Emulsion formulation overview
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Emulsion Formulation Overview
Jim McElroy
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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
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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
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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.
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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
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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
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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
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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
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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
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Processing
Method of Preparation Order of addition Rate of addition Energy effects
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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.
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Rate of Addition A significant improvement in the emulsion can
sometimes be seen by adding the aqueous phase at a slower rate.
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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
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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.
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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
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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
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Micro Emulsion Examples
Children's Vitamin drops Flavoring oils in cream sodas or colas Carnuba wax floor polishes Hair gels Dry Cleaning fluids
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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
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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.
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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
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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)
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Conclusions
Emulsions have unique chemistry and physical properties. Understanding this chemistry allows the formulator to create a unique formulation that meets end use requirements.