Chemistry Reference
In-Depth Information
Fatty acids and esters with limited water solubility are also often used as foam
inhibitors. Their mode of action is similar to that of the analogous alcohols. In addi-
tion, their generally low toxicity often makes them attractive for use in food appli-
cations. If the acids are neutralized to soaps, however, their antifoaming usefulness
disappears. 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 antifoaming material and
results in a loss of stabilization.
Metallic soaps of carboxylic acids, especially the water-insoluble polyvalent
salts such as calcium, magnesium, and aluminum can be effective as defoamers
in both aqueous and nonaqueous systems. In water, they are usually employed as
solutions in an organic solvent, or as a fine dispersion in the aqueous phase. Water-
insoluble organics containing one or more amide groups are found to be effective
antifoaming agents in a number of applications, especially for use in boiler systems.
It is generally found that greater effectiveness is obtained with materials containing
at least 36 carbon atoms compared to simple fatty acid amides. An example of such
a material would be distearoylethylenediamine:
C
17
H
35
CONHCH
2
CH
2
NHOCC
17
H
35
Alkyl phosphate esters are found to possess good antifoaming characteristics in
many systems because of their low water solubility and large spreading coefficient.
They also find wide application in nonaqueous systems such as inks and adhesives.
Organic silicone compounds are also usually found to be outstanding antifoaming
agents in both aqueous and organic systems. Because of their inherently low surface
energy and limited solubility in many organics, the silicone materials constitute one
of the two types of material that are available to modify the surface properties of
most organic liquids.
The final class of materials that have found some application as antifoaming
agents are the fluorinated alcohols and acids, which are related to those discussed
earlier. As a result of their very low surface energies, they are active in liquids
where the hydrocarbon materials have no effect.
8.5. A SUMMARY OF THE FOAMING AND ANTIFOAMING
ACTIVITIES OF ADDITIVES
As pointed out above, the mechanisms of action of foaming and antifoaming mate-
rials are quite often, understandably, opposite in nature. Foam stabilizers, for exam-
ple, may increase the surface viscosity, leading to slower liquid drainage and lower
gas permeability, or they may lower the critical micelle concentration of the pri-
mary foaming agent. They may also enhance the ''healing'' effects related to sur-
face elasticity and the Gibbs-Marangoni effect. Antifoaming agents, on the other
hand, may decrease surface viscosity and elasticity by displacing or disrupting
the structure formed by adsorbed surfactant molecules. In addition, they can retard
micelle formation and otherwise alter the surfactant properties of the system.
