emulsions Flashcards
Definition of Emulsions & types of emulsions
An emulsion is a thermodynamically unstable system consisting of at least two immiscible liquid phases, one of which is dispersed as globules in the other liquid phase stabilised by a third substance called emulsifying agent
the composition of emulsions includes
- dispersed phase or discontinuous phase
normally lower volume (30-40%) - dispersion medium or continuous phase
phase with generally larger volume (60-70%) - emuslifier/s (amphiphiles)
-> around 5-10%
-> two parts: hydrophilic head and hydrophobic tail
identification of emulsion types : o/w vs w/o
oil in water:
- water = continuous phase
- oil = discontinuous phase
water in oil:
- oil - continuous phase
- water = discontinuous
an emulsifier will do what
stabilise the two liquids to mi
we use the HLB system to select an appropriate emulsifier/combination of emulsifiers
HLB =
Hydrophile - Lipophile Balance
more hydrophilic surfactant = [higher/lower] HLB value (>10) and forms [w/o…o/w] emulsuion
more hydrophilic surfactant = higher HLB value (>10) and forms o/w emulsuion
more lipophilic = higher/lower HLB value (1-10) and forms [w/o…o/w] emulsion
more lipophilic = lower HLB value (1-10) and forms w/o emulsion
So how do emulsifiers stabilise emulsions?
[Interfacial film formation]
- Emulsifiers form a rigid film on the oil/water
interface, which prevent them from separation. - While mixing, it is encouraged to mix ONLY in
one direction i.e. clockwise.
– If you start mixing in opposite direction then
it will disrupt the interfacial film and
emulsions will BREAK! due to instability
- Manufacture of emulsion
methods of emulsion preparation:
- cold emulsification
- emulsification by heat
cold emulsification method
- making primary emulsion
-> dry gum method
-> wet gum method - secondary emulsion
-> dilution with continuous phases
-> manual mixing (with pestle and mortar) - use of mechanical stirrers
-> affect globule size
dry gum method:
- emulgent placed in mortar
- oil added to the emulgent and dispersed to form mucilage
- water added at a time with rapid continuous titration
wet gum method: (mix in 1 direction)
- emulsifier placed in mortar
- water added in and mixed
- oil is added and mixed with the mixture of water and oil to form a thick primary emulsions (until cream mixture approx 15 mins, when you can’t anymore)
- add the continuous phase slowly (still mixing) to make up the volume)
what is emulsification by heat:
the mixing of both phases at the same temperature (60-70C)
steps of emulsification by heat
- Oil soluble components are dissolved in oil (evaporating basin) & water soluble components in water (in a beaker)
- Oil phase is heated to melt the components in evaporating basin (metal dish/evaporating basin)
- Both phases heated to 70C separately
- Internal phase added to external phase at the same temperature
- Initially mixed slowly to avoid air bubbles
- High speed mixing with decrease in temperature
how do we identify emulsion type (4)
- dilution test
- dye solubility test
- conductivity test
- fluorescence test
dilution test
- addition of oil to the emulsion:
if oil mixes well with emulsion - w/o emulsion
if oil appears as droplets - o/w emulsion
- addition of water to the emulsion
water mixes well - o/w emulsion
water appears as droplets - w/o emulsion
dye solubility test
- add a few drops of red dye in emulsion
emulsion turns red = a type of emulsion
whether the dye is water or oil soluble varies: we check by adding to 2 systems and confirming this.
one system - oil in water, this fully turns red
the other system is water in oil / this partially turns red
conductivity test
- water is a good conductor of electricity, oil does not
- 2 electrodes placed in emulsions
- electrodes connected to a low voltage lamp: electric current passes through the emulsion
- to check whether o/w or w/o, we can use a conductivity meter, if there is no bulb. if there is a number = oil in water
- note: cannot use deionised water: use potable water as it will give 0 on the conductivity meter tpp
fluorescence test
- use a microscope to show fluorescence of oils on exposure to UV light
- o/w emulsion = spotty pattern (individual drops)
- w/o emulsions fluoresce (lots of colour) throughout the field
as oil has a tendency to fluorescence
extra filter paper test
- put a droplet of the emulsion on the paper and see if it acts like water or oil
- water should spread nice/evenly across
- oil won’t really spread
Stability of emulsion and its prevention
– Effect of homogenisation on emulsion stability
– Globule size
– Viscosity
3 types of emulsion instability
- physical
- chemical
- microbiological
physical stability emulsion examples
- Gravitational separation (reversible)
– Creaming
– Sedimentation - Coalescence
- Flocculation
- Cracking/Breaking (irreversible)
creaming rate of isolated spherical can be predicted by stokes equation. this is….
2gr^2 (pd-pc) / 9nc
https://www.notion.so/week-11-emulsions-13500bb3982d805a8948d7c5482120d8?pvs=4#13600bb3982d802e8086f44cbccab414
factors affecting creaming/sedimentation
https://www.notion.so/week-11-emulsions-13500bb3982d805a8948d7c5482120d8?pvs=4#13600bb3982d800ba772f1742853f1ad
inc globule size = inc creaming
dec viscosity = inc creaming
prevention of creaming methods and why do they work
reduce droplet size (r) using a homogeniser
reduce density different (delta p) by adding oils that have greater density than water
increase continuous phase viscosity (n) by adding thickening or gelling agent e.g. methylcellulose
what is breaking/cracking
breaking: due to coalescence and creaming combined, the oil separates completely from the water so that it floats at the top in a single continuous layer
The HLB of a mix of surfactants can be calculated as follows:
HLBmixture = f . HLBA + (1-f) . HLBB
HLBmixture is the HLB of a mix of two surfactants
and HLBA and HLBB are the HLBs of surfactants A and B respectively.
you can decide which one is A and which is B, just mention it in your working out though
examples of HLB calculations
https://www.notion.so/week-11-emulsions-13500bb3982d805a8948d7c5482120d8?pvs=4#13b00bb3982d80548aaef90e9fb96d8e
