. 7.11). Crosslinked microparticles ( microgels ) can be described as polymer particles with sizes in the submicrometer range and with particular characteristics, such as permanent shape, surface area, and solubility. The use of dispersion/emulsion aqueous or nonaqueous copolymerizations of formulations containing adequate concentrations of multifunctional monomers is the most practical and controllable way of manufacturing micro-gel-based systems (Funke et al., 1998). The sizes of CMP prepared in this way vary between 50 and 300 nm. Functional groups are either distributed in the whole CMP or are grafted onto the surface (core-shell, CS particles). 

Porous microparticles are the most common stationary phase particles used in modern HPLC. The role of pore size is a critical one, as the pores provide the surface with which the sample interacts. Particles with small pores exhibit a high surface area and therefore have greater retention. Large molecules like proteins, however, may be excluded from the small pores, and for those molecules a packing with a larger pore size is preferable. The difference between porous particles, pellicular particles, and porous microparticles is illustrated in Figure 3.19. Porous particles are seldom used owing to low efficiencies and are not discussed further. 

Combinations of cationic starch and anionic microparticles are useful commercial systems. Shear-sensitive flocculation occurs, allowing microscale reflocculation in the formed paper sheet, which improves dewatering and retention.63,75,76 The microparticles can be colloidal silica, aluminum silicate, poly(silicic acid) or bentonite of specific size and surface area.77 79 Cationic, anionic or polymeric aluminum-containing compounds can be additional components. A three-part coacervate system uses a high molecular weight anionic polyacrylamide, cationic starch and silica.80 Cooking cationic starch in the presence of an anionic silica hydrosol was reported to improve drainage and retention.81... 

A bidisperse structure can be advantageous because the effectiveness factor in the microparticles is often close to unity (their size being three to four orders of magnitude smaller than the usual size of the industrial catalysts). It is of interest to estimate the ratio of the conversion rates with mono- and bidisperse structures, having the same size, when the porous structure of the microparticles is identical to that of a monodisperse pellet. This ratio can easily be found when the micropore effectiveness factor is close to unity, as in the case for many industrial systems. Since the external surface area of the... 

When used mainly as a contacting device, the membrane material leads to considerably improved performance for transfer and/or reaction as compared to a classical packing. This is due to a better management of fluid dynamics and mixing conditions, to increased surface areas and to better controlled driving forces. In fact the membrane represents the assembly of a multitude of microsystems, which act in parallel and can be functionalized. With materials structured at the nanoscale level it would be possible to control phenomena at the molecular level. As compared to dispersed beds of microparticles, porous layers have the main advantage of keeping... 

Figure 6.29 shows some example linear sweep voltammograms assuming different scan rates (Osrrefers to the dimensionless scan rate Osr = F/RT)(yrl/D)). As the experimental time scale decreases, the diffusional behavior changes from near-steady-state to near-planar diffusion. With respect to the different shapes of microparticles, the mass transport-limiting current was found to be fairly consistent that is, a difference of less than 2% for sphere and hemispheres of equal surface area. 

We now turn to the question of texture within the particle, i.e., surface area, pore shape, and size distribution. Particles are formulated by agglomerating microparticles produced during a precipitation phase, as shown in Fig. 1.4. Approximately 100 m in size, these microparticles consist of a complex porous solid. Pores typically range from l.S to 15 nm in radius. Formerly termed micropores, these channels are now called mesopores. The name micropores is reserved for those less than 1.5 nm in radius, usually found in zeolites. 

Particles in the nanometer size range often show burst release phenomena as a result of large surface area and short diffusion distance of the drug [48], Zur Miihlen showed a direct correlation between microparticle size range and the extent of burst release and the release profile for Compritol microparticles loaded with tetracaine [48], That is probably one reason why Domb demands that the particles to be greater than 1 pm for controlled drug delivery [1],... 

The increase in surface area of the pHIPE is important to open up new application areas. One approach is to attach spherical particles to the surface of the monolithic material. This has been, for example achieved by electrostatic interaction between charged latex nanoparticles and a monolithic surface (62). In preliminary experiments we investigated if fimctional silica microparticles could be covalently adsorbed to the p(HIPE-g-GMA) surface so as to produce stable, functional pHIPE with an increased surface area. 

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