Particle ultracentrifugal sedimentation

Figure 3.40 Moving-boundary ultracentrifugation. The heavier particles (o) sediment more rapidly than the lighter particles (X) and as a result concentration boundaries develop as the supernatant fluid is cleared of particles. Monitoring the movement of a boundary will permit the determination of sedimentation coefficients.

In another type of experiment with the ultracentrifuge, the sedimentation coefficient of the particle is obtained. If the centrifugal force greatly exceeds the force due to the osmotic field, the particles will sediment, forming a boundary that will move down from the surface of the solution for positive values of (dp/dc2)v.. This movement will be opposed by the buoyancy of the particles and by the frictional force generated by their motion. It can be shown that this leads to a constant velocity (dr/dt) of the boundary and a sedimentation coefficient, s, is defined for the particle by s = (drldt)l(n2r, where r is the distance to the center of rotation (Schachman, 1959). For vanishing particle concentration,... 

Particle size Electron microscopy, light scattering, ultracentrifugal sedimentation, light microscopy... 

In this section the use of TEM, OM, and ultracentrifugal sedimentation methods for the particle size and shape of HPOPs will be illustrated. 

Particle Size by Ultracentrifugal Sedimentation and Comparison to TEM 21.7.4.1 Introduction... 

Ultracentrifugal sedimentation can be used to measure particle size in the range of 10 nm to 40 pm. The particles in dispersed form are injected into a rotating disc spinning up to 2400 RPM. The sedimentation is stabilized by density gradient fluid through which the particles settle. The principle is based on Stokes law ... 

The main advantage of particle size analysis by ultracentrifugal sedimentation over other methods such as dynamic light scattering is the enhanced resolution precision (typically 0.25%) and accuracy. Particle bands differing by only 5% in size may be differentiated. 

The following examples illustrate the applicability of ultracentrifugal sedimentation to particle size distribution of HPOP. Ultracentrifugal sedimentation has been used to determine the cause of the color strength loss of C.I. PR254 powder... 

Figure 21.50 Comparison of TEM and ultracentrifugal sedimentation particle size analysis of C.l. PV55.

Figure 21.51 Particle size of the large particle C.l. PBl 5 3 (crude pigment) by TEM and ultracentrifugal sedimentation.

Fig. 4. Effect of centrifugal force on the sedimentation constant of E. colt bacteriophages T5 and T6. The symbols refer to measurements at different pH, as indicated in the figure. Note that T6 gives two boundaries at pH 6.85 and pH 8.25. The Constance of Sjo indicates lack of orientation of the phage particles during sedimentation in the ultracentrifuge (256a).

For smaller particles, the sedimentation can be accelerated by the application of centrifugal fields. Indeed the first models of Svedbi g s ultracentrifuge have been applied to the determination of particle size in gold sols ... 

The ultracentrifuge has been used extensively, especially for the study of biopolymers, and can be used in several different experimental modes to yield information about polymeric solutes. Of the possible procedures, we shall consider only sedimentation velocity and sedimentation equilibrium. We shall discuss these in turn, beginning with an examination of the forces which operate on a particle setting under stationary-state conditions. 

Sedimentation coefficients are a measure of the velocity witli which a particle sediments in a centrifugal force field. Sedimentation coefficients are typically expressed in Svedbergs (.symbolized S), named to honor The Svedberg, developer of the ultracentrifuge. One S equals 10 . sec. 

There are various methods for the determination of the size distribution of organic pigment particles, the most common are sedimentation techniques in ultracentrifuges and specialized disk centrifuges as well as electron microscopy. These methods require considerable experimental skill, since the results depend largely on sample preparation and especially on the quality of the dispersion. 

Which of the following affect the speed of sedimentation of a particle in an ultracentrifuge ... 

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. 

A basic limitation of all these methods is the narrow range of particle sizes that can be investigated by sedimentation under gravity. Therefore, we turn next to a consideration of centrifugation, particularly the ultracentrifuge, as a means of extending the applicability of sedimentation measurements. 

The ultracentrifuge is an instrument in which a cell is rotated at very high speeds in a horizontal position. As we see below, the gravitational acceleration is easily increased by a factor of 105 in such an apparatus. Accordingly, the particle size that may be studied by sedimentation... 

Most probable settling velocity from sedimentation data Particle-size determination from sedimentation equation Sedimentation in an ultracentrifuge Solvation and ellipticity from sedimentation data Diffusion and Gaussian distribution Temperature-dependence of diffusion coefficients... 

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