Personal care emulsifiers are workhorse ingredients used in formulations to promote the formation of emulsions or colloidal dispersions. Emulsions are normally two immiscible liquid phases dispersed in one another.
When the external or continuous phase is water, the dispersion is called an oil in water emulsion, and a water in oil emulsion when oil is the continuous phase. The emulsion type can easily be determined by placing a sample in water and observing whether or not it easily disperses in water, indicating it is an oil in water emulsion.
An emulsifier can reduce the surface tension of a dispersed phase and normally consists of a hydrophobic (oil-loving) group and a hydrophilic (water-loving) group. Bancroft’s rule states that the phase in which an emulsifier is more soluble determines which phase is continuous. The more hydrophilic the emulsifier is, the more water solubility it has, indicating that it will normally form oil in water emulsions.
Oil in Water Emulsions: Lamellar Gel Network-Based
The most common type of oil in water emulsion based products sold globally are lamellar gel networks (LGNs) containing mixtures of hydrophilic and hydrophobic crystalline surfactants and soluble or swellable polymers. These emulsifiers normally contain saturated C12-22 chains and have melting points above 55C.
- The hydrophobic emulsifiers typically used include Cetyl, Stearyl, Behenyl alcohols, Stearic acid, Steareth 2, Sorbitan Stearate, and Glyceryl Stearate.
- Common anionic hydrophilic emulsifiers used include neutralized fatty acid soaps, Sodium Stearoyl Glutamate, Sodium Stearoyl Lactylate, and Potassium Cetyl Phosphate.
- Cationic emulsifiers used include Behentrimonium Chloride and Distearyl Dimethyl Ammonium Chloride.
- Commonly used nonionics include Steareth 20, PEG 40 Stearate, PEG 100 Stearate, Polysorbate 60, Cetearyl Glucoside.
The ratio of hydrophilic/hydrophobic emulsifiers used in a formulation is important because it helps form the correct packing parameter which results in the maximum amount of water being bound in the lamellar phase. The optimum ratio of hydrophilic/hydrophobic plus hydrophilic emulsifiers is typically around 25 percent using fatty alcohols, but can drop to around 10-15 percent if the hydrophobic emulsifier has a higher HLB than ~3. The total surfactant concentration will also determine the viscosity of the formulation. The interfacial tension of the LGN is very low and enables the easily emulsification of most types of oils.
Micellar Oil in Water Emulsions
The personal care emulsifiers or solubilizers used in making clear micellar oil in water emulsions are very different than those used to produce oil in water LGNs since their primary purpose is to solubilize, not thicken the formula. Micellar dispersions are the most common type of clear oil in water dispersions emulsified into water using emulsifiers, and or solvents.
If the particle size of the dispersed phase is <100 nm, the dispersion normally appears clear. These dispersions can have a particle size as low as 5nm. Normally you will need to use three to five parts solubilizer for one part fragrance oil and much higher when using less polar cosmetic emollients. Best in class ingredients include C12-C13 Pareth 9, Heptyl Glucoside, and Sorbitan Oleate Decylglucoside Crosspolymer.
Water in Oil Emulsions
The polarity of oils used is an important factor in determining the selection of the correct emulsifier. When using a non-polar oil phase that is mostly Silicone, Silicone based emulsifiers are generally preferred.
Best in class emulsifiers are Cetyl PEG/PPG-10/1 Dimethicone, Lauryl PEG-9 Polydimethylsiloxvethyl Dimethicone, and Lauryl PEG-10 Tris(trimethylsiloxy)silylethyl Dimethicone.
These polymeric emulsifiers form a very strong interfacial barrier that can emulsify over 80 percent water and have good compatibility with non-Silicone oils.
The best in class emulsifiers using non Silicone oils are PEG-30 Dipolyhydroxystearate, Polyglyceryl-2 Dipolyhydroxystearate, and Polyglyceryl 4 Diisostearate Polyhydroxystearate/Sebacate. These saturated polymeric emulsifiers are far superior to previously used materials which included Sorbitan Oleate, Oleth 3, Polyglyceryl 3 Isostearate and Oleate.
Personal Care Emulsifiers Trends
- Growing use of more green, sustainable, and biodegradable emulsifiers
- The use of ethoxylated/propoxylated emulsifiers is declining
- Green based non ethoxylated emulsifiers such as polyglyceryl esters and alkyl glucosides are becoming very popular
- Naturally produced biosurfactants are starting to be commercialized and may become more popular in the future as their price and quality improve
- Several companies are developing ethylene oxide plants using naturally produced ethanol as a feedstock in place of petroleum based ethylene. This will result in the production of more green ethoxylated emulsifiers in the future.
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As someone with a little prior knowledge of emulsions I read this as a refresher.
I would not call this a “Primer” since some basic definitions are not given eg hydrophilic, hydrophobic, HLB.
It would be useful to the new reader to have an explanation in the first paragraph of ‘Why do we need emulsions?”
The second paragraph is not well composed or proof read. The first sentence of it lacks punctuation and inverts the order of the statements. The second sentence has “… and observing whether or it easily disperses… ”
As for the rest of the content, I did find it useful to find out about current trends in emulsifiers.
Please could you tell me what the best emulsifier to use to thicken resorcinol and salicylic acid added to mainly oils such as Vaseline, lanolin with only a small amount of water to dissolve the resorcinol. I have been using DSG or distearate glycol. I feel the acids are not penetrating the skin as much as they should.
Petrolatum or Lanolin are not suitable vehicles to solubilize Salicylic acid and Resorcinol into.
I read on this link that recommends that phenoxyethanol not be formulated with non-ionic surfactants. Is this good advice?
https://www.americanpharmaceuticalreview.com/Featured-Articles/38885-Antimicrobial-Preservatives-Part-Two-Choosing-a-Preservative/
”benzyl alcohol [16], 2-phenoxyethanol [42], 4-chloroxylenol [20] and m-cresol [25] should not be formulated with non-ionic surfactants. Chlorobutanol [43] and 2-phenylethanol [37] are adversely affected by the presence of non-ionic surfactants, e.g. polysorbate 80.”