Particles characterization

Electron microscopy (see section B1.18) is very valuable in characterizing particles (see, for instance, figure C2.6.1). The suspension stmcture is, of course, not represented well because of tire vacuum conditions in tire microscope. This can be overcome using environmental SEM [241. 

Particle Size. Wet sieve analyses are commonly used in the 20 )J.m (using microsieves) to 150 )J.m size range. Sizes in the 1—10 )J.m range are analyzed by light-transmission Hquid-phase sedimentation, laser beam diffraction, or potentiometric variation methods. Electron microscopy is the only rehable procedure for characterizing submicrometer particles. Scanning electron microscopy is useful for characterizing particle shape, and the relation of particle shape to slurry stabiUty. 

The physicochemical forces between colloidal particles are described by the DLVO theory (DLVO refers to Deijaguin and Landau, and Verwey and Overbeek). This theory predicts the potential between spherical particles due to attractive London forces and repulsive forces due to electrical double layers. This potential can be attractive, or both repulsive and attractive. Two minima may be observed The primary minimum characterizes particles that are in close contact and are difficult to disperse, whereas the secondary minimum relates to looser dispersible particles. For more details, see Schowalter (1984). Undoubtedly, real cases may be far more complex Many particles may be present, particles are not always the same size, and particles are rarely spherical. However, the fundamental physics of the problem is similar. The incorporation of all these aspects into a simulation involving tens of thousands of aggregates is daunting and models have resorted to idealized descriptions. 

Unfortunately, the literature is relatively sparse with examples showing the water uptake profile onto crystalline, nonhydrating substances below RHq. This is most likely due to the difficulty in accurately measuring the small amounts of water that are sorbed. Alkali halides are an exception, however, likely due to their well-characterized particle morphologies [34—37]. Figure 2 shows a water uptake isotherm onto recrystallized sodium chloride [37]. Note that the amount of water sorbed as a function of relative humidity is normalized to the specific surface area of the sample. Since water is sorbed only to the external surface of... 

Glenister, P. R. (1974). Some useful techniques for the study of beer sediments. II. Characterizing particles of absorbent materials Particle counting. Proc., Am. Soc. Brew. Chem. 32, 11-12. 

The first field of application for SdFFF were latex beads, which were used either to test the channels or to produce separation results alternative to other separation techniques. PS nanoparticles used as model surfaces for bioanalytical work have been analyzed by SdFFF [39]. The appealing feature of SdFFF is its ability to characterize particle adlayers—by direct determination of the mass increase performed by observing the differences in retention between the bare and coated particles—with high precision and few error sources the mass of the coating is determined advantageously on a per particle basis. 

Automatic techniques for characterizing particle shape without operator error are also under development, based primarily on fiber optics with automatic signal processing. Kaye (Kl) has given a useful review of recent developments. 

In the second section of this chapter, techniques for measuring and characterizing particles are described. 

The total mass of particles per unit volume of air is one of the major parameters used to characterize particles in air and, along with size, is the basis of air quality standards for particulate matter (see Chapter 2). Methods of mass measurement include gravimetric methods, /3-ray attenuation, piezoelectric devices, and the oscillating microbalance. 

FIG. 4.13 Intrinsic viscosity of a protein solution (a) variation of the intrinsic viscosity of aqueous protein solutions with axial ratio a/b and extent of hydration mlb/m2 (redrawn from L. Oncley, Ann. NY Acad. Sci., 41, 121 (1941)) (b) superposition of the [r ] = 8.0 contour from Fig. 4.13a and the f/f0 = 1.45 contour from Figure 2.9. The crossover unambiguously characterizes particles with respect to hydration and axial ratio. 

Suppose your employer intends to develop a new laboratory to characterize particles in the colloidal size range. Your assignment is to prepare a list of the equipment that should be purchased for such a facility. A brief justification for each major item should be included along with a priority ranking based on the versatility of the method. Assume that your laboratory is already well stocked with such nonspecialized items as laboratory glassware, balances, and the like. 

The above-described pair problem is treated by the Smoluchowski equation [3, 19] - see Fig. 1.10. It operates with the probability densities (Fig. 1.11) and contains the recombination rate probability density to find a particle at a given point at time moment t gives us (by means of a trivial integration over reaction volume) the quantity of our primary interest - survival probability of a particle in the system with... 

