Microemulsion

A microemulsion is a stable mixture of liquids not soluble in each other (usually oil/water) that is formed spontaneously or with only a small initial energy input with the participation of a surfactant and usually also a co-surfactant. Under constant pressure and temperature conditions, microemulsions do not separate back into their individual phases; in this respect, they behave fundamentally differently from emulsions.

What are the properties of microemulsions?

In a microemulsion, two phases are present as a mixture with a very fine distribution of tiny droplets (between 10 and 100 nm). Although microemulsions are not solutions, they resemble them in their physical behavior. Unlike emulsions, they form spontaneously or with only a small amount of energy input, and in any case without intensive shaking or stirring. Accordingly, the phases do not separate again as long as the composition (e.g., the salt content) as well as the temperature and pressure remain unchanged.

Since the size of the droplets is below the wavelength of visible light (<380 nm), microemulsions are transparent. Due to light scattering effects, they are often milky-cloudy and shimmer bluish in transmitted light (opalescence).

Microemulsions can exist in a single phase or as a three-phase system with an aqueous and an organic excess phase. 

Are microemulsions real emulsions?

The term "microemulsion" has become established historically, but it is misleading because these are not emulsions in the true sense of the word. Emulsions form larger droplets and do not occur spontaneously, but must be mixed intensively by mechanical means. In addition, emulsions are thermodynamically unstable; their phases separate again over time, although the length of time this takes can vary greatly.

▶ Find out more in our glossary article on emulsions.

Where are microemulsions used?

Microemulsions can be used to mobilize organic substances that are otherwise immiscible with water in a very fine distribution without phase separation occurring. This capability offers a wide range of possible applications:

• Enhanced oil recovery (EOR): In surfactant flooding, the flooding mixture is pumped into the reservoir where, with optimal formulation of the solution, it forms a microemulsion with the oil and mobilizes it.
• Food industry: Microemulsions dissolve hydrophobic vitamins or flavors and extend the shelf life of sensitive ingredients. 
• Pharmaceuticals: Medicines based on microemulsions enable active ingredients to be transported in the body and ultimately absorbed at the site of action.
• Cosmetics and personal care: Microemulsions ensure good absorption through the skin and uniform distribution.
• Chemical synthesis: Microemulsion droplets can serve as tiny "reactors" to produce specific nanoparticles. In emulsion polymerization for latex, microemulsions ensure uniform, defined particle sizes.   

How is the formation of microemulsions related to interfacial tension?

In order to form an interface between two immiscible phases, work must be done, which is expressed in terms of interfacial tension (IFT). IFT is the work that must be done per unit area to enlarge an interface.

▶ Read our glossary article on interfacial tension.

Since the division of a quantity of liquid into droplets to form an emulsion is accompanied by an increase in the interfacial area, the interfacial tension must be overcome (by stirring, shaking, etc.). Surfactants used as emulsifiers reduce the interfacial tension, thus enabling the formation of smaller droplets and more stable emulsions.

In a microemulsion, the interfacial tension is reduced to nearly 0 mN/m, so that oil and water mix "voluntarily" and the mixture remains stable as long as the temperature and composition do not change (details below). The necessary drastic reduction in interfacial tension often occurs within a narrow temperature range and a small window for surfactant and salt content. 

What are typical formulations for microemulsions?

Surfactant solutions serve as the basis for the production of microemulsions. The concentration is often between 10% and 20%, which is generally higher than the dosage for classic emulsions. The mixtures also contain a co-surfactant, usually a short-chain alcohol (butanol, pentanol) or an amine. This component makes the surfactant film around the emulsified droplets more flexible and reduces the curvature pressure, which facilitates the formation of smaller droplets. 

Which measurement methods help in the production of microemulsions?

Measuring instruments known as tensiometers measure the interfacial tension (IFT) to evaluate the tendency to form conventional emulsions and their stability. Classic tensiometric approaches such as the ring method or plate method are helpful for formulating microemulsions, but they do not measure the relevant range of extremely low IFT values. Spinning drop tensiometers are the instruments of choice when it comes to measuring these values.

The spinning drop technique captures the IFT based on the deformation of an (oil) drop located in a rotating capillary filled with the specifically heavier (aqueous) phase. This method offers a particularly large measuring range down to 10^-6 mN/m. (For comparison: the IFT between water and sunflower oil is 20-35 mN/m). The concentration or temperature window for the formation of a microemulsion can be identified using a series of measurements.

▶ For more information on this topic, please refer to our application reports AR273 ("Ultralow Interfacial Tension in Enhanced Oil Recovery (EOR)") and AR288 ("From an Immiscible Water-Oil System to the Ultralow Interfacial Tension of a Microemulsion").

Is there a scientific, thermodynamic explanation for the formation of microemulsions?

The interaction of the phases and their interactions with surfactants and co-surfactants in the formation of microemulsions are complex, but can be described in a simplified and plausible way using the Gibbs-Helmholtz equation.  

The Gibbs-Helmholtz equation as a guideline for spontaneous processes

Whether a thermodynamic process occurs spontaneously is determined by the Gibbs energy G, also known as free enthalpy. The Gibbs-Helmholtz equation describes this quantity, or its change ΔG as follows:

If ΔG is negative, the process proceeds spontaneously, whereas if ΔG is positive, it must be maintained by supplying energy. 

ΔH is the energy released during a reaction (ΔH = negative) or required for it (ΔH = positive), which is referred to as enthalpy (but not free enthalpy, see above). S is entropy, a measure of the disorder of a system. In other words, S describes "freedom" – the degree to which a system can assume different states. T is the absolute temperature (Kelvin temperature).

Significance of the Gibbs-Helmholtz equation for microemulsions

The terms of the Gibbs-Helmholtz equation are often used to describe the Gibbs energy ΔG during emulsification in an approximate manner. First, the formation of small droplets during emulsion formation is associated with a positive enthalpy ΔH because energy must be expended to increase the surface area A against the interfacial tension γ:

Entropy increases during emulsification (positive ΔS) because an ordered system (two separate phases) is transformed into a disordered one (mixed phases).

If the interfacial tension γ is reduced to a value of nearly 0 by a suitable surfactant formulation, the entropy term TΔS becomes greater than the enthalpy contribution ΔH. As a result, ΔG becomes negative in the Gibbs-Helmholtz equation. In this case, a microemulsion forms because the division of a volume into small droplets is energetically favored.

Literature

• H.-D. Dörfler: Grenzflächen- und Kolloidchemie. Weinheim, New York, Basel, Cambridge, Tokyo 1994. pp. 281-293
• C. Solans, H. Kunieda (eds.): Industrial Applications of Microemulsions. Surfactant science series 66, New York 1997.
• B. Vonnegut: Rotating bubble method for the determination of surface and interfacial tension. Review of Scientific Instruments, 13 (1942). pp. 6–9. 

References

• [1] J. Zhang, G. Li, F. Yang, N. Xu, H. Fan, T. Yuan, L. Chen: Hydrophobically modified sodium humate surfactant: Ultra-low interfacial tension at the oil/water interface. Appl Surf Sci 2012, 259, pp. 774-779.