Chemistry Reference
In-Depth Information
TABLE 8.3. Effects of Organic Additives on the cmc and Foaming Characteristics
of Sodium 2-
n
-dodecylbenzene Sulfonate Solutions
(cmc)
cmc
Foam Volume
Additive
(g/L)
(%)
(mL at 2 min)
None
0.59
—
18
Lauryl glycerol ether
0.29
51
32
Laurylethanolamide
0.31
48
50
n-Decyl glycerol ether
0.33
44
34
Laurylsulfolanylamide
0.35
41
40
n-Octylglycerol ether
0.36
39
32
n-Decyl alcohol
0.41
31
26
Caprylamide
0.50
15
17
Tetradecanol
0.60
0
12
polymer may or may not affect the cmc of the surfactant, but it will undoubtedly
affect the rheology of the liquid phase. Since foam production is usually a rapid,
high-energy input process, new surface area is produced rapidly and the surfactant
molecules must diffuse rapidly to that new interface. A typical water-soluble poly-
mer will diffuse too slowly to be effective in foam formation; the great exceptions
are polymers that are themselves surface-active such as proteins and other struc-
tures mentioned in Chapter 7. If the polymer additive does not contribute to initial
foam formation, the increased solution viscosity in the lamellar film of the foam
will naturally retard drainage and therefore enhance foam persistence. If the poly-
mer can, over time, displace adsorbed surfactant from the air-liquid interface, the
added interfacial viscosity and monolayer rigidity will normally add to the stability
of the foam.
If polyelectrolytes are employed, the addition of di- and trivalent ions such as
calcium and aluminum may produce particularly rigid and stable foams. If the
foam is desirable, that is all well and good. In systems where foaming is not desir-
able, however, such effects can significantly complicate life. In wastewater treat-
ment plants, for example, the combination of proteins and metal ions such as
Al
3
þ
can produce particularly troublesome foams that can almost shut a system
down.
8.3. FOAM INHIBITION
Although the presence of some additives can enhance the foaming effectiveness and
persistence of a surfactant system, the properly chosen materials can also reduce or
eliminate foams. Such materials are termed ''foam inhibitors,'' if they act to prevent
the formation of foam, or ''foam breakers,'' if they increase the rate of foam
collapse. Foam breakers may include inorganic ions such as calcium, which coun-
teract the effects of electrostatic stabilization or reduce the solubility of many ionic