This general extraction scheme can be extended to the selective extractions with a variety of Au and Ag DENs [31]. Using a combination of long-chain thiols and carboxylic acids, Crooks group has shown it is possible to selectively extract Au and Ag monometallic DENs from mixtures of the two, and hence provided important chemical information about the shell of the nanoparticle [38]. In the case of the bimetallic DENs, selective extraction is a potentially powerful tool for characterizing particle surfaces, especially in the case of the core/shell DENs. 

Resolution is without question a key element in accurate and detailed particle characterization. Particle populations that cannot be resolved cannot, in any sense, be distinguished from one another. While deconvolution techniques can provide particle size distribution curves from low resolution systems, the deconvolution must be based on assumptions about instrumental band broadening and band shape. In general, any detailed information lost because of poor resolution cannot be recovered by mathematical manipulation alone. In all cases, the quality of a size distribution curve will increase with the intrinsic resolution exhibited by the system. 

For micrometer-sized particles subject to steric- or lift-hyperlayer-FFF, the driving forces are higher (10 14 to 10 8 N per particle) but are not balanced by back diffusion as in the normal FFF mode. Steric- and lift-hyperlayer-FFF provide powerful means for the investigation of hydrodynamic lift forces [79]. Here, retention times have been measured for well-characterized particles such as latex spheres under widely varying conditions, and the hydrodynamic lift force FL has been determined. 

Catalyst Characterization. Particle size distributions of oven dried products were determined with a Microtrac Particle Size Analyzer. PH, PM, and PS are the diameters corresponding to the 90th, 50th, and 10th percentiles, respectively, on the distribution curve. Particle microstructures were obtained with an ISI-SX-40 scanning electron microscope. 

For the different classes of colloidal materials, the characteristic length scale is generally termed particle size. Many techniques [3,4,5,6J have been developed for characterizing particle size and shape, including optical and electron microscopy, sedimentation, and various kinds of chromatography. Light scattering techniques have been utilized extensively because of their speed, versatility, and ease of use relative to the other methods. 

When a simple cross-flow of carrier liquid is used, the method is known as flow FFF and this has been represented as having the widest range of application of any single FFF technique [95,96]. This technique has been used to separate and characterize particles in the 0.01 to 50 pm size range. 

A non-integer dimension called the surface fractal dimension, Ds, has been used to characterize particle surface roughness. The value of Ds varies from 2 for a perfectly smooth surface to 3 for a very rough surface. Various techniques to obtain surface fractal dimension are explained in detail in the subsections below. 

Sample preparation must be done carefully because biases in sample preparation will lead to inaccurate results. The statistical diameters that are popular for characterizing particles in microscopy are based on a random orientation thus, biases in orientation due to improper sample preparation will affect the values. Any factors that cause the particles to preferentially orient on the microscope slide will affect the results. For example, spreading the particles out with a spatula may causes a preferential orientation. Another example is particles dispersed in a liquid when the liquid is sprayed or poured onto the microscope slide the particles could orient themselves with the flow lines of the liquid and this could lead to their non random orientation. 

The focus here is on the effects of dissolved natural organic matter (NOM) on the colloidal stability of particles in aquatic systems and, in particular, on the importance of the macromolecular nature of NOM in these effects. The approach used here has three components (1) modeling studies with mathematical polyelectrolytes, surfaces, and solvents (2) laboratory studies with well-characterized polyelectrolytes and particles and (3) laboratory studies with aquatic NOM, also using well-characterized particles. 

Well-characterized particles and polyelectrolytes used in laboratory experiments are termed model chemicals . They exist in the laboratory, but they may or may not be present in significant concentrations in the field. They are selected as models for aquatic environments and are used in laboratory experiments to determine the strengths and the limitations of the model simulations using mathematical chemicals. 

It is important to be able to characterize particles in a dispersion. Questions of interest include the particle size and charge. Ways to ascertain the glass transition temperature of the constituent polymer are discussed in the film formation section. 

Physical concepts, measurement techniques, and analysis methods are widely used in pharmaceutical research and development. In the area of powder technology, for instance, these include methods to characterize particles and tablets in terms of surface area, porosity, and mechanical strength. 

The correlation analyses of toner particle size and size distribution parameters and image quality characteristics of toner deposits as measured by the spectral dependence of contrast transfer function and noise show high coefficients of correlation Specifically the Wiener spectrum data appear to yield the weight geometric mean and standard deviation of the toner population in this study. Therefore the Wiener spectrum may be another analytical tool in characterizing particle populations. It must be pointed out that the analysis reported here is mainly empirical. Further work is needed to refine the models and to examine the limits of applicability of these tests. Factors such as particle clumping, non-uniform depositions and optical limitations are specific areas for examination. 

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