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Emulsions - Improving stability of emulsions

Q: 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 Diffusion 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

Q: 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

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

Diffusion

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
Flocculation/aggregation
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