Colloidal foams

The typical viscous behavior for many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels) is illustrated by the curves labeled structural in Figs. 3-5 and 3-6. These fluids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. In some materials this can be attributed to a reversible structure or network that forms in the rest or equilibrium state. When the material is sheared, the structure breaks down, resulting in a shear-dependent (shear thinning) behavior. Some real examples of this type of behavior are shown in Fig. 3-7. These show that structural viscosity behavior is exhibited by fluids as diverse as polymer solutions, blood, latex emulsions, and mud (sediment). Equations (i.e., models) that represent this type of behavior are described below. 

A gas suspended in a solid is also called a foam. This form of colloid is relatively rare in nature, unless we stretch our definition of solid to include rock, in which case pumice stone is a colloidal foam. Synthetic foams are essential for making cushions and pillows. There is also presently much research into forming metal foams, which have an amazingly low density. 

Under certain circumstances it is possible to utilize physical interactions to maintain surfaces at some minimum distances of separation as a result of an energy maximum in the interaction energy. The practical result of such long-range energy maxima is that, properly utilized, they can prevent or at least retard the natural tendency of surfaces to approach and join spontaneously, thereby reducing interfacial area. This effect is especially important in colloids, foams, emulsions, and similar systems. 

Gannon OK (1988) PhD Environmental reclamation through use of colloidal foam flotation, in-situ soil aeration and in-situ surfactant flushing, UMI Disseration Services Order Number 8910849... 

Chem. Descrip. Dehydrated egg whites, cellulose gum Uses Emulsifier, protective colloid, foaming agent for large commercial bakeries, angel food cakes, meringues, toppings, chiffon pies, candy Features Combines foaming props, of P-20 with exc. foam stability by addition of cellulose gum... 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

R. J. Akers, ed.. Foams Proceedings of a Symposium Organic d by the Sodety of Chemical Industy, Colloid and Suface Chemistry Group, Bmnel University, Uxbridge, U.K., Sept. 8—10, 1975, Academic Press, Inc., London, 1976. 

J. W. McBain and co-workers. Foaming of Aircraft-Fngine Oils as a Problem in Colloid Chemist NACA ARR No. 4105, National Advisory Committee for Aeronautics, Stanford, Calif., 1944. 

Among the methods of foam separation, foam fractionation usually implies the removal of dissolved (or sometimes colloidal) material. The overflowing foam, after collapse, is called thefoamate. The solid lines of Fig. 22-42 illustrate simple continuous foam fractionation. (Batch operation would be represented by omitting the feed and bottoms streams.)... 

For columns that are wider than several centimeters, reflux and feed distributors should be used, particularly for wet foam [Haas and Johnson, Am. Jn.st. Chem. Fng. J., 11, 319 (1965)]. Liquid content within the foam can be monitored conduc tometricaUy [Chang and Lemhch, J. Colloid Jnteiface Sci., 73, 224 (1980)]. See Fig. 22-44. TheoreticaUy, as the limit =K = 0is veiy closely approached, 2] = 3K [Lemhch,y. Colloid Jnteiface Sci, 64, 107 (1978)]. 

Foam Coalescence Coalescence is of two types. The first is the growth of the larger foam bubbles at the expense of the smaller bubbles due to interbubble gas diffusion, which results from the smaller bubbles having somewhat higher internal pressures (Adamson, The Physical Chemlstiy of Suifaces, 4th ed., Wiley, New York, 1982). Small bubbles can even disappear entirely. In principle, the rate at which this type of coalescence proceeds can be estimated [Ranadive and Lemhch,y. Colloid Inteiface Sci., 70, 392 (1979)]. 

The numerous separations reported in the literature include surfactants, inorganic ions, enzymes, other proteins, other organics, biological cells, and various other particles and substances. The scale of the systems ranges from the simple Grits test for the presence of surfactants in water, which has been shown to operate by virtue of transient foam fractionation [Lemlich, J. Colloid Interface Sci., 37, 497 (1971)], to the natural adsubble processes that occur on a grand scale in the ocean [Wallace and Duce, Deep Sea Res., 25, 827 (1978)]. For further information see the reviews cited earlier. 

Purified by Soxhlet extraction with pet ether for 24h, followed by dissolution in acetone MeOH H20 90 5 5(v/v) and recrystn [Politi et al. J Phys Chem%9 2345 1985. Also purified by two recrystns from absolute EtOH, aqueous 95% EtOH, MeOH, isopropanol or a 1 1 mixture of EtOHrisopropanol to remove dodecanol, and dried under vacuum [Ramesh and Labes J Am Chem Soc 109 3228 1987]. Also purified by foaming [see Cockbain and McMullen Trans Faraday 5oc 47 322 1951] or by liquid-liquid extraction [see Harrold J Colloid Sci 15 280 I960]. Dried over silica gel. For DNA work it should be dissolved in excess MeOH passed through an activated charcoal column and evaporated until it crystallises out. 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

Surfactants. These enhance the colloid stability against mechanical and chemical stresses, help to disperse fillers, aid in wetting and enhance foaming. The most common surfactants are dodecylbenzene sulphonates and potassium oleate. 

