Lincoln, October 2008
An emulsion is a two phase system consisting of two incompletely miscible liquids, one of which is dispersed as finite globules in the other.The particle size of the globules range from 0.1 to 10 microns.A surfactant system and mechanical energy are needed to join the phases.
Emulsions are usually referred to as:
oil-in-water (O/W) when the droplet is oil and water is the external phase
water-in-oil (W/O) when the droplet is water and oil is the external phase
3. Common Surfactants
Anionic - hydrophilic group has an anionic charge e.g. soaps, shampoo, detergents
Cationic - have a cationic charge e.g. preservatives, conditioners
Nonionic - no charge e.g. food additives
Amphoteric - contains two oppositely charged groups e.g. lysergic acid, psilocybin
Finely Divided Solids e.g. clays, bentonite (called a Pickering Emulsion)
Proteins - e.g. casein, egg yolks
Naturally Occurring e.g. lanolin, lecithin, acacia, carrageen and alginates
4. Emulsions are Thermodynamically Unstable
Emulsions are inherently unstable. All emulsions coalesce to reduce the total free energy of the system
the emulsion breaks
Surfactants facilitate the production of the emulsion and more importantly slow down its inevitable destruction.
5. Free Energy
Nature wants to reduce the value of free energy to zero.This is accomplished by a combination of 3 mechanisms.
Reduction in the total amount of interface.
Water drips in the shape of a sphere
Emulsions eventually coalesce
Foams eventually break
6. Free Energy
- Molecules at an interface will align in the easiest transition between two bulk phases.
In a solution of water , surfactant molecules align so that its polar groups are immersed in water and its chains are sticking out into the air phase
In an oil/water dispersion, surfactant molecules align so that its polar groups are immersed in water and its chains are sticking out into the oil phase
7. Droplet Size Distribution
Emulsions change their size distributions over time with the average droplet size shifting to larger values
A sharply defined distribution containing a the maximumfraction of small-diameter droplets is usually more stable
Continuous Phase: O/W emulsion can be partially controlled by clays and gums W/O emulsion by the addition of high-melting waxes and polyvalent metal soaps
Internal Phase:No impact to final emulsion viscosity
Droplet Size & Dist:The viscosity of emulsions having similar size distributions about a mean diameter is inversely proportional to the mean diameter
9. Predicting O/W or W/O Emulsion
Important parameters include:
Choice of emulsifiers
Method of Manufacture
Temperature (processing and storage)
The better the emulsifying system the less important the other factors
Method of Preparation
Order of addition
Rate of addition
11. Order of Addition
Placement of surfactants:
Ideally, lipophillic surfactant should be dispersed in the oil phase.Finer emulsions result when the hydrophilic surfactant is also dispersed in the oil phase.
Oil to water or water to oil:
If processing permits, addition of aqueous to the oil phase produces the finest emulsions.
If the oil phase is added to the aqueous phase, more energy will be required to produce small droplets.
12. Rate of Addition
A significant improvement in the emulsion can sometimes be seen by adding the aqueous phase at a slower rate.
13. Energy Effects (Processing)
Emulsions can be sensitive to energy input or energy removal from the system
Cooling rate can impact the system
Mechanical or heat energy will not overcome systemic problems with a formula
14. Temperature Effects (Shelf Life)
Temperature can affect:
The rheology of the system
The HLB of the emulsifiers
The ability of the emulsifier to adsorb or desorb from the droplet interface
The mechanical strength and the elasticity of the interfacial film.
15. Pickering Emulsion
It is an emulsion that is stabilized by solid particles (for example colloidal silica) which adsorb onto the interface between the two phases.
Generally the phase that preferentially wets the particle will be the continuous phase in the emulsion system.
Sunscreens fall typically into this category
16. Micro Emulsions
Oil, water and surfactants
High concentration of surfactantrelative to the oil
System is optically clear fluid or gel
Phases do not separate on centrifugation
System forms spontaneously
17. Micro Emulsion Examples
Children's Vitamin drops
Flavoring oils in cream sodas or colas
Carnuba wax floor polishes
Dry Cleaning fluids
18. Commonly used preservatives
Ingestible & Topical
Methyl, ethyl, propyl and butylparabens
Na, K & Ca Sorbate
Na, K & Ca Benzoate
Propylene glycol (15-30%)
Flavors w/ benzaldehyde
19. Chelating Agents as Preservative Enhancers
Alkaline earth metals such as Ca+ and Mg+ are important for the stabilization of the outer membrane of cellular organisms.Chelating agents sequester these ions.This contributes to the partial solubilization of the cell membrane which allow preservatives a pathway into the cell.EDTA is a typical chelating agent used in formulations.
20. Ingredients That Enhance Preservative Efficacy
Solutes (salts & high concentration of sugars)
Cationic and anionicsurfactants
Humectants (glycerin, propylene glycol)
Phenolic antioxidants (BHT)
Chelating agents (EDTA)
Low water activity
21. Ingredients That Hinder Preservative Efficacy
Sugars and alcohol sugars
Proteins, peptides, yeast extract
Natural gums & cellulose thickeners
Plant extracts (aloe vera, starch,)
High water activity
Surfactants (Tween 80)
Emulsions have unique chemistry and physical properties.Understanding this chemistryallows the formulator to create a unique formulation that meets end use requirements.