Thermodynamics microemulsions

Microemulsion Thermodynamically isotropic system of water, oil, and surfactant. 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

Isopar-M SSO-POEHO AIBN 1 1 > 10 50 uv Holtzscherer and Candau [3] Bkontinuous microemulsion. Thermodynamic stability Inverse- Microemulsion... 

This book is divided into five parts as follows Part I Historieal Perspeetive Part II Structural Aspects and Characterization of Microemulsions Part III Reactions in Microemulsions Part IV Applications of Microemulsions and Part V Future Prospects. The book opens with the chapter on the historical development of microemulsion systems by two leading authorities (Lindman and Friberg) who have significantly contributed to the field of microemulsions. In the next two chapters J. Th. G. Overbeek (the doyen of colloid science) and coworkers and E. Ruckenstein advance different approaches to describe the thermodynamics of microemulsion systems. While a full description of microemulsion thermodynamics is far from complete, the droplet type model predicts the experimental observations quite well. A theory that predicts the global phase behavior and the detailed properties of the phases as a function of experimentally adjustable parameters is still under development. 

The important properties unique to microemulsions - thermodynamic stability, ultra-low interfacial tensions, translucence, small and tunable microstructures - make microemulsions interesting for a variety of applications. Microemulsions find application as a reaction medium for formation of polymeric and inorganic nanoparticles, for the dispersion of drugs, food stuffs, agrochemicals, and cosmetic ingredients, and in detergency, the enhancement of oil recovery from reservoirs, and the extraction of contaminated solids (17). 

Microemulsions are treated in a separate section in this chapter. Unlike macro- or ordinary emulsions, microemulsions are generally thermodynamically stable. They constitute a distinctive type of phase, of structure unlike ordinary homogeneous bulk phases, and their study has been a source of fascination. Finally, aerosols are discussed briefly in this chapter, although the topic has major differences from those of emulsions and foams. 

A beautiful and elegant example of the intricacies of surface science is the formation of transparent, thermodynamically stable microemulsions. Discovered about 50 years ago by Winsor [76] and characterized by Schulman [77, 78], microemulsions display a variety of useful and interesting properties that have generated much interest in the past decade. Early formulations, still under study today, involve the use of a long-chain alcohol as a cosurfactant to stabilize oil droplets 10-50 nm in diameter. Although transparent to the naked eye, microemulsions are readily characterized by a variety of scattering, microscopic, and spectroscopic techniques, described below. 

These fascinating bicontinuous or sponge phases have attracted considerable theoretical interest. Percolation theory [112] is an important component of such models as it can be used to describe conductivity and other physical properties of microemulsions. Topological analysis [113] and geometric models [114] are useful, as are thermodynamic analyses [115-118] balancing curvature elasticity and entropy. Similar elastic modulus considerations enter into models of the properties and stability of droplet phases [119-121] and phase behavior of microemulsions in general [97, 122]. 

Modem scaling theory is a quite powerful theoretical tool (appHcable to Hquid crystals, magnets, etc) that has been well estabUshed for several decades and has proven to be particularly useful for multiphase microemulsion systems (46). It describes not just iuterfacial tensions, but virtually any thermodynamic or physical property of a microemulsion system that is reasonably close to a critical poiat. For example, the compositions of a microemulsion and its conjugate phase are described by equations of the foUowiug form ... 

However, the formal differences between microemulsions and macroemulsions are well defined. A microemulsion is a single, thermodynamically stable, equihbrium phase a macroemulsion is a dispersion of droplets or particles that contains two or more phases, which are Hquids or Hquid crystals (48). 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

A microemulsion (p.E) is a thermodynamically stable, transparent (in the visible) droplet type dispersion of water (W) and oil (O a saturated or unsaturated hydrocarbon) stabilized by a surfactant (S) and a cosurfactant (CoS a short amphiphile compound such as an alcohol or an amine) [67]. Sometimes the oil is a water-insoluble organic compound which is also a reactant and the water may contain mineral acids or salts. Because of the small dispersion size, a large amount of surfactant is required to stabilize microemulsions. The droplets are very small (about 100-1000 A [68]), about 100 times smaller than those of a typical emulsion. The existence of giant microemulsions (dispersion size about 6000 A) has been demonstrated [58]. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

Applied Surface Thermodynamics, edited by A. 14/. Neumann and Jan K. Spelt Surfactants in Solution, edited by Arun K. Chattopadhyay and K. L. Mittal Detergents in the Environment, edited by Milan Johann Schwuger Industrial Applications of Microemulsions, edited by Conxita Solans and Hironobu Kunieda... 

