Microemulsions: Thermodynamic and Dynamic Properties

Mixing two immiscible liquids (such as oil and water) using emulsifier and energy inputs has been the matter of study for decades. In early 1890’s extensive work have been carried out on macroemulsions (i.e. oil dispersed in water in the form of fine droplets or vice versa) (Becher, 1977) and several theories and methods of their formation have been vastly explored (Lissant 1976 and 1984). However going along the line, microemulsion systems were well opted because of their stability and isotropic nature. Microemulsions are basically thermodynamically stable, isotropically clear dispersions of two immiscible liquids such as oil and water stabilized by the interfacial film of any surfactant and/or cosurfactant. Although a microemulsion is macroscopically homogeneous, or quasi-homogeneous but structured microscopically. Microemulsions in comparison to micellar systems are superior in terms of solubilization potential and thermodynamic stability and offers advantages over unstable dispersions, such as emulsions and suspensions, since they are manufactured with little energy input (heat, mixing) and have a long shelf life (Constantinides, 1995). The term “microemulsion” was first coined by Schulman group (Schulman et al., 1959). However, ambiguity in the microemulsion terminology persists till today as some authors differentiate them from swollen micelles (which either contain low volume fraction of water and oil) and transparent emulsions (Prince, 1977, Malcolmson et al., 1998). One of the unique factors associated with microemulsions is the presence of different textures such as oil droplets in water, water droplets in oil, bicontinuous, lamellar mixtures etc., which are formed by altering the curvature of interface with the help of different factors such as salinity, temperature, etc. Such a variety in structure of microemulsion is a function of composition of the system. Phase study greatly helps to elucidate different phases that exist in the region depending upon the composition ratios. One peculiarity of microemulsions lies in the fact that these structures are interchangeable. Construction of phase diagram enables determination of aqueous dilutability and range of compositions that form a monophasic region (Fig. 1). One of the unique factor associated with microemulsions is the presence of different structures as classified by Winsor (Winsor, 1948). Winsor I (o/w), Winsor II (w/o), Winsor III (bicontinuous or middle phase microemulsion) and Winsor IV systems are formed by altering the curvature of interface with the help of different factors such as salinity, temperature, etc. Where Type I indicates