Colloidal systems light scattering

In this section we examine many of the methods that have been developed to make monodisperse materials. Whether or not they are ideal for ceramics fabrication, monodisperse particles are certainly important as model systems for testing theories of colloidal stability, light scattering, drying, and sintering. 

The study of the scattering of light by colloidal systems has a long history. The Tyndall effect describes the scattering of light by suspended particles. In fact, the first rigorous theory was that of Rayleigh in 1871. 

Neutron, X-Ray and Light Scattering Introduction to an Investigative Tool for Colloidal and Polymeric Systems, P. Lindner. T. Zemb, North-Holland. Amsterdam, 1991 Small Angle X-Ray Scattering, O. Glatter, O. Kratky. Eds., Academic Press, New York. 1982. 

Table 1.6 also lists the radius of gyration. This is an average dimension often used in colloid science to characterize the spatial extension of a particle. We shall see that this quantity can be measured for polydisperse systems by viscosity (Chapter 4) and light scattering (Chapter 5). It is therefore an experimental quantity that quantifies the dimensions of a disperse system and deserves to be included in Table 1.6. Since the typical student of chemistry has probably not heard much about the radius of gyration since general physics, a short review seems in order.

Comparing Equations (28) and (29) with Equations (3.35) and (3.36) reveals that plots of Kosm/RTc) versus c and (Kc/Re) versus c have identical intercepts, at least for monodisperse colloids (see Section 5.4d for a discussion of the average obtained for polydisperse systems), and that the slopes differ by a factor of 2, with the light-scattering results having the larger slope. 

Another colloidal system with light scattering characteristics that have been widely studied is the so-called monodisperse sulfur sol. Although not actually monodisperse, the particle size distribution in this preparation is narrow enough to make it an ideal system for the study of optical phenomena. 

The term /w/Ywcmulsion implies a system which (like an emulsion) contains droplets of oil or water, bui in which ihe droplets are too small to scatter light see also Colloid Systems. 

J.H. Schulman and J.A. Friend, Light scattering investigation of the structure of transparent oil-water disperse systems, II, J. Colloid Sci. 4 (1949) 497-509. 

As we shall see, the intensity, polarisation and angular distribution of the light scattered from a colloidal system depend on the size and shape of the scattering particles, the interactions between them, and the difference between the refractive indices of the particles and the dispersion medium. Light-scattering measurements are, therefore, of great value for estimating particle size, shape and interactions, and have found wide application in the study of colloidal dispersions, association colloids, and solutions of natural and synthetic macro-molecules. 

The application of light scattering to the characterisation of colloidal systems has advanced rapidly over the last few decades. This has been made possible by the development of (a) lasers as intense, coherent and well-collimated light sources, (b) sophisticated electronic devices for recording data, and (c) computers for the complex data processing that is involved. 

The radial distribution function plays an important role in the study of liquid systems. In the first place, g(r) is a physical quantity that can be determined experimentally by a number of techniques, for instance X-ray and neutron scattering (for atomic and molecular fluids), light scattering and imaging techniques (in the case of colloidal liquids and other complex fluids). Second, g(r) can also be determined from theoretical approximations and from computer simulations if the pair interparticle potential is known. Third, from the knowledge of g(r) and of the interparticle interactions, the thermodynamic properties of the system can be obtained. These three aspects are discussed in more detail in the following sections. In addition, let us mention that the static structure is also important in determining physical quantities such as the dynamic an other transport properties. Some theoretical approaches for those quantities use as an input precisely this structural information of the system [15-17,30,31]. 

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