The term “microemulsion” was first used by Schulman1 in 1959 to describe a multiphase system consisting of water, oil, surfactant and alcohol, which forms transparent solutions. MEs are thermodynamically stable micellar dispersions with a particle size of 10-100 nm that instantly form with simple mixing. They are low-viscosity systems containing an aqueous phase, oil phase, surfactant, and a cosurfactant.
The cosurfactant, typically a polyol or alkanol, helps reduce the interfacial tension, which in turn helps to reduce the size of the dispersed droplets and normally is water-insoluble. The presence of the cosurfactant also reduces the rigidity of the interfacial film, leading to the formation of a bicontinuous structure typical of microemulsions.
MEs can exist as oil in water, water in oil, or bicontinuous emulsions that are oil or water soluble. ME formation is dependent on surfactant type/structure and can be formed using ionic or nonionic surfactants. If the surfactant is ionic and contains a single hydrocarbon chain like sodium lauryl sulfate, MEs will only form if a co-surfactant and/or electrolyte is present. With double chain ionics and some non-ionic surfactants, a cosurfactant is frequently not necessary.
The main current marketed applications for MEs include makeup removers, stable oil in water emulsion based wipes, silicone free hair conditioning shampoos, and as fragrance and essential oil solubilization.
Note, Nano emulsions are also clear colloidal dispersions but are not true MEs since they are not thermodynamically stable and frequently require high shear processing to form.
Advantages of Microemulsions
- Easy to produce with minimal mixing
- Very stable
- Visually clear
- Can significantly increase the solubility of actives and fragrances
- Can increase skin penetration and the efficacy of actives
- Are excellent for removing all types of makeup
- Conditioning shampoos and moisturizing body washes
- Require high levels of surfactants
- Can be irritating
- Can sometimes be difficult to preserve
- Are thin and can only be thickened using water soluble polymers and still remain clear (not swellable polymers like Carbomer)
- Plantaren® 2000 (Decyl Glucoside-BASF)
- Labrasol® (PEG-8 Caprylic/Capric Glycerides-Gattefossé), surfactant
- Plurol® Diisostearique (Polyglyceryl-3 Diisostearate-Gattefossé), cosurfactant
- Plurol® Isostearique (Polyglyceryl-6 Isostearate-Gattefossé), cosurfactant
- Plurol® Oleique (Polyglyceryl-6 Dioleate-Gattefossé), cosurfactant
- Transcutol® CG (Ethoxydiglycol-Gattefossé), solvent
Marketed Microemulsion Systems
- Caprol® Micro Express (PEG 6 Caprylic/Capric Glycerides, Polyglyceryl-6 Dioleate, Glyceryl Caprylate/Caprate-Abitec), will form a microemulsion using 9 parts water and 1 part Express. Can dissolve actives into Express before water addition
- Barcleanse BC (Sorbitan Oleate, PEG 10 Laurate, Water, Decyl Glucoside-Barnet Products) is a bicontinuous micro emulsion
- Plantasil® Micro (Dicaprylyl Ether, Decyl Glucoside, Glyceryl Oleate-BASF) is positioned as a silicone free hair conditioning booster
- TEGO® Care LTP (Sorbitan Laurate, Polyglyceryl-4 Laurate, Dilauryl Citrate-Evonik)
- Lamesoft® OD (Coco-Caprylate, Lauryl Glucoside, Glycerin, Polyglyceryl-2 Dipolyhydroxystearate, Polyglyceryl-3 Diisostearate-BASF)
- Emulgade® CPE (Olus Oil, Glycerin, Lauryl Glucoside, Polyglyceryl-2 Dipolyhydroxystearate, Glyceryl Oleate, Dicaprylyl Carbonate-BASF) recommended for making wipe nanoemulsions
- Lamesoft® PO 65 (Coco-Glucoside, Glyceryl Oleate-BASF) recommended as a skin refatting agent in body washes
- Schulman, J. H.; Stoeckenius, W.; Prince, M. J. Phys. Chem. 1959, 63, 1677
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