Emulsion formulation overview

Emulsion Formulation Overview

Jim McElroy

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

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 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

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

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

Processing

Method of Preparation Order of addition Rate of addition Energy 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 Addition A 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 system

Cooling rate can impact the system Mechanical or heat energy will not

overcome systemic problems with a formula

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.

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

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

Micro Emulsion Examples

Children's Vitamin drops Flavoring oils in cream sodas or colas Carnuba wax floor polishes Hair gels Dry Cleaning fluids

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

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)

Conclusions

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