Microemulsion
- Also known as
- Winsor type
A microemulsion is a clear, thermodynamically stable mixture of oil, water and surfactant, often with a co-surfactant, that forms spontaneously. Its oil and water domains are only nanometers to tens of nanometers across, much smaller than the droplets of an ordinary emulsion.
What is a microemulsion?
A microemulsion is a mixture of oil and water held together by a surfactant film, usually with a co-surfactant. Despite the name, it is not simply an emulsion with small droplets. It is a distinct state: it forms on its own when the ingredients are combined in the right proportions, needs no high-shear mixing, stays stable indefinitely, and is typically transparent or slightly bluish because its structures are too small to scatter much light.
Microemulsion vs. emulsion
| Microemulsion | Emulsion | |
|---|---|---|
| Stability | Thermodynamically stable | Kinetically stable; separates over time |
| Formation | Spontaneous | Needs energy input |
| Structure size | Nanometers to tens of nanometers | Usually far larger |
| Appearance | Clear or translucent | Usually milky |
| Interfacial tension | Very low | Higher |
Winsor types
Microemulsion phase behavior is often described with Winsor’s classification:
- Type I: an oil-in-water microemulsion in equilibrium with excess oil.
- Type II: a water-in-oil microemulsion in equilibrium with excess water.
- Type III: a middle-phase microemulsion, often bicontinuous, in equilibrium with both excess oil and excess water. It is associated with ultralow interfacial tension.
- Type IV: a single microemulsion phase.
Salinity, temperature, surfactant structure and oil type move a system between types. In surfactant flooding, formulations are tuned toward the Type III region, where interfacial tension is lowest.
Where microemulsions are used
- Oil recovery and well treatment. For enhanced oil recovery, and in fracturing and acidizing additives to improve fluid penetration and cleanup.
- Cleaning and degreasing, where microemulsions dissolve oily soils.
- Agrochemicals and other formulations, where they deliver oil-soluble actives in a clear, stable concentrate.
Microemulsions at Locus
Locus’s AcidBoost is described by Locus as a biosurfactant microemulsion used in acid treatments to improve how acid contacts carbonate scale near the wellbore.
Frequently asked questions
No. A nanoemulsion has very small droplets but is still kinetically stable and eventually separates. A microemulsion is thermodynamically stable and forms spontaneously, which makes it a different state of matter.
Their oil and water domains are much smaller than the wavelength of visible light, so they scatter little light.
Sources
- microemulsion, IUPAC Compendium of Chemical Terminology (Gold Book)
- microemulsion; Winsor phase behavior, SLB Energy Glossary
- Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012