Surfactants Microemulsions Micelles

Description of the different mimetic systems will be the starting point of the presentation (Sect. 2). Preparation and characterization of monolayers (Langmuir films), Langmuir-Blodgett (LB) films, self-assembled (SA) mono-layers and multilayers, aqueous micelles, reversed micelles, microemulsions, surfactant vesicles, polymerized vesicles, polymeric vesicles, tubules, rods and related SA structures, bilayer lipid membranes (BLMs), cast multibilayers, polymers, polymeric membranes, and other systems will be delineated in sufficient detail to enable the neophyte to utilize these systems. Ample references will be provided to primary and secondary sources. 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Generation of nanoparticles under Langmuir monolayers and within LB films arose from earlier efforts to form nanoparticles within reverse micelles, microemulsions, and vesicles [89]. Semiconductor nanoparticles formed in surfactant media have been explored as photocatalytic systems [90]. One motivation for placing nanoparticles within the organic matrix of a LB film is to construct a superlattice of nanoparticles such that the optical properties of the nanoparticles associated with quantum confinement are preserved. If mono-layers of capped nanoparticles are transferred, a nanoparticle superlattice can be con-... 

Zana R. Dynamics of surfactant self-assemblies micelles, microemulsions, vesicles, and lyotropic phases. New York CRC Press 2005. 

Surfactants provide several types of well-organized self-assembhes, which can be used to control the physical parameters of synthesized nanoparticles, such as size, geometry and stability within liquid media. Estabhshed surfactant assembles that are commonly employed for nanoparticie fabrication are aqueous micelles, reversed micelles, microemulsions, vesicles [15,16], polymerized vesicles, monolayers, deposited organized multilayers (Langmuir-Blodgett (LB) films) [17,18] and bilayer Upid membranes [19](Fig. 2). 

Zana, R. (ed.), Dynamics of Surfactant Self-Assemblies Micelles, Microemulsions, Vesicles and Lyotropic Phases. CRC New York, 2005. 

A microemulsion, Fig. 1, has a similar organization to that characteristic of a micelle but employs, rather than one, multiple surfactant components, allowing for introduction of other additives into the hydrophobic core [11], As with micelles, microemulsions are optically transparent and can be easily studied by standard spectroscopic methods. One important use of such microemulsions is in the photoinduced initiation of polymerization of monomers with low water solubility many such reactions involve a mechanism occurring through photoinduced interfacial electron transfer. 

Garti, N. Aserin, A. Double emulsions stabilized by macro-molecular surfactants. In Micelles Microemulsions and Monolayers, Shah, D.O., Ed. Marcel Dekker, Inc. New York, 1998 333-362. 

On the contrary, this set of experimental results would provide some ground for a theoretical and thermodynamical explanation of the evolution swollen micelle-microemulsion. Indeed each type of structure seems to reflect a domination of one or other component of the free energy of these nonionics at room temperature. Although a calculation and a discussion of these energy effects are well beyond the scope of the present paper, we can point out the importance of the forces specific to the hydrocarbon chain and to the oil beside the pure hydration forces. Van der Waals forces would favour the formation of a water layer, while entropic effects seem very important as far as the transitions hank-lamella and lamella-globule are concerned. These effects due to the solvent concentration (but also to the nature of the oil (2,5) are quite evident from the fine evolution of the phase diagrams, especially for water/surfactant ratios in the range 0.5-1.2. 

VI.4. Solubilization in Solutions of Micelle-Forming Surfactants. Microemulsions... 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosucdnate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelle-forming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/wata systems is already wdl documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system.l The herical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now wdl-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e.g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

As a general description, a microemulsion is a homogeneous phase that contains a substantial fraction of both oil and water, their mixing being induced by the dissolved surfactant. In contrast with macroemulsions, microemulsion systems form spontaneously in solution. As with micelles, microemulsions are thermodynamically stable. As a result, in contrast to macroemulsions, microemulsion formation is reversible. For example, if changes in a system parameter (e.g., temperature) alter the microemulsion system, when the system parameter returns to its original state, so does the microemulsion system. In contrast to macroemulsions, a microemulsion forms virtually independent of the volume of the oil phase, but once formed, the microemulsion enhances the aqueous solubility of the oil phase. Most often, an excess oil phase persists in the presence of a microemulsion phase—the oil and water phases do not completely mix. 

The emulsion polymerization technique usually contains a micelle-forming surfactant and a water-soluble initiator in combination with a water-insoluble monomer. Polymerization takes place in the monomer-swollen micelles and latex particles. Therefore, the term emulsion polymerization is a misnomer the starting point is an emulsion of monomer droplets in water, and the product is a dispersion of latex particles. In the case of microemulsion polymerization, the monomer droplets are made very small (typical particle radius is 10-30 nm) and they become the locus of polymerization. In order to obtain such small droplets, a co-surfactant (e.g. hexanol) is usually applied. A microemulsion is thermodynamically stable... 

To control the size and size distribution, synthesis of magnetic nanoparticles in a W/0 microemulsion has been reported. The presence of surfactant molecules results in the formation of different sizes (1-10 nm) of micelles. The surfactant molecules organize themselves with the polar end inside in the water phase and the non-polar end in the oil phase. The micelles/droplets contain the aqueous solution of iron salts. The concentration and type of surfactants and metal ions, the pH, reducing agents and co-surfactants can all affect the particle growth and, consequently, the particle size distribution (Fig. 4) [82, 83]. 

The complexity of the equilibrium phases and nonequilibrium phenomena exhibited by multicomponent oil-water-surfactant systems is amply demonstrated in numerous contributions in this volume. Therefore, the need for theoretical (and computational) methods that make the interpretation of experimental observations easier and serve as predictive tools is readily apparent. Excellent treatments of the current status of theoretical advances in dealing with microemulsions are available in recent monographs and compendia (see, e.g., Refs. 1-3 and references therein). These references deal with systems consisting of significant fractions of oil and water and focus on the different phases and intricate microstructures that develop in such systems as the surfactant and salt concentrations are varied. In contrast, the present chapter focuses exclusively on simulations, particularly on a first level introduction to the use of lattice Monte Carlo methods for modeling self-association and phase equilibria in surfactant solutions with and without an oil phase. Although results on phase equilibria are presented, we spend a substantial portion of the review on micellization in surfactant-water mixtures, as this forms the necessary first step in the eventual identification of the most essential parameters needed in computer models of surfactant-water-oil systems. 

The use of surfactants or amphiphilic molecules in electrochemistry dates back over four decades [1,2]. Extensive research on electrochemistry in surfactant systems has been reported primarily in the last 20 years. Surfactant systems are ubiquitous. The aggregation of surfactant molecules may produce a variety of systems including micelles, monolayers and bilayers, vesicles, lipid films, emulsions, foams, and microemulsions. Developments in the area of electrochemistry in such association colloids and dispersions have been documented by Mackay and Texter [3]. Mackay [4] reviewed the developments in association colloids, particularly micelles and microemulsions. Rusling [5,6] also reviewed electrochemistry in micelles, microemulsions, and related organized media. This chapter focuses on microemulsions and does not deal with micelles, monolayers, emulsions, and other surfactant systems per se. 

The potential of microemulsions as a means of preparing liposomal solutions has not been realized in spite of the fact that the method offers significant advantages over existing methods. Instead, the publications so far have been concerned with simple surfactant solutions, with the process as follows. A micelle-forming surfactant (Sm) and a liposome-forming one (Sl) are combined at a concentration Sm > cmcsM (where cmc = critical micelle concentration) [43-45]. The liposome/micelle fraction Sl/(Sl + Sm) has attracted some attention [46]. 

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