Particle size measurement dynamic

Particle size measurement is one of the essential requirements in almost all uses of colloids. However, our discussion in Section 1.5 makes it clear that this is no easy task, especially since even the definition of particle size is difficult in many cases. A number of techniques have been developed for measuring particle size and are well documented in specialized monographs (e.g., Allen 1990). Optical and electron microscopy described in the previous section can be used when ex situ measurements are possible or can be acceptable, but we also touch on a few nonintrusive methods such as static and dynamic light scattering (Chapter 5) and field-flow fractionation (see Vignette II Chapter 2) in other chapters. 

After chemical derivatization, the integrity of the particles was confirmed by transmission electron microscopy (TEM), dynamic light scattering, and native gel electrophoresis. Measurements of the particle size by dynamic light scattering showed that the ferrocene-decorated CPMV particles (CPMV-Fc ) had an increase in radius of about 0.7 nm which is in good agreement with the size of the ferrocene moiety. 

As can be seen in the particle-size measurements (cf. Figure 14.4) and cryo-TEM images (cf. Figure 14.5), the mean particle sizes for Stober- and alkali-silicate sols vary around 21 and 40 nm, respectively, independent of the pH value. The smaller particles of the Stober-sol show a narrow distribution. As shown in Figure 14.5, the fumed silica consists of aggregates built by flame fusion of smaller spherical primary particles. Their real size cannot be measured exactly by the dynamic light scattering method... 

Spectroscopy provides the laboratory analyst, and the production engineer, the ability to perform particle size measurement over a very broad range of sizes in a timely manner. Spectroscopy in the form of diffraction and dynamic light-scattering technology have been demonstrated to provide information that can be applied to research and process studies as well as to the routine measurement demands of the plant. Examples of particle size measurement capability for the plastics industry from 0.003 to 3000 pm were presented, as well as a short discussion of means to convert the data from the newer technology to the classical sieve measurements. A final comment on the advancing science of spectroscopic particle size measurements was also presented. 

The concept of equivalence is well known and accepted in particle size measurement, and the paper applies this concept to the measure of the spread of the distribution. It characterises the actual distribution of particle size in the slurry by an equivalent, lognormal distribution described by a simple formula with two numerical parameters, the geometric mean (as a measure of the mean size) and the geometric standard deviation (as a measure of the distribution spread). The equivalence is by separation efficiency in a dynamic separator such as a hydrocyclone or a sedimenting centrifuge. 

In addition a quality factor, defined by 1X2 of Eq. (20.3) offers a measure of the width of the distribution of particle sizes measured by dynamic light scattering [23, 25]. 

Ansari, R. R., Suh, K., Dynamic Light Scattering Particle Size Measurements in Turbid Media, Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3251, 146-156. 

An alternative approach is the use of pH-sensitive fluorophores (Lichtenberg and Barenholz, lOSS). These probes are located at the lipid-water interface and their fluorescence behavior reflects the local surface pH, which is a function of the surface potential at the interface. This indirect approach allows the use of vesicles independent of their particle size. Recently, techniques to measure the C potential of Liposome dispersions on the basis of dynamic light scattering became commercially available (Muller et al., 1986). 

For a more detailed analysis of measured transport restrictions and reaction kinetics, a more complex reactor simulation tool developed at Haldor Topsoe was used. The model used for sulphuric acid catalyst assumes plug flow and integrates differential mass and heat balances through the reactor length [16], The bulk effectiveness factor for the catalyst pellets is determined by solution of differential equations for catalytic reaction coupled with mass and heat transport through the porous catalyst pellet and with a film model for external transport restrictions. The model was used both for optimization of particle size and development of intrinsic rate expressions. Even more complex models including radial profiles or dynamic terms may also be used when appropriate. 

Methods for analysis of the particle size distribution in the aerosol cloud include techniques such as time of flight measurement (TOE), inertial impaction and laser diffraction. Dynamic light scattering (photon correlation spectroscopy) is confined to particles (in suspension) in the submicron range. In addition to the size distribution, the particle velocity distribution can be measured with the Phase Doppler technique. 

It is our objective in this chapter to outline the basic concepts that are behind sedimentation and diffusion. As we see in this chapter, gravitational and centrifugal sedimentation are frequently used for particle-size analysis as well as for obtaining measures of solvation and shapes of particles. Diffusion plays a much more prevalent role in numerous aspects of colloid science and is also used in particle-size analysis, as we see in Chapter 5 when we discuss dynamic light scattering. The equilibrium between centrifugation and diffusion is particularly important in analytical and preparative ultracentrifuges. 

In contrast, in dynamic light scattering (DLS) the temporal variation of the intensity is measured and is represented usually through what is known as the intensity autocorrelation function. The diffusion coefficients of the particles, particle size, and size distribution can be deduced from such measurements. There are many variations of dynamic light scattering, and... 

Chu 1991 Schmitz 1990). For example, the dynamic version of the diffusing wave spectroscopy described in Vignette V is a form of DLS, although in diffusing wave spectroscopy the method of analysis is different in view of multiple scattering. Most of the advanced developments are beyond the scope of this book. However, DLS is currently a routine laboratory technique for measuring diffusion coefficients, particle size, and particle size distributions in colloidal dispersions, and our objective in this section is to present the most essential ideas behind the method and show how they are used for particle size and size distribution measurements. 

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