Colloidal systems problems

Nomenclature. Colloidal systems necessarily consist of at least two phases, the coUoid and the continuous medium or environment in which it resides, and their properties gready depend on the composition and stmcture of each phase. Therefore, it is useful to classify coUoids according to their states of subdivision and agglomeration, and with respect to the dispersing medium. The possible classifications of colloidal systems are given in Table 2. The variety of systems represented in this table underscores the idea that the problems associated with coUoids are usuaUy interdisciplinary in nature and that a broad scientific base is required to understand them completely. 

Foams are agglomerations of gas bubbles separated from each other by thin films (5). Mainly, the problem is concerned with one class of colloidal systems —gas dispersed in liquid—but liquid dispersed in gas, solids dispersed in liquid (suspensions), and liquids dispersed in liquids (emulsions) cannot be ignored. The dispersion of a gas into a liquid must be studied and observed by the food technologist to improve the contact between the liquid and gas phases, the agitation of the liquid phase, and most important, the production of foam 10). 

The major disadvantages of colloidal catalysts studied so far can be attributed to problems in controlling the metal colloid formation (control of particle size, particle size distribution, structure of metal colloids) and stabilization of the prepared particles, which are not yet completely solved. But it is exactly the stability of the nanoparticles, that is decisive for long-term usage during catalytic processes. Moreover for catalytic application, it is extremely important to preserve the large surface of such colloidal systems. 

The emulsion polymerization process has several distinct advantages. The physical state of the emulsion (colloidal) system makes it easy to control the process. Thermal and viscosity problems are much less significant than in bulk polymerization. The product of an emulsion polymerization, referred to as a latex, can in many instances be used directly without further separations. (However, there may be the need for appropriate blending operations,... 

COLLOID SYSTEMS. Colloids are usually defined as disperse systems with at least one characteristic dimension in the range 10 7 lo ll> centimeter. Examples include sals (dispersions or solid in liquid) emulsions (dispersion of liquids in liquids) aerosols (dispersions of liquids or solids in gases) /inum (dispersion of gases in liquids or solids) and gels (system, such as common jelly, in which one component provides a sufficient structural framework for rigidity and other components fill the space between the structural units or spaces). All forms of colloid systems are encountered in nature. Products of a colloidal nature arc commonly found in industry and are notably extensive in the food field. Foams, widely used in industrial products, but also the causes of processing problems are described in entries on Foam and Foamed Plastics. 

The surface or interfacial phenomena associated with colloidal systems such as emulsions and foams are often studied by means of experiments on artificially prepared flat surfaces rather than on the colloidal systems themselves. Such methods provide a most useful indirect approach to the various problems involved. 

As diffusion proceeds, concentration and concentration gradient changes will take place as illustrated in Figure 2.4. To ensure that the broadening of the boundary is due to diffusion only, very accurate temperature control (to avoid convection currents) and freedom from mechanical vibration must be maintained. The avoidance of convection is a problem common to all kinetic methods of investigating colloidal systems. 

This restriction, actually, corresponds with the weak coupling approximation employed throughout the previous section. It simplifies all equations significantly, and allows to obtain an analytical solution to the problem. Also, the standard, DLYO theory is well developed in this limiting situation, what alleviates comparison of our calculations with the standard, DLYO results used widely for the description of the electrostatic interaction in colloidal systems. 

Aggregation of liposomes both in vitro and in vivo is one of their main stability problems. According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, or theory of colloidal stability, a colloidal system is stable if the electrostatic repulsion forces between two particles are larger than the attraction van der Waals forces. Therefore charged liposomal formulations are highly desirable. Manipulation of... 

In principle, it would be desirable to study the swelling for many different vermiculites and many different cations. However, as we shall see, the study of even one swelling system becomes a complicated many-variable problem. The swelling seems to be most pronounced and homogeneous for vermiculites with x = 1.3 and M = C4H9NH3, so we choose to investigate this ideal clay colloid system as rigorously as possible and focus our studies on the Eucatex minerals in hand. 

It is because of the subdivision of matter in colloidal systems that they have special properties. The large surface-to-volume ratio of the particles dispersed in a liquid medium results in a tendency for particles to associate to reduce their surface area, so reducing their contact with the medium. Emulsions and aerosols are thermodynamically unstable two-phase systems which only reach equilibrium when the globules have coalesced to form one macro-phase, for which the surface area is at a minimum. Many pharmaceutical problems revolve around the stabilisation of colloidal systems. 

Theoretical molecular statistic calculations of adsorption thermodynamic properties for colloid systems are generally believed to be a quite cumbersome problem. This is mainly... 

A basic problem which can be encountered in the application of photophysics to colloidal systems are difficulties involved in the measurement of true luminescence spectra and determination of luminescence quantum yields of molecules in light scattering media. Gade and Kaden have produced a theory for this effect which can be used to take account of readsorption and re-emission effects in suspensions. 

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