1. Experiment is contained in the WACKER's Experimental Kit.

Yes

 2. Experimental procedure has been modified

Yes, with major additions

 3. A separate experimental procedure has been devised

Yes

 4. Video clip available

Yes (wmv1, wmv2 or mov1, mov2)

 5. Flash animation available

Yes

 6. Other materials: Worksheet 3, Worksheet 10, Slide DH6

Silicones as Antifoams

TopDown 1 Materials, Chemicals, Time Needed
  • 2 vials with snap-on lids
  • 3 test tubes and test-tube holders
  • Glass tub
  • Paper clip
  • Dropping pipet

Allow a total of about one hour for the following experiments.

TopDown 2 Procedure and Observations

Variant A:
Take two small vials with snap-on lids, and fill each two-thirds with water. Add several drops of detergent. Close the lids and shake the vials vigorously. Observe how the foam changes as time passes.

Clear solutions are formed when detergents are added. When the vials are shaken, the upper third fills with foam. This initially consists of small spherical bubbles that gradually unite to form larger polyhedral bubbles.

Leave one of the vials containing foam to serve as a reference. To the other, add one drop of Antifoam Emulsion SILFOAM®SRE from WACKER's Experimental Kit. Observe the foam.

The foam disappears very quickly when the antifoam emulsion is added. The foam can be heard popping as the air escapes. A slightly hazy solution is formed. Shaking the vial again causes some foam to form, but it collapses immediately.

TopDown Variant B:
Fill a medium-sized glass dish with water and carefully place a paper clip (or sewing needle) on the surface of the water. Then add several drops of water from a pipet at the edge of the glass dish and observe the paper clip/needle. When water is added, the paper clip/needle continues to float on the water.

Now add several drops of the surfactant solution from Variant A. When the surfactant solution is added, the paper clip/needle slowly sinks further and further into the surface and then suddenly sinks to the bottom. Perform the same experiment on the second solution from Variant A (surfactant solution + antifoam emulsion). Again, the paper clip/needle sinks.

TopDown Variant C:
Thoroughly shake up a mixture of 2 ml water and 0.5 ml olive oil in a test tube. Place the test tube in the stand and observe how the liquids separate. Repeat the experiment on two further samples, by adding some surfactant solution from Variant A to one and some surfactant and antifoam solution from Variant A to the other.

Yellowish emulsions are formed on shaking. Without surfactant solution, the emulsion separates again quickly and an oil phase and a water phase are formed (see left test tube in photo above right) With surfactant solution, the emulsion lasts longer and no obvious separation occurs (middle test tube). The same observation is made for the solution containing antifoam agent. Unlike the pure surfactant solution, however, virtually no foam is formed (right test tube).

TopDown 3 Discussion of Results

Note: The theory behind the mechanism of surfactants and foam formation is discussed in Part 5 Supplementary Information. This section should perhaps be read before the discussion of Variants A to C.

Variant A:
Surfactant solutions tend to foam.
As observed in the experiment, the bubbles are first spherical and then polyhedral. Adding the antifoam emulsion destroys the foam and prevents further foaming. The low amount of antifoam added has no effect on the other properties of the surfactant solution, such as the stabilization of emulsions and the lowering of the surface tension of the water (see also Variants B and C).

TopDown Variant B:
The paper clip/needle floats on the water because the surface tension of the water is greater than the force of gravity acting on the paper clip.
The pond skater (see photo) owes its particular skills in this regard to surface tension.

Additional force (pressure from above on the paper clip/needle) or a reduction in the surface tension brought about by added surfactant, forces the paper clip/needle to sink.

TopDown Variant C:
Surfactants can stabilize oil-in-water emulsions because their structure allows the formation of grease/surfactant micells. That is why these emulsions are much more durable that the pure oil-in-water emulsion. Thanks to this effect (formation of grease/surfactant micells) and the lowering of the surface tension of the water, surfactants constitute the most important basic ingredient for detergents.

The surfactant solution containing antifoam agent behaves exactly like the surfactant solution, except for the slight foam formation. This is due to the very good spreading power of the silicone antifoam, which forces the surfactant particles out of the bubble skin and thereby causes them to collapse.

