Foam - Improving stability of foams

12 important questions on Foam - Improving stability of foams

How can you improve the stability of a foam?

  1. Formation of dense films (close packing the surface active agents, barrier to coalescence and disproportionation)
  2. Formation of strong interfacial films (proteins and polysaccharides, slows coalescence)
  3. Increase viscosity and thickness of aqueous film (thicker film, slower drainage)

How are dense films with globular proteins created?

Using globular proteins (whey protein, ovalbumin) --> When the surface/interface is not completely covered, air flow takes place and Ostwald ripening and coalescence can occur. There should be a low electrostatic repulsion between the proteins, to get a high surface coverage. This is very sensitive to changes in pH and salt.

How are dense films with flexible proteins created?

Flexible proteins (caseins/gelatin) --> have a hydrophilic and hydrophobic side.

  • Hydrophobic ends at interface as they are surface active, are responsible for surface coverage.
  • Hydrophilic ends stick out in the aqueous phase. These groups form an "additional layer" in the aqueous phase
  • Stability of foam determined by the size of the layer/thickness of film between the air bubbles
    • Flexibility of the chains
    • Repulsion between the chains (pH, salt)
      • Repulsion higher --> chains become more straight --> thicker film --> larger steric repulsion

What is the effect of gravity in foams?

Gravity pulls down liquid and air bubbles rise.

  • Drainage --> very large molecules (protein aggregates, complexes, etc.) drain down through aqueous films due to gravity --> less surface coverage --> diffusion of air --> coalescence
  • Large molecules also drag down liquid with them --> amount of water in the foam decreases --> liquid layer underneath the foam will be formed

How can you create a strong interfacial film?

Proteins + polysaccharides
  • Difficult to add polysaccharide after formation --> so needs to be present already in the liquid before foam formation
  • Proteins need to be small enough to diffuse to the interface --> to make the foam
  • Solution needs to be liquid (not too thick and viscous) to mix in air (incorporate air) --> PS lead to increase in viscosity so amount of PS than can be added is limited
  • PS not surface active --> need to be attached by proteins
  • Preformation of complexes by electrostatic attraction

What kind of complexes of proteins + polysaccharides have to be made to create a strong interfacial film?

  • Complexes (50-100 nm) --> small enough to diffuse to interface --> PS co-adsorbed at interface, where they provide a strong network
    • Network formation: stronger film (elastic)
    • Increase in size of interfacial layer --> limits coalescence and disproportionation
  • Coacervates (>100 nm) --> larger so diffuse slower to the interface --> foam formation more difficult. If they are at the interface, will drain down quickly as they are heavy. Foam formation and foam stability is low for coacervates

How can the viscosity of the aqueous phase be increased to get a more stable foam?

Addition of polysaccharides.
  • Polysaccharides need to stay in the aqueous film (no interaction with the surfactant/protein, requires similar charge or no charge).
    • Size/thickness of the aqueous liquid film
    • Viscosity of the film (diffusion)
  • Need to be stuck in the aqueous film, not drain down (to prevent drainage)

What is the size/thickness of the aqueous liquid film dependent on?

  • Concentration of polysaccharides
  • Water holding capacity of the polysaccharides --> Higher water holding capacity will keep more water in aqueous film, more water --> thicker aqueous film, PS can easily stay in the film
  • Charge density --> higher charge: more space between polysaccharides (repulsion), can only be done when more water is hold in the film --> larger aqueous film

Does addition of polysaccharides in the aqueous film always lead to a better stability?

  • More stable --> thicker aqueous film due to polysaccharides being in there
  • Less stable
    • When there is repulsion between PS and proteins --> PS pushed out of aqueous film and drain down
    • Proteins first have to diffuse to the interface, when there is repulsion between PS and proteins --> proteins less mobile, PS may prevent proteins to diffuse to the interface --> more difficult to form the foam and if is formed there is less surface coverage

What do surface active components do to make an emulsion/foam more stable?

They interact with both hydrophilic and lipophilic phases --> lower the surface tension of the interface

How can emulsion/foams be made more stable?

  • Increasing continuous phase viscosity
  • Increasing repulsion
  • Decreasing droplet size
  • Decreasing density difference
  • Making stronger interfacial films

What is the life span of emulsions and foams?

Emulsions have a life span of months or even years, foam have a short life span (minutes).

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