Microemulsions thermodynamic definition

A thermodynamic definition of microemulsions can be obtained from a consideration of the energy and entropy terms for formation of microemulsions. The process of formation of microemuision from a bulk oil phase (for a O/W microemuision) or from a bulk water phase (for a W/O microemuision) is shown schematically in Figure 15.2. 

A distinction has to be made between kinetically and thermodynamically stable emulsions. Mixtures of water, an oil, a surfactant, and a cosurfactant in proper proportions can spontaneously form transparent, themiodynamically stable emulsions, called microemulsions. The definition is not quite unequivocal. Thermodynamically unstable but kinetically stable systems which form spontaneously have also been called microemulsions [117]. Micellar solutions have been included in the definition of microemulsions and the debate on microemulsion structure is ongoing. 

Microemulsions are a convenient medium for preparing microgels in high yields and rather uniform size distribution. The name for these special emulsions was introduced by Schulman et al. [48] for transparent systems containing oil, water and surfactants, although no precise and commonly accepted definitions exist. In general a microemulsion may be considered as a thermodynamically stable colloidal solution in which the disperse phase has diameters between about 5 to lOOnm. 

The most common definition of a microemulsion characterises it as a thermodynamically stable, transparent, optically isotropic, freely flowing surfactant mixture, often containing co-surfactants (e.g. alcohol) and added salts [37]. We restrict the definition further to non-crystalline (disordered) aggregates, since crystalline isotropic phases are better considered as liquid crystalline mesophases. Indeed, the most succinct description of a microemulsion would involve its microstructure. However, this has proven to be a very equivocal issue. So much so that until very recently it was widely believed that microemulsions were devoid of microstructure hence the thermod)mamic definition. 

Thus, microemulsions have nothing in common with macroemulsions, and in many cases it is better to describe the microemulsion system as swollen micelles. The best definition of microemulsions is as follows [3] System of Water -I- Oil -I- Amphiphile that is a single Optically Isotropic and Thermodynamically Stable Liquid Solution. Amphiphiles refer to any molecule that consist of a hydrophobic and hydrophilic portions, for example surfactants and alcohols. 

Our own involvement in microemulsion research was very much influenced by the contacts with the Swedish masters in the field of phase behaviour, Ekwall and Friberg, and at a later stage Shinoda, as well as by our previous experience of studying molecular interactions and association phenomena for other types of surfactant systems. Regarding the stability issue, we found it useful to suggest a definition [32] of a microemulsion as a system of water, oil and amphiphile which is a single isotropic and thermodynamically stable liquid solution . While this definition certainly provided nothing new, we felt it contributed to eliminate some confusion. 

The most general definition of a template is as a structure-directing agent. In surfactant solutions the final templated polymers can be either discrete nanoparticles or mesostructured bulk materials as a consequence of polymerization, respectively, in the non-continuous or continuous domains of the template. Thermodynamically stable media, such as microemulsions, equilibrium vesicles, or lyotropic mesophases are especially useful as templates because of their structural definition and reproducible morphologies. The mesostructure of a thermodynamically stable template is defined by composition and temperature, but this same feature makes the structure unstable to changes in temperature, pH, or concentration. The aim of template synthesis is to transfer the self-organized template structure into a mechanically and chemically stable, durable, and processable material. 

The term microemulsion was initially introduced in 1959 by Schulman who suggested the following definition A microemulsion is formed on addition of an aliphatic alcohol (co-surfactant) to an ordinary emulsion. Generally, micro-emulsions are defined as thermodynamically stable homogeneous mixtures of oil and water stabilised by surfactants and, in some cases, by co-surfactants. Thus, a water in oil microemulsion, i.e. a reversed micellar solution, is a transparent, isotropic and thermodynamically stable fluid in which nanometer-sized water droplets are dispersed in a continuous oil phase. 

Microemulsions are systems consisting of water, oil, and amphiphile(s) that constitute a single optically isotropic and thermodynamically stable liquid solution [6]. Using this definition of microemulsions, it follows that solutions of micelles or reverse micelles with solubilized oil and water, respectively, should also be referred to as microemulsions, and these systems are therefore included in the present chapter. 

The essential of the emulsion definition is the multiphase feature distinguishing emulsions from micro-emulsions, which by definition are single-phase liquids (3). This distinction, although not appreciated immediately (4), is essential. With modem mechanical emulsifiers and a judicial choice of components, it certainly is possible to produce liquid dispersions with the dimension of the dispersed phase less than that of a microemulsion of large dimensions, but such emulsions are not microemulsions. They are thermodynamically unstable and, hence, emulsions. 

