Foam-Stabilizing Organic Additives

Since micelles can solubilize organic additives and thereby remove them from the interface, much larger amounts of foam-stabilizing additives are required to stabilize the foam of aqueous solutions above their CMC than below their CMC. 

The most effective additives for increasing the stability of the foam produced by surfactant solutions appear to be long-chain, often water-insoluble, polar compounds with straight-chain hydrocarbon groups of approximately the same length as the hydrophobic group of the surfactant. Examples are lauryl alcohol for use with sodium dodecyl sulfate, Af,/V-bis(hydroxyethyl) lauramide for use with dodecylbenzenesulfonate, lauric acid for use with potassium laurate, and N,N-dimethyldodecylamine oxide for use with dodecylbenzenesulfonate and other anionics. 

Polar additives may also increase foam stabilization by solubilizing foamicidal oils (Schick, 1957), since micelles containing solubilized polar additives (Chapter 4, Section IB4) have increased solubilization power for nonpolar materials. 

As described in Chapter 4, Section IB5, surfactants interact with polymers to form complexes, the strength of the surfactant-polymer interaction being dependent 

---

Foam films and a foam from the aqueous and organic phases of an extraction system containing a 30% solution of tri-buthyl phosphate (TBP) in kerosene and nitric acid (1 mol dm 3) have been studied in a parallel mode [137]. The reasons for foaming and the effect of emulsion formation on foam stability were elucidated. Thus, a foam with a measurable lifetime was obtained when TBP was in concentration of 0.8 mol dm 3 which corresponded to the concentration of black spot formation. When the volume ratio of the organic to the aqueous phase was 1 5, the foam formed in the system was stabilised additionally by a highly disperse O/W emulsion. This was due to the reduced rate of drainage. These results are confirmed by the experimental data acquired with a specially constructed centrifugal extractor [136]. It makes it possible to perform an extraction process under conditions close to those in industry. 

The surface activity of silicones is often exploited by using them as additives. For this reason, aspects of the two most important additive forms, copolymers and surfactants, are also included in this discussion. These two classes come together in the relatively low molecular weight PDMS-poly(alkylene oxide) block and graft copolymers that are commonly used as polyurethane foam stabilizers. Other short-chain silicone surfactants designed for aqueous systems and other silicone-organic copolymers are also available. 

Additives for foam stability are generally divided into three main classes electrolytes, polar organic molecules (surfactants), and macromolecular compounds. 

Ultrafine particle size platy talcs have been used extensively as recycling additives. The highly adsorbant talcs attract inks, adhesives and other organics that may be part of the recycled stock. Talcs also function in flotation deinking processes as foam stabilizers, and since talc tends to float on water, it helps to carry the ink particles to the surface. The talc-waste particles are large enough to be easily removed by filtration or other cleaning methods. 

Not only does foam stabilization by polar organic additives seem to go hand in hand with the effect of the additive on the cmc of the surfactant there is also a correlation with the relative amount of additive that is located in the interfacial film. The greater the mole fraction of additive adsorbed at the interface, the more stable is the resulting foam. Many of the most stable foaming systems were found to have surface layers composed of as much as 60-90 mol% additive. 

Another possible solution to the problem of high temperature stability is the use of additives. Not exactly a stranger to petroleum people (as evidenced by use in gasoline and lubricants) they generally fall into two classes metallic and non-metallic. The former, for the most part are metal salts of sulfonates or naphthenates, whereas the latter are either amines or amine derivatives (later other organics may prove more effective) Use of additives in jet fuels, however, must of necessity be approached with caution. As surface active materials, many have a variety of uses and properties. Hence, they must not introduce new problems such as foaming at high altitudes, emulsification, or interference with low temperature flow. These could easily be severe limitations, but additives are under serious consideration thruout the industry... 

Fatty acids and esters with limited water solubifity are also often used as foam inhibitors. Their mode of action is similar to that of the analogous alcohols. In addition, their generally low toxicity often makes them attractive for use in food apphcations. Organic compounds with multiple polar groups are, in general, found to be effective foam inhibitors. The presence of several polar groups generally acts to increase the surface area per molecule of the adsorbed foam breaker and results in a loss of stabilization. 

---