Wetting agents. These facilitate the wetting of surfaces and aid colloidal stability without foaming. Naphthalene sulphonate/formaldehyde is the most common wetting agent. 

Process leaks of sugars, fats, colloidal materials, pectins, emulsions, and proteins cause stable foams in the boiler, leading to carryover and a further contamination cycle. 

Where contamination from edible oils, fats, solvents, and the like occurs, the development of stable foams or colloids, saponification of fatty materials into crude soaps, and a very serious risk to heat transfer surfaces may result. 

NOTE Colloidal clays also can form deposits, and suspended solids have a significantly higher effect on foaming than TDS. 

Foods such as sugars, colloidal pectins, and mayonnaise emulsions are also prime sources of foaming. 

Process leaks from food and beverage production or wood leachates often produce sugars, colloidal materials, pectins, emulsions, and proteins that cause stable foams in the boiler. These lead to carryover and further steam-condensate line contamination. The temporary use of a demulsifier or defoamer as part of the water treatment program may be of particular benefit, but again the condensate is unsuitable for return to the boiler. Other process leaks include ... 

Free caustic alkalinity usually is not recommended for jet-type electrode, as foaming conditions may develop rapidly because of the high recirculation rate. Where high alkalinity is present and FW contamination from colloidal or organic matter takes place, the foaming that develops quickly causes the boiler to be shut down. 

In addition, even where foaming is not a specific problem in a boiler, carryover may occur, especially in lower pressure boilers with very high TDS (i.e., over 10,000 to 15,000 ppm TDS) because of the collapse of surface bubbles. This leads to BW aerosol generation and entrainment of the spray in steam. Under these circumstances, antifoam agents such as polyamides are useful in preventing these entrainment problems. Furthermore, the antifoaming action of polyamides is often enhanced by protective colloid materials such as tannins, and consequently, formulations containing polyamide emulsions in an alkaline tannin base are available. 

Another family of polyols is the filled polyols.llb There are several types, but die polymer polyols are die most common. These are standard polyether polyols in which have been polymerized styrene, acrylonitrile, or a copolymer thereof. The resultant colloidal dispersions of micrometer-size particles are phase stable and usually contain 20-50% solids by weight. The primary application for these polyols is in dexible foams where the polymer filler serves to increase foam hardness and load-bearing capacity. Other filled polyol types diat have been developed and used commercially (mainly to compete with die preeminent polymer polyols) include the polyurea-based PEID (polyhamstoff dispersion) polyols and the urethane-based PIPA (poly isocyanate polyaddition) polyols. 

Almost all urethane materials are synthesized without the use of solvents or water as diluents or earners and are referred to as being 100% solids. This is true of all foams and elastomers. There are many products, however, which do utilize solvents or water, and these are known as solvent-borne and waterborne systems, respectively. In the past, many coatings, adhesives, and binders were formulated using a solvent to reduce viscosity and/or ease application. However, the use of volatile solvents has been dramatically curtailed in favor of more environmentally friendly water (see Section 4.1.3), and now there are many aqueous coatings, adhesives, and associated raw materials. Hydrophilic raw materials capable of being dispersed in water are called water reducible (or water dispersible), meaning they are sufficiently hydrophilic so as to be readily emulsified in water to form stable colloidal dispersions. 

Colloids are classified according to the phases of their components (Table 8.9). A colloid that is a suspension of solids in a liquid is called a sol, and a suspension of one liquid in another is called an emulsion. For example, muddy water is a sol in which tiny flakes of clay are dispersed in water mayonnaise is an emulsion in which small droplets of water are suspended in vegetable oil. Foam is a suspension of a gas in a liquid or solid. Foam rubber, Styrofoam, soapsuds, and aerogels (insu-... 

A foam is a colloid formed by suspending a gas in a liquid or a solid matrix, while a sol is a suspension of a solid in a liquid. Some examples of foams are styrofoam and soapsuds examples of sols are muddy water and mayonnaise. 

The area of colloids, surfactants, and fluid interfaces is large in scope. It encompasses all fluid-fluid and fluid-solid systems in which interfacial properties play a dominant role in determining the behavior of the overall system. Such systems are often characterized by large surface-to-volume ratios (e.g., thin films, sols, and foams) and by the formation of macroscopic assembhes of molecules (e.g., colloids, micelles, vesicles, and Langmuir-Blodgett films). The peculiar properties of the interfaces in such media give rise to these otherwise unlikely (and often inherently unstable) structures. 

Figure 6.2. (a). Colloidal silica network on the surface of spores from Isoetes pantii (quill wort). Scale = 20 pm. (b). Polystyrene networks and foams produced as a biproduct of colloidal latex formation. Both types of colloidal system are typical of the diversity of patterns that can be derived from the interactions of minute particles. Scale (in (a)) = 50pm. 

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