Emulsions

Jim McElroy

Lincoln, October 2008

DefinitionsAn 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 phasewater-in-oil (W/O) when the droplet is water and oil is

the external phase

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

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.

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

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

Droplet Size DistributionEmulsions 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

RheologyContinuous 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

Predicting O/W or W/O Emulsion

Important parameters include:

Choice of emulsifiersPhase-Volume RatioMethod of ManufactureTemperature (processing and storage)

The better the emulsifying system the less important the other factors

ProcessingMethod of Preparation

Order of additionRate of additionEnergy effects

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.

Rate of AdditionA significant improvement in the emulsion can sometimes

be seen by adding the aqueous phase at a slower rate.

Energy Effects (Processing)Emulsions can be sensitive to energy input

or energy removal from the systemCooling rate can impact the systemMechanical or heat energy will not overcome systemic

problems with a formula

Temperature Effects (Shelf Life)

Temperature can affect:The rheology of the systemThe HLB of the emulsifiersThe ability of the emulsifier to adsorb or desorb from the

droplet interfaceThe mechanical strength and the elasticity of the

interfacial film.

Pickering EmulsionIt 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

Micro EmulsionsOil, water and surfactantsHigh concentration of surfactant relative to

the oilSystem is optically clear fluid or gelPhases do not separate on centrifugationSystem forms spontaneously

Micro Emulsion ExamplesChildren's Vitamin dropsFlavoring oils in cream sodas or colasCarnuba wax floor polishesHair gelsDry Cleaning fluids

Commonly used 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

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.

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

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)

ConclusionsEmulsions have unique chemistry and

physical properties. Understanding this chemistry allows the formulator to create a unique formulation that meets end use requirements.