Foam formation

Question 1

What is a foam? And the differences with emulsions?

Answer 1

The dispersed phase is air, water-aire surface.

1. Air bubbles are much larger than emulsion droplets.
2. Density difference between dispersed and continuous phase is much larger in foam than in emulsions.
3. Solubility of gas in water is usually much higher than oil in water: this leads to faster creaming/disproportionation(= Ostwald ripening in emulsions)
4. Foam: air dispersed in water / Emulsions: oil droplets free to move within the continuous phase.

Question 2

The stability of the foams is related more to ….?

Answer 2

The film between the air bubbles and the interfacial properties.

Question 3

Production of foam:

Answer 3

1. Requieres the creation of a lot of surface area, you need a surface active agent to decrease the surface tension. These surface active agents have to diffuse fast to the surface in order to keep the aire bubbles stable as well as well surface coverage.

Question 4

What do we use as surface active agents?

Answer 4

*Proteins: the energy input determines how much protein goes to the surface, the amount of foam that can be made. It determines the volume of the foam, the bubble size –> Foamability.

Question 5

Foamability:

Answer 5

Changes with the pH. In the case of globular proteins, the highest stability is close to the iso-electric point and decreases for pH values further the pI where the charge is higher. For flexible proteins as caseins, stability is the highest far for the iso-electric point when the charge is high and lowest at the pI.

Question 6

Foam stability:

Answer 6

1. Interfaces are stable and the aire diffusion between the bubbles is limited.
2. Air bubbles are stuck in space and only separated by a thin aqueous film.
3. Due to the large density difference, the bubbles cream and the water drains down and this leads to a large increase in the volume fraction of the air bubbles. (max packing >74%).
4. Foams become unstable when the air is able to flow form one air bubble to another air bubble and finally disappear into the air. This can happen because disproportionation or by coalescence.
   The air bubbles will grow and the entrapped air volumen in the foam will decrease.

Question 7

How is the foam stability gives in a graphic?

Answer 7

Change of the volume over the time. Not say so much about the bubble size.

Question 8

Foam ability and foam stability: surface active agents differences

Answer 8

1. Foam formers: small molecules(fast diffusion), large hydrophobic groups (hydrophobicity, composition of proteins –>to absorb well to the interface)
2. Foam stabilizers: large molecules (thick interface, more stable–>prevent air diffusion ) protein coverage (effect of pH, salt).