The classification of microemulsions based on size is not adequate. Whether a system is transparent or translucent depends not only on the size but also on the difference in refractive index between the oil and the water phases. A microemulsion with small size (in the region of 10 nm) may appear translucent if the difference in refractive index between the oil and the water is large (note that the intensity of light scattered depends on the size and an optical constant that is given by the difference in refractive index between oil and water). Relatively large microemulsion droplets (in the region of 50 nm) may appear transparent if the refractive index difference is very small. The best definition of microemulsions is based on the application of thermodynamics, as discussed below. 

Microemulsion systems with their inner structure in the colloidal domain have been the subject of many theoretical and experimental studies due to their very broad applicability [1 ]. The name microemulsion was first introduced by Hoar and Schutman in 1943 as the name for a clear or transparent system obtained by titration of a milky white emulsion with a medium-chain length alcohol (e.g., 1-pentanol or 1-hexanol) [5]. A more general definition of the term microemulsion was given later by Danielsson and Lindman, who described it as a system, composed of water, oil and an amphiphilic component, being an optically isotropic and thermodynamically stable liquid solution [6],... 

Scientifically described for the first time in 1943 by Hoar and Schulman (2), the latter author coined the term microemulsion in 1959 to describe these optically isotropic transparent oil and water dispersions (3). Since this early work, many experimental and theoretical efforts have shown that these dispersions are actually solutions, namely thermodynamically stable equilibrium phases (4). Consequently, the most widely, but still not universally accepted definition of a microemulsion is that of a thermodynamically stable mixture of oil and water. Occasionally, the term microemulsion (5) or miniemulsion (6) is used to describe long-lived emulsions with ultra-small droplet sizes (30-100 nm). Sometimes, stable emulsions may be created by agitation of systems while passing through regions of the phase diagram where microemulsion phases form however, the final state is in the emulsion region (7, 8). In this present chapter, we use the most widely accepted definition of microemulsions, namely equilibrium phases of oil and water (9). 

Microemulsions [512, 513] are special types of emulsions that form spontaneously and have very small particles. Microemulsions are optically clear, thermodynamically stable dispersions of two immiscible liquids obtained by the use of carefully adjusted surface-active molecules (surfactants). Both liquids in a microemulsion will be present in regions of the same order of magnitude, with the dispersed phase on the order of 10-100 nm. Aggregates of surface-active molecules, or micelles, form into colloidal-sized clusters in such a way that hydrophilic groups are directed toward the water. These definitions [514] are general in nature, but they suffice for the current purpose the interested reader is directed to texts on this... 

Emulsions are formed when two immiscible liquids are mixed with each other. The most familiar forms are oil-in-water emulsions (0/W emulsions), which consist of oil droplets in water, and water-in-oil emulsions (W/O emulsions), where an aqueous solution is emulsified in an outer oil phase. In specific cases, two immiscible liquids form transparent systems, which are termed microemulsions. According to lUPAC, a microemulsion is a thermodynamically stable emulsion. This definition applies to a few systems made up of oil, water and emulsifiers, which spontaneously form transparent mixtures. Over and above this narrow definition, however, in patents and scientific literature the name microemulsion is applied to other transparent to translucent systems, which are not created spontaneously but by special production procedures. 

Microemulsions are thermodynamically stable, isotropic transparent mixtures of at least a hydrophilic, a hydrophobic and an amphiphilic component. The first microemulsion structures termed at that time "oleophatic hydro-micelle" were discovered in 1943 by Hoar and Schulman (Hoar Schulman 1943). The term microemulsion was introduced by Schulman and coworkers in 1959 describing optically isotropic transparent solutions consisting of water, oil, surfactant and alcohol (Schulman et al., 1959). A more recent definition was given by Danielsson and Lindman "A microemulsion is a system of water, oil and an amphiphile which is a single optically isotropac and thermodynamically stable liquid solution" (Danielsson Lindman 1981). Herein, "water" corresponds to a polar phase that is classically an aqueous solution that can contain electrolytes and other... 

Historically, microemulsions were discussed as a separate family of colloids that formed essentially spontaneously and were thermodynamically stable. However, microemulsions must, by definition, contain at least three components—solvent, amphiphile, and dispersed phase—and quite often contain a fourth, the so-called cosolvent. More recent experimental and theoretical work has tended to move them into the larger family of surfactant aggregates, their complex composition notwithstanding. That convention will be followed here, although there still remain a number of points of contention that need to be resolved on their classification as surfactant mesophases on a par with classical liquid crystals. 

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