Emulsions Flashcards
Define: Emulsion
Dispersion of two or more immiscible liquids in which one of the other liquids is dispersed in other as small droplets
Define: Foam
Air bubbles dispersed in a liquid
How do we classify emulsions
Based off their spatial distribution, i.e what’s in what
What does this image depict
Oil in water (o/w) e.g. salad dressings
What does this image depict
Oil in water in oil (o/w/o)
What does this image depict
Water in oil (w/o) e.g. margarines
What does this image depict
Water in oil in water (w/o/w)
What are the applications for emulsions
- Fat reduction (water in oil) - Encapsulation
What effects the type of emulsion formed
- Concentration - Type of emulsifier
What is the disperse phase
Droplets
What is the continuous phase
What the droplets are in
Why are emulsions thermodynamically unstable
All emulsions have different levels of stability. this is because we have increased the surface area
Stable emulsion
Droplets that don’t change size and are homogeneously dispersed throughout
What defects can occur in an oil in water emulsion
Creaming, flocculation, coalescence, phase seperation
What is Ostwald ripening
Smaller droplets in solution dissolve and deposit on larger droplets to reach a more thermodynamically stable state where they have a reduces surface area: volume ratio. Irreversible instability. requires the dissolution of the soluble oil phase
What defects can occur in water in oil emulsions
Flocculation, coalescence, sedimentation (instead of phase separation due to density difference)
How can we prevent / slow the instability of an emulsion
- Thickeners e.g. xanthan , carrageenan. increases viscosity so the droplets cant flow as much - Decrease droplets size (Stokes law). Smaller droplets will take longer to rise - Change the emulsifier to impact the charge on the droplet - disperse emulsion to prevent Ostwald ripening.
What defects can occur in foams
- Drainage - Ostwald ripening - Coalescence (promoted by Ostwald ripening and film drainage)
How can we prevent / slow the instability of a foam
- Increase the continuous phase viscosity or solidify it - Smaller bubble size - Gas solubility - Choice of surfactant / emulsifier - Very wet / dry limit
Name three types of emulsifiers
- Amphiphilic molecules (surfactant) - Polymeric emulsifiers (proteins) - Particulate emulsifiers
Describe how an amphiphilic molecule functions as an emulsifier
Hydrophilic head binds with water Hydrophobic tail binds with the oil. Surrounds the droplet
What occurs when to much emulsifier is added
Emulsifier micelle is formed as it is more thermodynamically favourable
Name four surfactants
- Mono and diglycerides of fatty acids - Lecithin - Polysorbates - Citric acid esters
What is the HLB
- Hydrophobic Lipophilic Balance
Why is knowing the HLB important
- Predicts the behaviour of an emulsifier - Helps select an emulsifier appropriate fir intended application - Hydrophobic emulsifiers stabilise w/o emulsions - Hydrophilic emulsifiers stabilise o/w - This is because of the arrangement at the interface
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Why are polymeric emulsifiers better foam stabilisers than small molecule surfactants
They are larger molecules so when they dissolve they create a thicker, more viscoelastic solution. This thick interface makes it harder for the bubbles to move therefore increases stability. As a result it can cope with greater changes in the microstructure
How to particulate emulsifiers create a more stable emulstion
Absorb at the interface forming different structures depending on the type of particle. e.g. fat crystals
How does competitive absorption affect a product
Competition for the interface which can impact the size and stability of a system and therefore shelf life stability and product quality
List pro’s and cons of each of the types of emulsifier
- Surfactants: smaller emulsion droplets formed. Can be less stable - Proteins: larger droplets, more stable - Particles: cannot make particles small enough so we get larger droplets. Therefore more creaming, coalescence, oily mouthfeel. Different flavour profiles due to the way they’re digested.
What needs to be considered when selecting an emulsifier
- Microstructure - Stability - Label declaration.
Define: Interfacial tension
Energy required to increase the size of the interface between two adjacent phases which do not mix completely with each other
What are the two main methods of measuring surface tensions
- Optical. - Force tensiometry
What in a homogeniser affects the final microstructure
- Type of homogeniser - The flow of fluid - Composition - Emulsifier amount - How much energy we put in
What factors need to be considered when considering droplet breakup and microstructure
- Viscosity ratio - Type of homogenizer (this is determined by the material properties) - flow fields e.g. inertia forces and cavitation
What equipment can be used to make emulsions and foams
- Rotor-stator. sheer creates smaller droplets. Makes a course emulsion - Ultrasound continuous emulsification - High pressure homogenisation - Membrane emulsification
Name the advantages and disadvantages of a rotor-stator
- Simple setup - Low operational cost - All scales - combination of steps, e.g mix pasteurise emulsify and cool - >1micron - Variance in droplet size - Long process time can cause off flavours to be produced
Name the advantages and disadvantages of a high pressure homogenisator
- <1 micron - Requires a course emulsion to begin - Can be processed numerous times to reduce the droplet size
Explain the energy input vs droplet size balance
- Small droplet size requires high energy input which is expensive. - More energy causes heat generation which can cause undesirable affects
How can we measure an emulsion
- Droplet size and size retention. affects creaming and stability - Viscosity - Texture - Zeta potential (measure charge)
Give two examples of an oil-in-water emulsion.
- Cream
- Mayonnaise
Give two examples of a water-in-oil emulsion.
- Butter
- Margarine
Define:
Creaming
The upward movement of droplets due to having a lower density than the surrounding liquid.
Define:
Sedimentation
The downward movement of droplets due to having a higher density than the surrounding liquid.
Define:
Flocculation
Two or more droplets coming together to form an aggregate in which the droplets retain individual integrity.
Define:
Coalescence
Two or more droplets come together to form a larger droplet.
As the concentration of PGPR increases, the interfacial tension decreases.
Why is there a plateau at 100?
At this point, interfacial tension is not reduced any further with increasing amounts of PGPR because the interface is saturated with surfactants.
