An emulsion is a two-phase system with one liquid dispersed as globules in another liquid, requiring a surfactant and energy to join the phases. Emulsions can be oil-in-water or water-in-oil. Common surfactants include anionic, cationic, nonionic, and proteins. Emulsions are thermodynamically unstable and surfactants help slow their breakdown over time. Factors like emulsifier choice, phase ratio, temperature, and processing method impact emulsion stability and type.

1. Emulsion Formulation
Overview
Jim McElroy

2. Definitions
 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
maximum fraction of small-diameter droplets is
usually more stable

8. Rheology
 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
Phase-Volume Ratio
Method of Manufacture
Temperature (processing and storage)
The better the emulsifying system the less
important the other factors

10. Processing
Method of Preparation
Order of addition
Rate of addition
Energy effects

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 surfactant relative 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
Hair gels
Dry Cleaning fluids

18. Common Preservatives
Ingestible & Topical
 Methyl, ethyl, propyl
and butylparabens
 Sorbic acid
 Na, K & Ca Sorbate
 Benzoic acid
 Na, K & Ca Benzoate
 Sodium metabisulfite
 Propylene glycol (15-
30%)
 BHT, BHA
 Flavors w/
benzaldehyde
Topical Only
 Formaldehyde donors
 Essential Oils
 Monoglyceride
 Phenol
 Mercury compounds

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)
 Esters
 Cationic and anionic surfactants
 Humectants (glycerin, propylene glycol)
 Phenolic antioxidants (BHT)
 Chelating agents (EDTA)
 Fragrances
 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,…)
 Vitamins
 Clay compounds
 High water activity
 Surfactants (Tween 80)

22. Conclusions
 Emulsions have unique chemistry and
physical properties. Understanding this
chemistry allows the formulator to
create a unique formulation that meets
end use requirements.