Emulsions Flashcards
What is an emulsion?
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 an emulsifying agent
Different phases of emulsions (3)
- Dispersed phase or discontinuous phase - normally lower volume, 30-40%
- Dispersion medium or continuous phase - phase with generally larger volume (60-70%)
- Emulsifiers / amphiphiles - around 5-10%; two parts, hydrophilic head and hydrophobic tail
Types of emulsions (2)
o/w
w/o
Emulsifying agents:
One VS combination
One: Significantly high amount needed
Combination of emulsifiers: Lower conc. required, better protection against instability
How to select an appropriate emulsifier / combination of emulsifiers?
Using HLB system
Hydrophile - Lipophile Balance
Used to classify non-ionic surfactant
All non-ionic surfactants have a HLB value
Higher HLB = More hydrophilic
Lower HLB = More lipophilic
What HLB value and type of emulsion do hydrophilic surfactants form?
High HLB value (>10)
Form O/W emulsion
What HLB value and type of emulsion do hydrophobic surfactants form?
Low HLB value (1-10)
Form W/O emulsion
How do emulsifiers stabilise emulsions?
Interfacial film formation
* Emulsifiers form a rigid film on the oil/water interface, which prevents separation
* While mixing, it is encouraged to mix ONLY in one direction
Methods of emulsion preparation (2)
- Cold emulsification
- Emulsification by heat
How does cold emulsification work?
a. Making primary emulsion
i. Dry gum method
1) Emulgent is placed in the mortal
2) Oil is added to the emulgent and is dispersed to form mucilage
3) Water is added at a time with rapid continuous titration
ii. Wet gum method
1) Emulsifier is place in a mortar
2) Water is added to emulsifier and mixed
3) Oil is added and mixed with mixture of water and oil to form a thick primary emulsion
4) Stop when it becomes creamy mixture and at the clicking sound
5) Add the continuous phase slowly with continued mixing to make up the volume
b. Secondary emulsion
i. Dilution with continuous phase
ii. Manual mixing (pestle and mortar)
c. Mechanical stirrers
i. Affect globule size
How does emulsification by work?
a. Heating of 2 phases
i. Oil soluble components are dissolved in oil (evaporating basin)
ii. Water soluble components in water (in a beaker)
iii. Oil phase is heated to melt the components in evaporating basin (metal dish / evaporating basin)
iv. Both phases headed to 70 degrees Celsius separately
v. Internal phases added to external phase at the same temperature
vi. Initially mixed slowly to avoid air bubbles
vii. High speed mixing with decrease in temperature
Extemporaneous dispensing Practice (Aqueous cream)
Dilution test
a. Addition of oil to the emulsion
i. Oil mixed well with emulsion w/o emulsion
ii. Oil appears as droplets - o/w emulsion
b. Addition of water to the emulsion
i. Water mixed well - o/w emulsion
ii. Water appears as droplets - w/o emulsion
Dye solubility test
a. Add few drops of Red dye in emulsion
i. Emulsion turns red
ii. Dye water or oil soluble
iii. How to identify type of dye
b. Continuous phase appears red - emulsion type?
c. No change in colour - emulsion type?
Conductivity test
a. Water is a good conductor of electricity
b. Oils do not conduct electricity
c. Two electrodes placed in emulsion
d. Electrodes connected to a low voltage lamp
e. Electric current passes through emulsion
f. Which emulsion will conduct electricity?
Fluorescence test
a. Use microscope
b. Fluorescence of oils on exposure to UV light
c. O/W emulsions exhibit spotty pattern
d. W/O emulsions fluoresce throughout the field
Types of emulsion stability
-
Physical
a. Gravitational separation (reversible) - creaming VS sedimentation
b. Coalescence
c. Flocculation
d. Cracking / Breaking (irreversible) - Chemical
- Microbiological
How to prevent creaming
Reduce droplet size - homogeniser
Reduce density difference - add oils that have a density greater than the density of water
Add thickening or gelling agent e.g. methylcellulose
Creaming definition
Formation of upward or downward layer
Properties of creaming:
1. Reversibility
2. Agitation
3. Emulsifying firm
4. Internal phase globules
5. Effect of phase volume ratio
- Reversible
- Reconstitute
- Intact
- Partial or no coalescence
- No or little
Breaking definition
Separation of emulsion to upward oily layer and downward aq layer
Properties of breaking:
1. Reversibility
2. Agitation
3. Emulsifying firm
4. Internal phase globules
5. Effect of phase volume ratio
- Irreversible
- Not reconstituting
- Destroyed
- Complete fusion
- in o/w if oil >74%
Summary
- Emulsions are made up of oil, water and emulsifying agent.
- Two types: o/w/ and w/o
- Emulsion manufactured by cold emulsification or by heating of two phases
- Identification of emulsions by conductivity, dye solubility and dilution tests
- Types of emulsion instability and their mechanisms
- Strategies for Prevention of emulsion instability
