Emulsions Formulation Overview

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A brief overview of emulsion chemist


<ul><li>1.Emulsions<br />Jim McElroy<br />Lincoln, October 2008<br /></li></ul> <p>2. Definitions<br />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.<br />Emulsions are usually referred to as:<br />oil-in-water (O/W) when the droplet is oil and water is the external phase<br />water-in-oil (W/O) when the droplet is water and oil is the external phase<br />3. Common Surfactants<br />Anionic - hydrophilic group has an anionic charge e.g. soaps, shampoo, detergents<br />Cationic - have a cationic charge e.g. preservatives, conditioners<br />Nonionic - no charge e.g. food additives<br />Amphoteric - contains two oppositely charged groups e.g. lysergic acid, psilocybin<br />Finely Divided Solids e.g. clays, bentonite (called a Pickering Emulsion)<br />Proteins - e.g. casein, egg yolks<br />Naturally Occurring e.g. lanolin, lecithin, acacia, carrageen and alginates<br />4. Emulsions are Thermodynamically Unstable<br />Emulsions are inherently unstable. All emulsions coalesce to reduce the total free energy of the system<br />the emulsion breaks<br />Surfactants facilitate the production of the emulsion and more importantly slow down its inevitable destruction.<br />5. Free Energy<br />Nature wants to reduce the value of free energy to zero.This is accomplished by a combination of 3 mechanisms.<br />Reduction in the total amount of interface.<br />Water drips in the shape of a sphere<br />Emulsions eventually coalesce<br />Foams eventually break<br />6. Free Energy<br /></p> <ul><li>Molecules at an interface will align in the easiest transition between two bulk phases.</li></ul> <p>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<br />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<br />7. Droplet Size Distribution<br />Emulsions change their size distributions over time with the average droplet size shifting to larger values<br />A sharply defined distribution containing a the maximumfraction of small-diameter droplets is usually more stable<br />8. Rheology<br />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<br />Internal Phase:No impact to final emulsion viscosity<br />Droplet Size &amp; Dist:The viscosity of emulsions having similar size distributions about a mean diameter is inversely proportional to the mean diameter<br />9. Predicting O/W or W/O Emulsion<br />Important parameters include:<br />Choice of emulsifiers<br />Phase-Volume Ratio<br />Method of Manufacture<br />Temperature (processing and storage)<br />The better the emulsifying system the less important the other factors<br />10. Processing<br />Method of Preparation<br />Order of addition<br />Rate of addition<br />Energy effects<br />11. Order of Addition<br />Placement of surfactants: <br />Ideally, lipophillic surfactant should be dispersed in the oil phase.Finer emulsions result when the hydrophilic surfactant is also dispersed in the oil phase.<br />Oil to water or water to oil:<br />If processing permits, addition of aqueous to the oil phase produces the finest emulsions.<br />If the oil phase is added to the aqueous phase, more energy will be required to produce small droplets.<br />12. Rate of Addition<br />A significant improvement in the emulsion can sometimes be seen by adding the aqueous phase at a slower rate.<br />13. Energy Effects (Processing)<br />Emulsions can be sensitive to energy input or energy removal from the system<br />Cooling rate can impact the system<br />Mechanical or heat energy will not overcome systemic problems with a formula<br />14. Temperature Effects (Shelf Life)<br />Temperature can affect:<br />The rheology of the system<br />The HLB of the emulsifiers<br />The ability of the emulsifier to adsorb or desorb from the droplet interface<br />The mechanical strength and the elasticity of the interfacial film.<br />15. Pickering Emulsion<br />It is an emulsion that is stabilized by solid particles (for example colloidal silica) which adsorb onto the interface between the two phases. <br />Generally the phase that preferentially wets the particle will be the continuous phase in the emulsion system.<br />Sunscreens fall typically into this category<br />16. Micro Emulsions<br />Oil, water and surfactants<br />High concentration of surfactantrelative to the oil<br />System is optically clear fluid or gel<br />Phases do not separate on centrifugation<br />System forms spontaneously<br />17. Micro Emulsion Examples<br />Children's Vitamin drops<br />Flavoring oils in cream sodas or colas<br />Carnuba wax floor polishes<br />Hair gels<br />Dry Cleaning fluids<br />18. Commonly used preservatives <br />Ingestible &amp; Topical<br />Methyl, ethyl, propyl and butylparabens<br />Sorbic acid <br />Na, K &amp; Ca Sorbate<br />Benzoic acid<br />Na, K &amp; Ca Benzoate<br />Sodium metabisulfite<br />Propylene glycol (15-30%)<br />BHT, BHA<br />Flavors w/ benzaldehyde<br />Topical Only<br />Formaldehyde donors<br />Essential Oils<br />Monoglyceride<br />Phenol<br />Mercury compounds<br />19. Chelating Agents as Preservative Enhancers<br />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.<br />20. Ingredients That Enhance Preservative Efficacy<br />Solutes (salts &amp; high concentration of sugars)<br />Esters<br />Cationic and anionicsurfactants<br />Humectants (glycerin, propylene glycol)<br />Phenolic antioxidants (BHT)<br />Chelating agents (EDTA)<br />Fragrances<br />Low water activity<br />21. Ingredients That Hinder Preservative Efficacy<br />Sugars and alcohol sugars<br />Proteins, peptides, yeast extract<br />Natural gums &amp; cellulose thickeners<br />Plant extracts (aloe vera, starch,)<br />Vitamins<br />Clay compounds<br />High water activity<br />Surfactants (Tween 80)<br />22. Conclusions<br />Emulsions have unique chemistry and physical properties.Understanding this chemistryallows the formulator to create a unique formulation that meets end use requirements.<br /></p>