Organized surfactant assemblies

The fluidity (nanoviscosity) in an organized surfactant assembly on soUds can be substantially different from that in the bulk aqueous phase and hence, the diffusional resistance experienced by the probe in the micelle will be considerably different from that faced in the bulk solution [ 145]. Measurement of the viscosity or fluidity of the interior of a micelle is based on measurement of fluorescence properties that depend on the mobihty of the probe in the interior. A commonly used method for such studies involves the intramoleciflar... 

Structure and Properties of Different Organized Surfactant Assemblies... 

Other Useful Applications. It is well known that there are many other important applications of surfactants and organized surfactant assemblies in separation science. Many specific separation processes such as secondary and tertiary oil recovery (500-502), tar sand extraction (503). gas scrubbing and purification (504) and different electrophoretic techniques utilize surface active agents (505). However, space limitations and the existence of several recent review articles preclude further discussion of these applications in this particular overview. 

Reverse micelle and microemulsion solutions are mixtures of a surfactant, a nonpolar fluid and a polar solvent (typically water) which contain organized surfactant assemblies. The properties of a micelle phase in supercritical propane and ethane have been characterized by conductivity, density, and solubility measurements. The phase behavior of surfactant-supercritical fluid solutions is shown to be dependent on pressure, in contrast to liquid systems where pressure has little or no effect. Potential applications of this new class of solvents are discussed. 

FULTON SMITH Organized Surfactant Assemblies in Supercritical Fluids 93... 

El Seoud, O.A. Effects of organized surfactant assemblies on acid-base equilibria. Adv. Colloid Interface Sci. 1989, 30(1-2), 1-30. 

It is well known that surfactants form several types of well-organized assemblies that provide specific size, geometrical control, and stabilization to particulate assemblies formed within the organized surfactant assemblies. The host surfactant assemblies that are available for the formation of nanoparticles are summarized in Table 1. The aqueous micellar solutions, reverse micelles, microemulsions, vesicles, monolayers, Langmuir-Blodgett films, and bilayer lipid membranes are typical surfactant assemblies that are often employed to prepare nanoparticles [9,10]. 

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. 

Functionalized polyelectrolytes are promising candidates for photoinduced ET reaction systems. In recent years, much attention has been focused on modifying the photophysical and photochemical processes by use of polyelectrolyte systems, because dramatic effects are often brought about by the interfacial electrostatic potential and/or the existence of microphase structures in such systems [10, 11], A characteristic feature of polymers as reaction media, in general, lies in the potential that they make a wider variety of molecular designs possible than the conventional organized molecular assemblies such as surfactant micelles and vesicles. From a practical point of view, polymer systems have a potential advantage in that polymers per se can form film and may be assembled into a variety of devices and systems with ease. 

Other examples of organized molecular assemblies of interest for photocatalysis are (1) PC-A, PC-D or D-PC-A molecules where PC, A and D fragments are separated by rigid bridges (2) host-guest complexes (3) micelles and microemulsions (4) surfactant monolayers or bilayers attached to solid surfaces, and (5) polyelectrolytes [19]. 

Figure 1.2 September 2007 cover of a leading international chemistry journal dedicated to sol-gel materials shows mesostructured silicas derived from the cooperative assembly of soluble sol-gel precursors and organic surfactant molecules. (Reproduced from acs.org, with permission.)...

Because of the hmited solubility in different solvents surfactants form different types of surfactant assemblies in solutions and on solids. These organized assemblies are formed when different proportions of surfactants, oils, cosurfactants and water are mixed together. The types of surfactant aggregate formed depends on its chemical structure and the nature of the medium. 

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). 

Naseem, B. et al.. Interaction of flavonoids within organized molecular assemblies of anionic surfactant. Colloids Surf. B, 35, 7, 2004. 

A brief description of the structural features and relevant properties of different organized assemblies formed from surfactant molecules is presented. Next, the use and application of these organized surfactant systems in separation science is surveyed. Several possible new areas for future developments employing these ordered media are mentioned. 

Micelles and other organized surfactant aggregates are increasingly utilized in analytical applications (1.)- They interact with reagents and alter spectroscopic and electrochemical properties which, in turn, often results in increased sensitivities. Organized assemblies have also been employed in separation processes. Gas, liquid and thin layer micellar chromatographic techniques have been developed (2). 

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