Oil in water (o/w) emulsifiers are among the most common ingredients used in skin care products and serve a multitude of functions in formulations. These include solubilizing oils/fragrances into water, acting as primary emulsifiers, and as wetting/dispersing agents. By far their most important role is in helping to emulsify and stabilize oil in water emulsions.
Emulsifiers are molecules consisting of polar (hydrophilic) and nonpolar (lipophilic) groups that promote and the formation of stable emulsions. Oil in water emulsifiers work by lowering the surface tension of water from 72.8 millinewtons per meter (mN/m) at 20°C to that of the oil phase, which typically is between 30-50 mN/m. The lower the difference in the interfacial tension surface tension between the phases, the easier it is to form the emulsion.
Emulsifiers must be more soluble in the water than the oil phase in order to form oil in water emulsions. This is referred to as Brancroft’s rule. The exceptions to this rule are oil in water Pickering emulsifiers, which are submicron particles that preferentially disperse in water and absorb at the water-oil interface to stabilize emulsions, even though most don’t reduce surface tension.
In simple emulsions, finely dispersed droplets of the oil phase are surrounded by an emulsifier layer or interfacial film which helps keep the droplets separated. Emulsifiers are classified as being nonionic (no charge), anionic (negative charge) or cationic (positive charge). They also can be surface tension-reducing nonionic or charged polymers. It is commonplace to use mixtures of emulsifiers to achieve optimal results.
It’s critical to know the type of oil in water emulsion you want to make when choosing the emulsifier you need to use. Types include microemulsions, nano/micellular, and lamellar gel network (LGN)-based emulsions.
LGN-based systems are the most common oil in water emulsions used in skin care formulations. LGNs are combinations of low and high hydrophilic-lipophilic balance (HLB) crystalline surfactants that can form lamellar bilayer structures that can swell, thicken, and bind water. These structures have a very low interfacial tension that can effectively emulsify and stabilize oil phases. LGNs need a high HLB surfactant to disperse and swell the low HLB surfactant.
Ideally both the low and high HLB surfactants should have a melting points of >50°C to form heat-stable LGNs. The most commonly used low HLB materials include:
- Cetyl
- Stearyl alcohol
- Eicosanol (C20)
- Behenyl alcohol (C22)
- Glyceryl Stearate (40-60% mono)
- Steareth 2
- Sorbitan Stearate
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The high HLB surfactant can be nonionic, anionic, or cationic and generally C16-22 saturated derivatives work the best. The high HLB surfactant should be also water soluble or dispersible.
The ratio of surfactants is critical to stability because only certain ratios permit LGNs to form due to the packing parameter or geometry requirements for the surfactants. The optimum ratio is typically around 15-30% high HLB surfactant as a percent of total surfactant used. The typical total surfactant use level to form a cream like consistency is 6-8%.
Recommended high HLB surfactants include:
- PEG 40 Stearate
- Steareth 20
- Cetearyl Glucoside
- Polyglyceryl 2 or 3 Stearate
- Polyglyceryl 6 Distearate
- Sodium Stearoyl Glutamate
- Sodium Lauroyl Lactylate
- Distearyldimethyl Ammonium Chloride
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Emulsifiers used to solubilize oils or form nano-emulsions require different types of surfactants due to the nature of the micelle formed, which is highly dependent on the polarity of the oil used. Non-polar oils form structures in which the oil resides in the micellular core where the hydrocarbon tail groups are concentrated. The surfactant should have some solubility in the oil phase to emulsify well. Highly polar oils like fragrances are located between the polar head groups near the outside of the micelle. Intermediate polarity oils reside in the Palisade layer of the micelle located between the hydrocarbon chains.
The typical surfactant use level to solubilize 1% fragrance is 3-5%. For 1% non-polar oils, this can range from 5% to 20%. Recommended solubilizers for polar oils include:
- Polyglyceryl 10 Oleate
- Polyglyceryl 10 Stearate
- C12-C13 Pareth 9
- Sorbitan Oleate Decylglucoside Crosspolymer
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Recommended options for non-polar oils include:
- Polyglyceryl 10 Oleate/Polyglyceryl 2 Oleate (80/20 mixture)
- Sucrose Laurate/Glycerin (70% anhydrous oil in polyol emulsion-oil, 20% Glycerin, 10% Sucrose Laurate).
Microemulsions are another type of clear oil in water micellular dispersion that instantly form with simple mixing due to their extremely low interfacial tension. They require higher concentrations of emulsifier versus typical micellular solutions. Unlike other oil in water emulsions, they are thermodynamically stable. They consist of an aqueous phase, oil, surfactant, and a cosurfactant.
The cosurfactant, typically a polyol or alkanol, helps reduce the particle size of the oil droplets. Below is a model microemulsion formulation for emulsifying non-polar to medium polarity oils.
Ingredient | % |
DI water | 17 |
PEG-8 Caprylic/Capric Glycerides | 13 |
Polyglyceryl-3 Diisostearate | 7 |
Squalane | 63 |
Further reading:
- Beauty Multitasking: Multifunctional Cosmetics Trends and Challenges
- Contemporary Formulation: Best-in-Class Natural/Synthetic Water-Based Thickeners
- Crackling Creams: Creating a Sensory Delight [VIDEO]
- Smooth moves: Water in silicone emulsions [VIDEO]
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Dear Mr. Deckner,
Very nice article on working with oil/water emulsions in cosmetics area. I am working on agrochemical emulsions. Wonder if same principles work there as well. Will you be kind enough to explain/comment on the same. also if possible could you suggest some good literature read for working on agrochemical emulsions?
with warm regards,
For VVF India ltd.
Vilas kakade.
Vice president R &D.
An emulsion is an emulsion, it doesn’t matter what the end application is. The principles remain the same.
I have not previously worked on agrochemical emulsions.
Dear Mr. Deckner,
Very informative, thank you!
I am trying to form a good oil in water emulsion. But the water system is salt-saturated and has a high pH (>11). The oil phase is either mineral oil or diesel.
I am trying to find the best emulsifier that can work in achieving the stability of these fluids.
Can you by any chance provide me the commercial product name for this so that I can purchase.
Thank you.