CHEM 470 Understanding Emulsions I. Definitions A. Any heterogeneous system which has at least one immiscible or barely miscible liquid dispersed in another liquid in the form of tiny droplets. A. Becher, Encyclopedia of Emulsion Technology B. Emulsions are the most common type of delivery system used for cosmetic products. A. Quick and convenient delivery to hair and skin B. semisolid materials consisting of hydrophilic and hydrophobic portions C. O/W and W/O a. these two portions make up the internal and external phases i. internal phase is composed of the materials that make up the tiny dispersed (or emulsified) droplets ii. external phase is made up of the rest of the materials (also called a continuous phase), and is the more abundant of the two A. Most common emulsion is one in which oil is dispersed in water = oil in water emulsion (o/w) B. water-in-oil emulsion (w/o) a. depends on concentration of each material in the system, the type of emulsifier and the processing steps used to create the emulsion
D. Size A. Particles which make up the emulsion s internal phase are polydispers (variable sizes), and their average is often used for emulsion classification E. Multiple emulsions a. Average particle diameter is less than 100 = micellar emulsion b. Particle diameter = 100 to 2000 is known as microemulsion c. Larger particles = macro emulsion most common A. more complex than two-phase emulsions B. more than one internal phase (emulsions within emulsions) a. e.g. water can be dispersed in an oil which can further be dispersed in another water phase water-in-oil-in-water (w/o/w) emulsion b. >4 internal phases II. Emulsion Components A. Will eventually separate into two phases, but the goal is to design a system that will remain stable over the useful life of the product 1. 3 components: oil phase, aqueous phase, emulsifier B. Oil phase A. fats, oils and waxes B. derivatives: fatty alcohols and acids, esters, hydrocarbons, glycerides, and silicones
a. action (skin): emollients-feel, soften, improve moisture-retaining properties b. action (hair): conditioning-appearance C. Aqueous phase A. humectant materials like glycerine or propylene glycol, water-soluble polymers (thicken or provide conditioning), preservatives, colorants, electrolytes, plant extracts, hydrolyzed proteins B. reduces greasy feel and reduces cost D. Emulsifiers A. stabilize the dispersion of the internal phase and continuous phase B. surfactants reduce interfacial tension between the two phases a. e.g. stearic acid b. oil particles stabilize because they are incorporated into the lipophilic part of the micelles stable emulsion C. emulsions are still thermodynamically unstable (eventually will separate) a. e.g. addition of polysorbate 80 to mineral oil and water (macroemulsion) yields an emulsion stable for ~ 2 days b. some emulsions are stable for years III. Emulsion Destabilization A. Four primary mechanisms: creaming, flocculation, coalescence and inversion (occur simultaneously) A. Creaming due to different densities in emulsion droplets a. e.g. unhomogenized milk in which the fatty cream rises to the top b. can be reversed by agitation B. Flocculation internal phase droplets form a weak, reversible association
a. Caused by inadequate surface charge on the micelles, which reduces repulsive forces between them b. Particles are distinct, no change in size i. e.g. two billard balls touching one another, while they are touching, an association is formed that can be easily disrupted by moving one of the balls ii. can be reversed by agitation C. Coalescence when two internal phase droplets get close enough, they combine to form one larger particle a. Irreversible b. If enough particles coalesce, the result is complete separation c. Ostwald ripening internal phase particles ten to combine to a uniform size, which can also lead to phase separation D. Inversion external phase becomes the internal phase and vice versa a. Changes physical characteristics of emulsion E. Other factors a. Elevated storage temperatures accelerate separation b. Evaporation of aqueous phase c. Microbial contamination d. Undesired reactions F. prevention a. ph and viscosity checks b. light scattering c. conductivity measurements d. NMR IV. The Step-Wise Approach A. Determine type of product desired determines emulsion used, composition and concentration of oil and water phases, type of emulsifier
B. Type of product A. Hand cream, antiperspirant, moisturizing cream, shaving cream (common o/w macroemulsion) a. Common o/w macroemulsion chosen for applications requiring a relatively small amount of fatty material b. e.g. hand creams are typically o/w emulsions approximately 10-25% oil phase B. emollient creams, hairdressings, sunscreens, cleansing creams (certain types-w/o emulsion) a. w/o emulsion better when a large amount of oil Is needed b. greasy, longer lasting residue, but doesn t dry out C. microemulsions are good for products which need clearness a. developed 1940s, formed by first making a w/o emulsion, then mixing that emulsion in a water phase b. useful for encapsulating materials that give prolonged release when applied to a surface (e.g. skin) c. microemulsions demonstrate better stability than macroemulsions and appear to be less irritating d. microemulsion gels ringing gel i. when shaken, it resonantes and a ringing sensation can be felt ii. relaxers, permanent waves
D. multiple emulsions not fully explored a. potential use in prolonged ingredient release, odor masking, stable delivery of materials such as vitamins and enzymes b. less stable C. Composition/concentration A. References Cosmetic Bench Reference (1998) or Harry s Cosmetology (1982) D. Emulsifier HLB system (Hydrophobic-Lipophilic Balance) A. Scale from 0-20 to rate emulsifiers, based on their affinity for oil and water B. Low HLB more liphophilic, high HLB more hydrophilic C. HLB rating 3-8: w/o emulsion D. HLB rating 10-20: o/w emulsion E. Originally developed for nonionic emulsifiers, but can be applied to other surfactant types F. Calculate the required HLB value for a given oil system and type of emulsion desired a. e.g. mineral oil requires a material with approximately a 4.5 HLB to create a w/o emulsion b. to make an o/w emulsion with mineral oil requires an emulsifier with an HLB value of about 11 c. HLB value can be done experimentally (published charts) i. Does not give information about concentrations required can be done through experimentation only G. introduction of silicone does not always work well with emulsions a. new method: 3-D HLB i. uses phase diagrams to refine the HLB values of materials that will be used in systems with silicones H. HLB is not the only system a. It is known that a material s ability to emulsify changes with temperature I. Phase Inversion Temperature (PIT) system was developed
a. Similar to HLB except it rates materials based on the temperature at which the emulsifier causes o/w emulsions to invert to w/o emulsions J. Cohesive Energy Ratio (CER) system a. Rates materials on the basis of thermodynamic principles b. Useful for anionic surfactants V. Conclusion A. Emulsions are very useful and will remain a large part of the cosmetics industry