Microemulsions
Definition and properties of microemulsions
In 1943 the first microemulsions were produced by HOAR & SCHULMAN. They first described the formation of stable, optically clear, liquid systems after adding an alcohol to a defined mixture of water, oil and a soap.
In 1959, SCHULMAN et al. then coined the term microemulsion for such systems.
Microemulsions consist of two liquids that are immiscible in the concentrations used, usually water and oil, at least one surfactant and often a co-surfactant. Their characteristic properties include a homogeneous, spatial structure (isotropy), transparency or weak opalescence, thermodynamic stability and low viscosity.
They differ from classic emulsions (macroemulsions) in their very small droplet size of between 10 and 200 nm. As with macroemulsions, O/W or W/O microemulsions can also be formed depending on the ratio of the water to oil phase.
Formation of microemulsions
Minimal interfacial tension is a prerequisite for the formation of thermodynamically stable systems. This can only be achieved if the positive interfacial energy between two phases is compensated by the negative free energy during the mixing of the components.
An added emulsifier attaches itself to the oil-water interface and reduces the interfacial tension. However, only a few surfactants are capable of
reducing this to the minimum required for microemulsion formation. This is often only possible by adding a co-surfactant.
A second, non-ionic surfactant with a different HLB value is used for this purpose.
The co-surfactant penetrates between the surfactant molecules that surround the droplets of the inner phase in a monomolecular layer. This changes the geometric packing of the layer and thus also its curvature. The interface increases and the
degree of dispersion of the droplets rises. However, it must also be taken into account that molecules from the oil phase can also penetrate the surfactant layer and thus influence the formation of the interface.
The question of whether the formation of microemulsions depends on the type of production is still the subject of intense debate. While there is much to suggest that it is a purely thermodynamically driven process, ROSANO et al. believe that the formation of microemulsions is influenced by the order in which the components are added.
The choice of emulsifier is crucial
The choice of emulsifier is decisive in determining whether
O/W or W/O microemulsions are formed. If the HLB value is between four and seven, W/O microemulsions are formed, while O/W systems are formed at higher values between nine and twenty.
Since the properties of emulsifiers depend on temperature, change in emulsifiers also influences the micellar behaviour of the surfactant. As the temperature rises,
the interactions of a hydrophilic surfactant can decrease to such an extent that a phase reversal from an O/W to a W/O system occurs. The emulsifier becomes increasingly hydrophobic and forms a W/O microemulsion. This transition point is referred to as the phase inversion temperature. A characteristic feature of microemulsions is the reversibility of this temperature behaviour.
We focus on PEG-free microemulsion systems and use long-chain polyglycerol emulsifiers for this purpose. These are more skin-friendly and ideally reflect the requirements of modern cosmetics.
Please contact us – we will be happy to advise you.
Literature:
Attwood, D.: Microemulsions. In: Kreuter, J. (Hrsg.): Colloidal drug delivery systems. Marcel Dekker, New York, Basel, Hong Kong 1994
Dörfler, H.-D.: Grenzflächen- und Kolloidchemie. VCH Verlagsgesellschaft mbH Weinheim, New York, Basel, Cambridge, Tokyo 1994
Rosano, H.L., Cavallo, J.L., Chang D.L., Whittam, J.H.: Microemulsions: A commentary on their preparation. J. Soc. Cosmet. Chem. 39 (1988) 201-209
Solans, C., Garcia-Celma, M.J.: Surfactants for microemulsions. Curr. Opin. Colloid Interface Sci. 2 (1997) 464-471