Percolation in microemulsions and concomitant microstructural changes are the focal points of this review. A complete understanding of percolation phenomena in reverse microemulsions will require an understanding of droplet interactions and the associated thermodynamics of droplet fusion, fission, aggregation to form clusters of varying fractal... 

In 1959, J. H. Schulman introduced the term microemulsion for transparent-solutions of a model four-component system [126]. Basically, microemulsions consist of water, an oily component, surfactant, and co-surfactant. A three phase diagram illustrating the area of existence of microemulsions is presented in Fig. 6 [24]. The phase equilibria, structures, applications, and chemical reactions of microemulsion have been reviewed by Sjoblom et al. [127]. In contrast to macroemulsions, microemulsions are optically transparent, isotropic, and thermodynamically stable [128, 129]. Microemulsions have been subject of various... 

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 influence of surfactant structure on the nature of the microemulsion formed can also be predicted from the thermodynamic theory by Overbeek (17,18). According to this theory, the most stable microemulsion would be that in which the phase with the smaller volume fraction forms the droplets, since the osmotic term increases with increasing i. For w/o microemulsion prepared using an ionic surfactant, the hard sphere volume is only slightly larger than the water volume, since the hydrocarbon tails of the surfactant may interpenetrate to a certain extent, when two droplets come close together. For an oil in water microemulsion, on the other hand, the double layer may extend to a considerable extent, depending on the electrolyte concentration... 

The term microemulsion is applied in a wide sense to different types of liquid liquid systems. In this chapter, it refers to a liquid-liquid dispersion of droplets in the size range of about 10-200 nm that is both thermodynamically stable and optically isotropic. Thus, despite being two phase systems, microemulsions look like single phases to the naked eye. There are two types of microemulsions oil in water (O/W) and water in oil (W/O). The simplest system consists of oil, water, and an amphiphilic component that aggregates in either phase, or in both, entrapping the other phase to form... 

As is often the case, we have become involved in microemulsions somwehat by accident. In the last five years or so we have been making systematic studies of the thermodynamic properties of aqueous organic mixtures and of electrolytes in these mixed solvents. Of particular interest were our heat capacity measurements. With a differential flow microcalorimeter it is possible to... 

A microemulsion is defined as a thermodynamically stable and clear isotropic mixture of water-oil-surfactant-cosurfactant (in most systems, it is a mixture of short-chain alcohols). The cosurfactant is the fourth component, which effects the formation of very small aggregates or drops that make the microemulsion almost clear. 

Microemulsions are thermodynamically stable mixtures. The interfacial tension is almost zero. The size of drops is very small, and this makes the microemulsions look clear. It has been suggested that microemulsion may consists of bicontinuous structures, which sounds more plausible in these four-component microemulsion systems. It has also been suggested that microemulsion may be compared to swollen micelles (i.e., if one solubilizes oil in micelles). In such isotropic mixtures, short-range order exists between droplets. As found from extensive experiments, not all mixtures of water-oil-surfactant-cosurfactant produce a microemulsion. This has led to studies that have attempted to predict the molecular relationship. 

Part II starts with the possibilities of ACE for characterizing the relevant physicochemical properties of drugs such as lipophilicity/hydrophilicity as well as thermodynamic parameters such as enthalpy of solubilization. This part also characterizes interactions between pharmaceutical excipients such as amphiphilic substances (below CMC) and cyclodextrins, which are of interest for influencing the bioavailability of drugs from pharmaceutical formulations. The same holds for interactions of drugs with pharmaceutical vehicle systems such as micelles, microemulsions, and liposomes. 

Microemulsions are thermodynamically stable, homogeneous, optically isotropic solutions comprised of a mixture of water, hydrocarbons and amphiphilic compoxmds. The microemulsions are usually four- or three-component systems consisting of surfactant and cosurfactant (termed as emulsifier), oil and water. The cosurfactants are either lower alkanols (like butanol, propanol and hexanol) or amines (Hke butylamine, hexylamine). Microemulsions are often called swollen micelles (Fig. 3) and swollen re-... 

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