Microemulsions are dispersions consisting of oil, water, and surfactants. They differ from other dispersions in that they spontaneously form stable structures. A distinction is made between different phases, which are called Winsor phases. One method of measuring microemulsions is the static multiple light scattering method.
What are microemulsions?
Microemulsions are dispersions of oil, water and, surfactants that differ from conventional emulsions in their properties. Unlike conventional emulsions, in which oil and water do not normally mix, microemulsions spontaneously form stable structures. These consist of tiny droplets of one phase (e.g. oil) dispersed in another phase (e.g. water) and surrounded by a surfactant layer.
The chemical structure of surfactants is crucial for the formation and stability of microemulsions. Surfactants are amphiphilic molecules that have hydrophilic (strong affinity for water) and lipophilic (strong affinity for oil) regions. This structure enables surfactants to stabilize oil droplets in aqueous media by reducing the interfacial tension between oil and water. The choice of surfactants and their concentration influence the structure of the microemulsion. This can affect the solubility of compounds in oil droplets or micelles. Polarity and solubility are important factors in the formulation of microemulsions for various applications.
One of the most outstanding physical properties of microemulsions is the tiny size of oil droplets or micelles, which are often in the nanometer range. This small size results in a large surface area and high stability of the microemulsion. Due to their small droplet size and low viscosity, microemulsions can be relatively fluid. This property can be advantageous when uniform distribution of ingredients is required. Microemulsions can serve as reaction media in which chemical reactions can take place. The tiny droplets or micelles provide a large surface area for reactions, while the presence of surfactants can make the environment more accessible for certain reactions.
Phase behavior of microemulsions
The Winsor stages, named after P.A. Winsor, describe the different phase distributions that can occur in microemulsions (see Figure 2). Winsor I is an oil-in-water microemulsion in which the water phase is continuous and small oil droplets are stabilized by surfactants, and there is also excess oil present; This state typically occurs with hydrophilic surfactants. Winsor II is the opposite, a water-in-oil microemulsion in which the oil phase is continuous, the surfactants stabilize the water droplets, and excess water is present; this occurs with lipophilic surfactants. Winsor III describes a three-phase system consisting of an excess oil phase, an excess water phase, and a bicontinuous microemulsion middle phase containing both oil and water channels; this state occurs with surfactants that have similar affinity for oil and water. All types are thermodynamically stable. By adjusting the temperature and salt content of the water phase, the system switches between the three different types.
Microemulsion mixtures are often sought after for enhanced oil recovery (EOR) processes due to their high crude oil yield. In general, such mixtures consist of a thermodynamically stable dispersion of crude oil, surfactant and water or brine. The phase behavior of the mixture changes from Winsor I to Winsor III to Winsor II when the temperature, salinity and pressure are adjusted. The Winsor III phase is best suited for enhanced oil recovery processes because it has the lowest interfacial tension (10−2…10−4 mN/m) and the surfactants are used most efficiently. Therefore, the phase behavior is being intensively researched to find the right surfactants, the right salinity and the right temperature for ideal production conditions.