 

TopDown 4 Tips and Comments

  • For the paper clip/needle experiment, ensure that they are totally dry and clean, as otherwise they will sink immediately.
  • The experiments described here may be supplemented by those from the booklet accompanying WACKER's Experimental Kit and from the CHEMIE S II textbook (see 6 References) on the subject of surfactants.
  • The antifoam experiments described here supplement and deepen standard classroom surfactant experiments and address the practical requirements imposed on modern detergents.
  • The explanation of the mechanism underlying antifoams is a prime example of the relationship between particle structure and material properties and should also be taught as such.
  • The three variants lend themselves to classroom experiments and could, for example, be conducted and evaluated by the group learning method. The general theory could then be discussed by everyone at the end.

TopDown  5 Supplementary Information

These experiments serve to study the influence of surfactants on the surface tension of water and the resultant effects such as foam formation, emulsifiability etc. In this connection, the action of silicone fluids as antifoams is studied for its practical application.
Thus, silicone fluids are administered to cows suffering from bloat. Bloat is a swelling of the stomach that is caused by excessive foaming. The cause of the excessive foaming is plants that contain sapoic acid. These react with water and intestinal gas to produce a foam.
Silicones are especially useful here because, with a few exceptions, they have no toxic properties at all.

The structure and mechanism of surfactants
A force is generated at the water/air interface that is based on forces of attraction (dipole-dipole interactions and hydrogen bonds) between the water molecules. Since each water molecule at the surface interacts more strongly with the other water molecules below it, i.e. inside the liquid, and beside it in the boundary layer than it does with the molecules above it in the air, this leads to what is known as surface tension. It shows up as the attempt by the liquid to minimize its surface area.
Surfactants are substances that lower the surface tension of a liquid. As shown in the model of the surfactant particle, it has a hydrophilic and a hydrophobic part.

TopDown At the water’s surface, the surfactant particles align themselves such that the hydrophilic head projects into the water and the hydrophobic tail projects out of the surface of the water. Since the forces of attraction between the nonpolar hydrocarbon groups of the surfactant particles are much lower than between the water molecules, the surface tension of the water is lowered. Inside the liquid, micells form (see diagram below left).Emulsions are stabilized by surfactants, dirt particles from grease and oil are removed from surfaces and emulsified or dispersed in the liquid phase (see diagrams a and b).
TopDown Foam
The accumulation of surfactant particles at the surface of the water means that it becomes more and more like air, a fact which leads to the formation of foam bubbles (spherical foam – see diagram below left). Progressive drainage of the liquid causes the bubbles to become thinner and thinner. Consequently, they huddle closer together, causing mutual deformation and forming polyhedra (polyhedral foam – see experimental description, Variant A) until they eventually collapse.
The air bubbles remain stable, though, if the surfactant prevents the liquid from draining completely from the lamella.
There are applications for surfactants in which their tendency to foam is undesirable. For example, the cleaning action of modern washing machines relies critically on foam suppression.
Silicone fluids act as antifoams. Antifoams have a low surface tension, poor solubility in the medium to be defoamed and positive penetration and spreading coefficients. The particles (molecules) of the antifoam displace the surfactant molecules from the surface of the lamella and replace them with a new film of lower surface tension and lower cohesive forces (see diagram below right).
Their action can be enhanced by adding hydrophobic solids. In such cases, the antifoam liquid acts as a vehicle for transporting the solid particles into the foam lamella. There, they act as foreign bodies that both absorb the surfactant molecules and diminish the cohesive forces. Pyrogenic silica is an ideal solid for silicone fluids.
TopBottom  6 References
  • M. Tausch, M. von Wachtendonk (editors), CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg (1993), (1998)
  • M. Tausch, M. von Wachtendonk (editors), STOFF-CHEMIE S I, FORMEL-UMWELT, C.C. Buchner, Bamberg (1996), (1997)
  • W. Held et al., Learning by Doing – School Experiments with WACKER Products (handbook accompanying WACKER's Experimental Kit), Wacker Chemie AG, Munich, 2007
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