An emulsion is most accurately defined as a dispersion of liquid droplets in a second immiscible liquid. Temporary emulsions may be formed by mixing/agitating the two normally immiscible liquids, however, the stability of temporary emulsions produced in this way is poor. Emulsifiers are surface active materials (surfactants) that are used to assist in the formation of an emulsion and to stabilise the emulsion.
There are several different types of emulsions. They are loosely described by their phase relationship and/or by their appearance:
Oil-in-Water
Water-in-Oil
Multiple emulsions
Macro-emulsions
Micro-emulsions
The appearance of the emulsion is dependant upon the particle size of the
discontinuous phase.
Particle size is listed in nanometers (nm)
Particle size | Appearance |
|---|---|
> 1 | White |
0.1 -1.0 | Blue White |
0.05 - 0.1 | Translucent |
< 0.05 | Transparent |
Water in Oil or Oil in Water
The basic oil-in-water and water-in-oil emulsions have different physical characteristics, and are easily distinguished from one another:
Multiple Emulsions
Multiple Emulsions are complex polydispersed systems where both oil in water and water in oil emulsion exists simultaneously. They find application in areas such as foods, personal care products and agrochemical formulations.
Macroemulsions
Macroemulsions are emulsions in which their appearance is typically an opaque milky white or blue white emulsion. The emulsion particle size tends to be higher than 50nm. They can form spontaneously with limited energy input but may also require significant energy/shear input depending on the surfactant system and concentration. They are generally considered to be thermodynamically unstable but can be stabilised by the correct emulsifier selection.
Microemulsions
Microemulsions are emulsions where the particle size of the disperse phase is typically 5 - 50 nm, the emulsion is totally transparent. They generally form spontaneously with no need for energy input and are generally considered to be thermodynamically stable. They require significantly higher concentrations of emulsifier than a typical macroemulsion.
The basic characteristics of an emulsion system:
Emulsions are inherently unstable
To form emulsions, a large energy input is required
This can be a combination of mechanical and chemical energy
One key role of the surfactant/emulsifier is to lower the energy formation
The other key role of the surfactant/emulsifier is to stabilise the emulsion
Below is a simplistic view of an emulsion particle, protected by surfactant molecules partitioned at the interface of the internal and external phase.
Emulsifiers lower the interfacial energy between the two immiscible liquids, thereby helping the formation of an emulsion. For an emulsifier to be effective at reducing droplet size and stabilising an emulsion, it needs to be located at the interface. The emulsifier must not be too soluble in either phase, otherwise it will migrate to that phase. If the emulsifier migrates away from the interface, the emulsion is destabilised.
Emulsifier Selection
Required emulsion type, ie O/W, W/O, W/O/W
Nature of the oil phase, ie. polar, non-polar
Regulatory restrictions, ie. Food approved, biodegradability
Other factors to consider
The chemical “affinity” between surfactant hydrophobe and oil phase
Blends of surfactants are more effective than single emulsifier of correct HLB
Blends of anionics and nonionics can be more effective than either type alone
The possibility of chemical reaction between the emulsifiers and the oil phase, solutes in the oil phase, or solutes in the aqueous phase
Processes that indicate instability within an emulsion
Creaming
Flocculation or aggregation
Coalescence
Phase inversion
Ostwald ripening
Emulsion Breakdown Processes
External factors can also lead to breakdown of the emulsion
Temperature
Alters emulsifier solubility
Nonionics become less water-soluble with increasing temperature
Anionics become more water soluble with increasing temperature
HLB effectively decreases with increasing temperature, when the emulsifier system is solely nonionic
Surfactant moves away from oil/water interface thus destabilising emulsion
Water quality
Presence of electrolytes impact on surfactant solubility
Salting out solutes include NaOH, CaCO3, MgSO4
Certain anionics interact with hard water
Solubility changes cause interface partitioning changes
Anionic/nonionic systems resistant to hard water
Methods to increase the emulsion stability
Each of the individual causes of emulsion breakdown can be controlled by formulation technique and surfactant selection. Some corrections are easy to implement, some require a radical reformulation of the emulsion system
Reduce Creaming and Sedimentation by:
Density adjustment - type of solvent or oil in disperse phase
Lower mean particle size of the disperse phase - mode of formation of the emulsion
Increase viscosity of the continuous phase - addition of protective colloids, attention to structure of surfactant hydrophobe
Reduce Flocculation by:
Increasing surfactant concentration
Reinforce the entropic barrier - nonionic/polymeric surfactant species
Reinforce the electrostatic barrier - ionic surfactant species
Reduce Coalescence by:
Prevention of creaming
Increase the interfacial film thickness and elasticity
Use of polymeric stabilisers
Reduce Phase Inversion by:
Review and revise phase ratio
Increase phase inversion temperature
Review ionic/nonionic ratio
Increase EO number of nonionic component
Decrease carbon number of hydrophobe
Further Reading: The HLB Concept