Chemistry Reference
In-Depth Information
voir for extended defoaming action by adsorption as a surface monolayer. When the
solubilization limit was reached, additional defoaming effect was obtained by the
lens spreading mechanism.
So far the discussion of foams and defoaming has centered on aqueous systems.
While most organic liquids will do not form stable foams, the presence of polymers
and oil-soluble amphiphiles can result in persistent foams, whether wanted or
unwanted.
A classical and commonly encountered example of unwanted foaming in organic
liquids is that of used frying oil. While a fresh vegetable oil or fat will not form
foam, the frying process brings about several chemical changes that result in foam-
ing systems. One effect is that the components of the material being fried, espe-
cially water, will slowly bring about saponification of the oil to produce free
fatty acids and monodiglycerides in the system. Both materials are amphiphilic,
of course. If neutralization of the fatty acids occurs, soap is formed. A second effect
of the frying process is oxidation of the unsaturated fatty acid chains. At frying tem-
peratures, the polyunsaturated acids such as linoleic and linolenic are particularly
susceptible to oxidation, introducing peroxide and hydroxyl groups that enhance the
amphiphilic character of the materials with the observed result. Such reactions also
produce unwanted flavor changes in the fried product, of course. For those reasons,
among others, frying oils have limited useful lifetimes and are often stabilized by
the addition of antioxidants.
Another basically nonaqueous system that tends to produce unwanted foam is
that of lubricating oils. Many such products contain amphiphilic materials for the
purposes of reducing friction and corrosion control. However, since the oil is
recycled, the presence of foam can foul the recycling mechanism. It has been
found that silicone fluids, some of the few materials having the required character-
istics of limited solubility and adequate surface tension lowering in organic liquids,
act as foaming agents below their solubility limit, but inhibited foam formation
when that limit was exceeded.
8.4. CHEMICAL STRUCTURES OF ANTIFOAMING AGENTS
Materials that are effective as antifoaming or defoaming agents can be classified
into eight general chemical classifications, with the best choice of material depend-
ing on such factors as cost, the nature of the liquid phase, and the nature of the
foaming agent present. One of the most common classes of antifoaming agents is
polar organic materials such as highly branched aliphatic alcohols. As noted earlier,
linear alcohols, in conjunction with surfactants, can increase foam production and
stability due to mixed monolayer formation and enhanced film strength. The
branched materials, on the other hand, reduce the lateral cohesive strength of the
interfacial film, which increases the rate of bubble collapse. The higher alcohols
also have limited water solubility and are strongly adsorbed at the air-water inter-
face, displacing surfactant molecules in the process.
