Sedimentation aggregation technique

Sedimentation (qv) techniques, whether based on gravitational forces or centrifugation, derive the particle size from the measured travel rates of particles in a Hquid. Before the particle analysis is carried out, the sample is usually dispersed in a medium to break down granules, agglomerates, and aggregates. The dispersion process might involve a simple stirring of the powder into a Hquid, but the use of an ultrasonic dispersion is preferred. 

Before beginning a size determination, it is customary to look at the material, preferably under a microscope. This examination reveals the approx size range and distribution of the particles, and especially the shapes of the particles and the degree of aggregation. If microscopic examination reveals that the ratios between max and min diameters of individual particles do not exceed 4, and indirect technique for particle size distribution based on sedimentation or elutria-tion may be used. Sedimentation techniques for particle size determination were first used by Hall (Ref 2) in 1904, He showed that the rate of fall of individual particles in a fluid was directly related to the particle size by the hydrodynamic... 

The techniques of field-flow fractionation appear to be well suited to colloid analysis. The special subtechnique of sedimentation FFF (SdFFF) is particularly effective in dealing with colloidal particles in the diameter range from 0.02 to 1 using the normal or Brownian mode of operation (up to 100 jU-m using the steric-hyperlayer mode). As a model sample for the observation of aggregate particles by SdFFF, of... 

Recent improvements in techniques of sedimentation and diffusion analysis have permitted their application to materials in the molecular-weight range of the Schardinger dextrins. These measurements supplement other types of determinations in that (1) they are not particularly sensitive to the presence of low molecular-weight impurities, and (2) the sedimentation and diffusion constants can be extrapolated to infinite dilution to eliminate aggregation and interaction effects. 

A technique called differential centrifugation is commonly used to fractionate particles into pellet and supernatant. The pellet is an aggregate of all sedimenting components, while the supernatant is a purified portion of the sample containing only the slowest sedimenting components. In this way, differential centrifugation is a purification technique in which large particles are removed from the supernatant. 

Chromatographic silica was studied by FFF on several previous occasions. An early study using sedimentation-steric FFF yielded size and size distribution information (14) but undoubtedly incurred errors because it preceded recent calibration techniques developed to account for hydrodynamic lift forces (20). More recent work with flow FFF has provided size and size distribution information and has clearly signaled the presence of aggregates in one commercial sample (15). In a study (16) using sedimentation-steric FFF combined with microscopy, we developed a strategy for obtaining not only size distributions, but also porosity and porosity distributions. Some of the data derived from this study are summarized in Table VI. 

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