Emulsions Formulation Overview

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A brief overview of emulsion chemist

A brief overview of emulsion chemist

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  • 1. Emulsions
    Jim McElroy
    Lincoln, October 2008
  • 2. 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
  • 3. 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
  • 4. 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.
  • 5. 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
  • 6. 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
  • 7. 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
  • 8. 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
  • 9. 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
  • 10. Processing
    Method of Preparation
    Order of addition
    Rate of addition
    Energy effects
  • 11. 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.
  • 12. Rate of Addition
    A significant improvement in the emulsion can sometimes be seen by adding the aqueous phase at a slower rate.
  • 13. 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
  • 14. 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.
  • 15. 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
  • 16. 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
  • 17. Micro Emulsion Examples
    Children's Vitamin drops
    Flavoring oils in cream sodas or colas
    Carnuba wax floor polishes
    Hair gels
    Dry Cleaning fluids
  • 18. 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
  • 19. 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.
  • 20. 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
  • 21. 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)
  • 22. Conclusions
    Emulsions have unique chemistry and physical properties. Understanding this chemistry allows the formulator to create a unique formulation that meets end use requirements.