Solid dispersions foams

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). 

Nanoparticles are frequently used as a suspension in some kind of solvent. This is a two phase mixture of suspended solid and liquid solvent and is thus an example of a colloid. The solid doesn t separate out as a precipitate partially because the nanoparticles are so small and partially because they are stabilised by coating groups that prevent their aggregation into a precipitate and enhance their solubility. Colloidal gold, which has a typical red colour for particles of less than 100 nm, has been known since ancient times as a means of staining glass. Colloid science is a mature discipline that is much wider than the relatively recent field of nanoparticle research. Strictly a colloid can be defined as a stable system of small particles dispersed in a different medium. It represents a multi-phase system in which one dimension of a dispersed phase is of colloidal size. Thus, for example, a foam is a gas dispersed in a liquid or solid. A liquid aerosol is a liquid dispersed in gas, whereas a solid aerosol (or smoke) is a solid dispersed in a gas. An emulsion is a liquid dispersed in a liquid, a gel is liquid dispersed in a solid and a soils a solid dispersed in a liquid or solid. We saw in Section 14.7 the distinction between sol and gel in the sol gel process. 

The adsorption mechanisms of surfactant at interfaces have been extensively studied in order to understand their performance in many processes such as dispersion, coating, emulsification, foaming and detergency. These interfaces are liquid-gas (foaming), liquid-liquid (emulsification) and liquid-solid (dispersion, coating and detergency). 

In foams with charged gas/liquid interfaces, as in other disperse systems, various electrokinetic phenomena are possible to occur. Such are the change in the transport numbers of ions, electroosmosis, streaming potential and surface conductivity. While these phenomena are largely studied in disperse systems with solid disperse phase, the first electrokinetic observations in foams have been reported only recently. 

The above description of foam destabilization mechanisms is only applicable to liquid foods. In solid dispersions, these mechanisms are significantly slowed or even eliminated. As mentioned earlier, solid matrixes such as aerated confectionery wiU hold air for up to years. Structure stabilization occurs for example in chocolate foams when the product is cooled to solidify and trap the bubbles formed within. The baking... 

Examples of gas-in-liquid dispersions are the foams or the boihng hquids. Gas-in-solid dispersions are the various porous media such as filtration membranes, sorbents, catalysts, and isolation materials. 

When the aqueous system contains finely divided solids, then foaming of the system may be influenced greatly by the nature of the dispersed solid particles. If the particles have a surface that is hydrophobic, and if the particles are divided finely enough, then the particles may adsorb onto the surface of any air bubbles introduced into the system and stabilize them against coalescence. They adsorb at the air-solid interface from the aqueous system because their solid-aqueous solution interfacial tension, ySL, is high and their solid/(nonpolar) air interfacial tension, ySA, is low because of their nonpolar surface. Consequently, their contact angle, 0, with the aqueous phase, from equation 6.3... 

The formation of systems with solid backbones is often the result of aggregation processes taking place in suspensions and sols which lead to the development of spacial networks and final conversion of disperse systems into materials with valuable properties (Chapter IX, 2). In some cases, e.g. during solidification of metal alloys, the processes of structuring accompany formation of new phases. The systems with solid dispersion medium also form upon the solidification of a continuous phase in foams, emulsions, suspensions and sols. 

Systems with a solid dispersion medium are represented by rocks, minerals, a variety of construction materials. Most such systems are of the S,/S2 types. Various synthetic and natural porous materials (with closed porosity), such as pumice and solid foams (e.g. styrofoam, bread), belong to the G/S type. The systems of L/S type include natural and synthetic opals and... 

Verreck, G. Decorte, A. Heymans, K. Adriaensen, J. Cleeren, D. Jacobs, A. Liu, D. et al. The effect of pressurized carbon dioxide as a temporary plasticizer and foaming agent on the hot stage extrusion process and extrudate properties of solid dispersions of itraconazole with PVP-VA 64. Eur. J. Pharm. Sci. 2005, 26(3-4), 349-358. 

In Chapter 1 the importance of the various classes of colloidal systems to modern science and technology was indicated in a general way. Because of the wide variety of colloidal systems one encounters, each having certain unique features that distinguish it from the others, it is convenient to discuss each major classification separately. For that reason, chapters have been devoted to specific systems such as solid dispersions, aerosols, emulsions, foams, lyophilic colloids (i.e., polymer solutions), and association colloids. There is a great deal of overlap in many aspects of the formation, stabilization, and destruction of those systems, and an effort will be made not to repeat more than is necessary. However, for purposes of clarity, some repetition is unavoidable. 

Solid Solid dispersions Solid emulsions Solid foams... 

Colloids can be further categorized as aerosols (liquid or solid dispersed in gas), foams (gas dispersed in liquid or solid), emulsions (liquid dispersed in another liquid), sols (solid dispersed in liquid or in another solid), and gels (liquid dispersed in a solid). Table 13.5 li.sts the different types of colloids and gives one or more examples of each. 

The disjoining pressure is a concept commonly used in the field of foams and emulsions (fluid systems) and is completely analogous to what is referred to as surface forces when considering interactions in thin films separated by two solid phases (e.g. solid dispersions). The primary difference is simply based on the historical origins of the different schools working on the subject and the actual parameters measured when conducting experimental investigations with these different systems. As the present work is focussed on fluid systems, we will adopt the former in order to be historically consistent. 

Foaming agents are surfactants that allow creation of dispersions of gaseous substances in liquid or solid food. Foaming gases are carbon monoxide and carbon dioxide (E290) and in some countries natural saponins. 

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