Emulsions: Principles and preparation - Physical stability of emulsions - Emulsifiers

8 important questions on Emulsions: Principles and preparation - Physical stability of emulsions - Emulsifiers

What are the 2 mechanisms by how surfactants form a barrier to prevent two droplets to come very close?

  1. Steric hindrance
  2. Electrostatic repulsion
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What is the Bancroft rule?

Surfactant usually are dissolved in the continuous phase. As 2 droplets approach each other, the continuous phase gets expelled from the slit formed between the droplets. The continuous phase, while flowing out of the slit, drags surfactant molecules along the interface, leading to a concentration gradient of surfactant molecules at the interface. When surfactants are soluble in the continuous phase, this concentration gradients tends to be rapidly compensated by a flow of liquid coming in the slit, and the droplets separate.
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Is the adsorption of low molecular weight surfactants reversible?

Yes
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Is the adsorption of amphiphilic biopolymers (such as proteins) reversible?

No
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The adsorption of proteins is very time consuming, why?

  • Large molecules --> diffusion through the aqueous phase towards the interface is slowly
  • When it arrives at the interface --> the process of re-folding takes time as well
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Why are plant proteins not good emulsifiers?

They have a relatively low solubility at low or neutral pH and heavy processing necessary to obtain plant protein isolates.
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Mention different types of emulsifiers

  • Low molecular weight emulsifiers (surfactants)
  • Amphiphilic biopolymers
    • Proteins
    • Dairy proteins
    • Plant proteins
    • Polysaccharides
  • Particles
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How can particles act as an emulsifier?

Small particles that are not completely wetted by either oil or water have the tendency to absorb at the interface between the two phases. This can cause stabilisation of the interface: the particles partially stick out of the interface, giving steric hindrance for coalescence, just as non-ionic surfactants do. The colloidal particles form a very strong membrane around emulsion droplets. Emulsions stabilised with such particles are referred to as Pickering emulsions.
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