Microemulsion
A microemulsion is an emulsion whose disperse phase (e.g. oil or water) forms such small domains (“droplets”) that visible light is not scattered on them. This makes microemulsions transparent like water, while normal emulsions are opaque like milk. The domain diameter is in the lower nanometer range (<350 nm).
The term is not exactly defined and serves more to understand the mostly surprising three- or single-phase formation in emulsions. It goes back to Schulman .
A typical microemulsion consists of two immiscible liquids 1 and 2 and a surfactant . A surfactant is a chemical compound with two chemically very different molecular parts 1 and 2. If the concentration is high enough, the surfactant forms micelles , spherical structures in which the surfactant particles are arranged in such a way that part 1 is always oriented inwards and part 2 is always oriented outwards . As a result, the micelles can take up liquid 1 in their interior, while the molecular part 2 interacts more strongly with liquid 2. If the absorption capacity of the micelles is exhausted, the rest of the liquid 1 forms an excess phase.
Microemulsions can occur alone or together with two other phases, so-called excess phases. Excess phases consist of either aqueous or organic components. If the three phases have completely separated, there are three completely clear, apparently homogeneous liquids separated by 2 phase boundaries. Due to the high concentration of disperse phase, a bluish discoloration occurs in the microemulsion when one looks at the liquid perpendicular to the incident light. This is known as opalescence (see figure).
If the surfactant alone, together with the oil used, does not form a microemulsion with the oil / water content and the temperature applied, this is often achieved by adding another surfactant, a cosurfactant. A systematic investigation of the microemulsion formation showed that the (three- or single-phase) microemulsion formation is strongly dependent on temperature and composition, but only very slightly dependent on pressure. If the substances used ( hydrophilic and lipophilic chain length of the surfactant, salt concentration ( hydrotropic and lyotropic salts), carbon chain length of the oil or the organic component) are known, it can be reproducibly produced for a given temperature.
Microemulsions are stable and form spontaneously. The nanodisperse structure is formed with minimal stirring effort. Normal emulsions, the much larger domains of which are often only created by laborious emulsification, are sensitive to temperature and shock. Heating with subsequent cooling generally leads to an irreversible change in the disperse structure, which can break the emulsion.
use
As early as the 1920s, microemulsions were used in car care to apply a layer of wax. In pharmacy, microemulsions are used to formulate water-insoluble active ingredients. Another application is ternary oil production , in which an aqueous surfactant solution is pressed into the oil-bearing layer. The surfactant solution forms a (mobile) microemulsion with immobile petroleum and thus allows the deposit to be further exploited.
literature
- Kahlweit, Strey: Phase behavior of ternary systems of the water-oil-nonionic amphiphile type (microemulsions) , Angewandte Chemie 97 (1985), 655-669
- Dörfler: Interfaces and colloid-disperse systems , Berlin 2002