Colloidal particle scattering

A FIGURE 12.23 The Tyndall Effect When a light beam passes through a colloidal suspension (left), it is visible because the colloid particles scatter some of the light. The beam is not visible in pure water (right), nor would it be visible in a noncolloidal solution. 

The polymer concentration profile has been measured by small-angle neutron scattering from polymers adsorbed onto colloidal particles [70,71] or porous media [72] and from flat surfaces with neutron reflectivity [73] and optical reflectometry [74]. The fraction of segments bound to the solid surface is nicely revealed in NMR studies [75], infrared spectroscopy [76], and electron spin resonance [77]. An example of the concentration profile obtained by inverting neutron scattering measurements appears in Fig. XI-7, showing a typical surface volume fraction of 0.25 and layer thickness of 10-15 nm. The profile decays rapidly and monotonically but does not exhibit power-law scaling [70]. 

Turbidimetry and nephelometry are two related techniques in which an incident source of radiation is elastically scattered by a suspension of colloidal particles. In turbidimetry, the detector is placed in line with the radiation source, and the... 

In practice, the invariant can be used for the purpose of calibration to absolute scattering intensity by means of samples for which the absolute invariant can easily be computed. For this purpose colloidal suspensions of noble metals with known volume concentration are suitable [96], All the noble metal particles must be small enough so that they really contribute to the observed particle scattering. They must not agglomerate. 

Finally, we have designed and synthesized a series of block copolymer surfactants for C02 applications. It was anticipated that these materials would self-assemble in a C02 continuous phase to form micelles with a C02-phobic core and a C02-philic corona. For example, fluorocarbon-hydrocarbon block copolymers of PFOA and PS were synthesized utilizing controlled free radical methods [104]. Small angle neutron scattering studies have demonstrated that block copolymers of this type do indeed self-assemble in solution to form multimolecular micelles [117]. Figure 5 depicts a schematic representation of the micelles formed by these amphiphilic diblock copolymers in C02. Another block copolymer which has proven useful in the stabilization of colloidal particles is the siloxane based stabilizer PS-fr-PDMS [118,119]. Chemical... 

If a beam of light is passed through a colloidal solution, the colloidal particles will reflect or scatter the light so that the beam becomes visible when the solution is viewed from the side against a dark background. This Tyndall effect is used in the ultramicroscope. 

Photon correlation spectroscopy (PCS) has been used extensively for the sizing of submicrometer particles and is now the accepted technique in most sizing determinations. PCS is based on the Brownian motion that colloidal particles undergo, where they are in constant, random motion due to the bombardment of solvent (or gas) molecules surrounding them. The time dependence of the fluctuations in intensity of scattered light from particles undergoing Brownian motion is a function of the size of the particles. Smaller particles move more rapidly than larger ones and the amount of movement is defined by the diffusion coefficient or translational diffusion coefficient, which can be related to size by the Stokes-Einstein equation, as described by... 

This is partly for ease of processing but frequently fillers are used to produce colour and/or opacity as well as to improve mechanical properties and it is for this reason also that colloidal particles are used because they provide good light scattering at low addition levels. However we will concentrate on changes in mechanical properties. 

Particles of a size of less than 2 turn are of particular interest in Process Engineering because of their large specific surface and colloidal properties, as discussed in Section 5.2. The diffusive velocities of such particles are significant in comparison with their settling velocities. Provided that the particles scatter light, dynamic light scattering techniques, such as photon correlation spectroscopy (PCS), may be used to provide information about particle diffusion. 

The nature and relevance of colloids is one of the main current research topics (Birdi, 2002). They are an important class of materials, intermediate between bulk and molecularly dispersed systems. Colloid particles may be spherical but, in some cases, one dimension can be much larger than the other two (as in a needle-like shape). The size of particles also determines whether they can be seen by the naked eye. Colloids are not visible to the naked eye or under an ordinary optical microscope. The scattering of light can be suitably used to see such colloidal particles (such as dust particles, etc.). Their size then may range from 1() 4 to 1() 7 cm. The units used are as follows ... 

The basic mechanism is that a polyphenol molecule with at least two binding sites attaches to two proteins and bridges them together. Additional polyphenol molecules attach this structure to additional protein molecules and eventually the complex grows so large that it is no longer soluble. At this point, it becomes a colloidal particle and scatters light. The... 

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