Surface Area and Particle Size

The study of fine particles in pharmaceutical applications involves a number of different techniques. Micromeritic investigations involve surface areas, particle sizes and their distributions, the nature of solid surfaces, and particle shapes [4]. Scientists working in this field realize that a number of techniques are necessary to fully investigate a system and that an interdisciplinary approach is essential. This ability to correlate data from different techniques allows a more thorough understanding of the system, process, or problem being investigated. 

Thus, the specific surface area, particle size and particle size distribution may be varied. In this type of silica, primary particles are linked into linear chains and a non-porous structure is produced (figure 1.2). 

Ideally, electrical precipitators generally achieve collection efficiencies of more than 99% for a full range of particle size. The efficiency depends on the ratio of the collector surface area particle size and dielectric properties and the volumetric gas flow rate times the charged particle migration speed induced by the applied electrical field. 

Carbon black [1333-86-4] is virtually pure elemental carbon (diamond and graphite are other forms of nearly pure carbon) in the form of near-spherical colloidal particles that are produced by incomplete combustion or thermal decomposition of gaseous or Uquid hydrocarbons. Its physical appearance is that of a black, finely divided pellet or powder, the latter sometimes small enough to be invisible to the naked eye. Its use in tires, mbber and plastic products, printing inks and coatings is related to the properties of specific surface area, particle size and structure, conductivity and color. 

Table 2,3. Surface area, particle size and oil absorption of some Cabot grades...

S the potential sites for adsorption, determined by the specific surface area (particle size) and crystallinity of the mineral. 

Aerogels prepared by Pekala s base-catalyzed route have been studied extensively for the effect of process parameters such as the concentration of monomers and the catalyst as well as the pH of the solution. Both structure and properties such as density, surface area, particle size, and pore size distribution are influenced by those variables. Low-density RF aerogels ( 0.03 g cm ) prepared by that method exhibit high porosities (>80%), high surface areas (400-900 m g ), and ultrafine pore size (<500 A). 

Powder X-ray diffraction is an excellent analytical tool for identification and physical characterization of crystalline samples. It can be used for chemical identification (by matching unknowns with X-ray patterns of known reference samples), for crystal structure information, and for crystallite size determination. Such information is extremely valuable for pigment characterization. Many pigments exhibit polymorphism and each morph can have dramatically different physical properties (shade, opacity, heat stability, surface area, particle size and shape, melting point). These properties define to a large extent the performance characteristics of the pigment and therefore its applications. 

The properties of the synthetic silicas are related to the BET surface area, particle size, and particle shape, as is the case with carbon black, but also to silanol group density. Table 1.8 summarizes the characteristics of synthetic silicas. In rubber processing applications, the degree of reinforcement obtained with silicas is related, like carbon black, to the external surface area. The internal surface area is not conducive to reinforcement it is believed that the internal surface is inaccessible to large polymer molecules and thus excluded from reinforcement. 

Major properties characterising these fillers are surface area (particle size) and structure (bulkiness). The commercial grades of carbon black and their characteristics are given in Table 9.1 [3]. When discussing mixing one more property must be considered, the mixing ease, which will be part of the discussion that follows. 

Many of the chemical and physical properties of mineral fillers are important in their application in thermoplastics. These include purity, specific gravity, hardness, electrical, thermal and optical properties, surface area, particle shape and size. The determination and importance of many of these has been covered in several reviews [65,66]. Only a brief coverage is given here for the less ambiguous properties such as specific gravity, hardness and standard thermal and optical properties, with most attention being concentrated on properties such as size and shape which have been found to give particular problems in measurement and interpretation. 

In this section, various issues concerning solid particles are presented. The analysis covers the most important particle properties (surface area, particle shape and size distribution, mechanical strength, and density) as well as the behavior of a single particle in suspension (terminal velocity) and of a number of particles in fluidization state. Finally, the diffusion of molecules in a porous particle (diffusion coefficients) is also discussed. 

The physical characteristics that are really important to a catalyst are surface area, particle size distribution, and particle density. These properties have been extensively discussed in Section 3.9. In Table 5.2, the surface area, pore volume, and mean pore radii are presented for some common catalysts. 

Due to the variety in porous structure, particle size and surface area, pure silica gels and powders find a very wide range of applications. Variation in preparation methods and parameters allows the tailoring of the substrate properties for specific application needs. The main features in the silica applications are its porosity, active surface, hardness, particle size and the viscous and thixotropic properties. Although most applications are based on a combination of those, a classification according to the main properties of interest may be set up. For references, the reader is referred to the works of Iler6 and Unger7 and to the references cited in chapter 8. 

In the selection of the stationary and mobile phase, a variety of chemical and physical factors of the chromatographic system that may contribute to the variation in the resolution and recovery of natural products need to be considered. The stationary phase contributions relate to the ligand composition, ligand density, surface heterogeneity, surface area, particle size, particle size distribution, particle compressibility, pore diameter, and pore diameter distribution. The mobile phase contributions relate to the type of organic solvents, eluent composition, ionic strength, pH, temperature, loading concentration, and volume. 

The properties of nanoparticles depend on surface morphology, specific surface area, particle size distribution, bulk density, drug incorporation, capacity, release, hydrophobicity, bioadhesiveness, and biodegradability. Nanoparticles (microspheres) loaded with the drug product can be formulated using copolymers, e.g., poly(lactide-co-glycolide) (PLG) or poly(lactide-co-ethylphosphate), by solvent extraction/evaporation technique. 

The thermal activity of a series of nano- and micron-particle grade anatase and rutile titanium dioxide pigments, with various densities of surface treatments, particle size, and surface area, have been determined by chemiluminescence in monomodal metallocene polyethylene [57]. 

Steps that cause changes in surface area, particle size, bulk and tap density or homogeneity. 

The properties of fumed silica can be determined during the production process by adjusting the feed rates of the three components — hydrogen, air and silane — to the burner. Basically, the specific surface area (particle size) is determined by the flame temperature. 

Carbon black can be described qualitatively by a series of properties particle size (and surface area) particle size distribution stmcture (particle aggregates) and surface activity (chemical functional groups such as carboxyl and ketones). 

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