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