Emulsions - Improving stability of emulsions
6 important questions on Emulsions - Improving stability of emulsions
What are 5 ways to improve the stability of an emulsion?
- Increase viscosity of continuous phase or give it a yield stress (slower creaming, slower coalescence) --> adding thickener
- Increase repulsion between droplets (electrostractic repulsion: pH, salt, or steric repulsion (polymer addition); slows aggregation, coalescence)
- Formation of a strong interfacial film (add proteins: slow coalescence)
- Smaller droplets (less creaming, but still coalescence)
- Reduce density difference between oil and water (less creaming) --> increase density oil phase, or decrease density aqueous phase
Emulsion stability is a combination of:
- Emulsion formation, droplet size determined by
- Low interfacial tension
- Fast diffusion to interfaces
- Interfacial thickness needs to prevent coalescence, determined by size of surfactants
- Small proteins --> fast diffusion --> good emulsion formation
- Large proteins --> slow diffusion --> may prevent formation
- Thickness of interface
- Small proteins -> thin interface --> may not prevent coalescence
- Large proteins --> more space between oil droplets --> prevent coalescence
Effect of pH on droplets
- Low charge (close to pI)
- No repulsion between proteins
- Closer packing at interface
- Lot of proteins needed
- No repulsion between oil droplets
- Dependent on strength of the interface, interface may also rupture and coalescence may take place
- High charge (low pH or high pH)
- More repulsion between proteins
- Less protein coverage
- Due to charge of proteins --> electrostatic repulsion between oil droplets
- Droplets cannot approach each other, less aggregation/flocculation, prevents coalescence and slows down creaming
Electrostatic repulsion causes stabilisation --> electrostatic stabilization.
What does increasing the interfacial thickness lead to?
- Increase repulsion between droplets (steric repulsion)
- Stronger interfacial film
Will prevent coalescence, as droplets cannot merge.
How can a the interfacial thickness be increased?
- Proteins as emulsifier --> aggregated proteins is thicker film
- Combination of proteins and polysaccharides --> polysaccharides are larger, so thicker interface together with proteins. The PS are often not surface active themselves, so need proteins.
How can the interfacial thickness be increased by protein/polysaccharide co-absorption?
- Layer-by-layer --> first proteins on interface, then PS added (secondary layer). Attractive interaction needed between proteins and PS. Steric stabilization to prevent coalescence.
- Preformed protein/polysaccharide mixtures --> proteins and PS missed together before emulsification. Attractive interaction proteins and PS --> complex formation
- Complexes --> 50-100 nm --> good size to diffuse to the interface and stabilise
- Coacervates --> >100 nm --> too large to diffuse to the interface and stabilize
- Then oil added --> complexes/coacervates have to diffuse to the interface and absorb there --> slow diffusion due to large size --> difficult to form an emulsion
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