Self-assembled surfactant aggregates

Clint, J. H., Surfactant Aggregation, Chapman and Hall, New York, 1992. (Graduate level. An advanced monograph on surfactant solutions and on self-assembly [ aggregation ] in surfactant solutions. A research reference at the graduate level.)... 

Micelles are dynamic structures where a frequent exchange of monomers between micelles and the bulk solution occurs. Thermodynamics of the self-assembly of surfactants or aggregates is determined by the free energy of transfer of a surfactant monomer from water to the micelle. 

Air-Water Interface. Organized films of surfactants and phospholipids at the air-water interface are of interest in biophysics, general interfacial chemistry, and have relevance to the self-assembling aggregates, which are viewed as having potential applications in non-linear optics and as microelectronic devices (122). FT-IR spectroscopy has recently been applied to the problem of obtaining information about amphiphiles at the air-water interface. 

Considerations of the packing parameter concept of Israelachvili et al. [1] suggest that double-chain surfactants, which form the basis of measurements described in this article, cannot readily form spherical micelles. With double-chain surfactants, a more likely aggregate structure is the formation of bilayer vesicles, which can be also thought of as a dispersed lamellar phase (La) as such the vesicular dispersed form is likely to be preferentially formed at low concentrations ( 1 mmol dm-3) of surfactant. Furthermore, it is necessary to consider the possibility, unlike in the case of micelles, that such vesicles, formed by self-assembly of surfactant monomers, will not be thermodynamically stable. The instability is then likely to be in the direction of growth to a thermodynamically-stable lamellar phase from the vesicles. This process will be driven, at least initially, by fusion of two vesicles. 

The structural units of surfactant-containing liquids are self-assembled aggregates, such as spherical or cylindrical micelles, or bilayers. These supramolecular structural units can then further self assemble into ordered phases, with cubic, hexagonal, smectic, or other symmetry. Consequently, the structural and flow properties of such liquids are amazingly rich. Laws of mass action, combined with geometric packing arguments, allow rationalization, if not prediction, of the phase behavior of many surfactant solutions. 

In dealing with self-assembly of surfactants and lipids, we must consider two aggregation states monolayers and bilayers. The latter class are ubiquitous in biological membranes, discussed in detail in Chapters 5 and 7. If the constituent monolayers in the bilayer are made up of identical molecules, the local geometry of both monolayers must be identical. In this case the mean... 

Vasilescu, M. Caragheorgheopol, A. Caldararu, H. Aggregation numbers and micro structure characterization of self-assembled aggregates of poly(ethylene oxide) surfactants and related block-copolymers, studied by spectroscopic methods. Adv. Colloid Interf Sci. 2001, 89, 169-194. 

There has been much interest in studying surfactant aggregation in polar solvents other than water over the last few decades. In a large number of studies various surfactant systems have been mapped and evidence for self-assembly of surfactants in some nonaqueous polar solvents has been published. During the last few years more detailed information on the structure of the aggregates and on the characteristics of the aggregation processes have been provided. 

Surfactants have been used for over 1000 years in everyday applications, for example as emulsifiers in cleaning and in foods. They occur widely in nature, where as a bilayer they constitute a vital structural unit of biological membranes. Their functionality derives from the molecular structure, with a polar (hydrophilic) head-group, which conveys water-solubility, being attached to a non-polar (hydrophobic) tail, which drives the formation of self-assembled aggregates (micelles). Other chapters in this volume detail the wide variety of chemical structures that can form the polar groups (ionic, nonionic, zwitterionic, etc.) and tail structures. 

The latter are limited to hydrocarbon, perfluorocar-bon and polydimethylsiloxane chains. While the formation of micelles is well known, surfactants also form a wide variety of liquid crystalline phases in water which are much less familiar. Almost all surfactants that form micelles also form liquid crystals, while many do not form micelles but do form liquid crystals. Thus, liquid crystal formation by surfactants is more widespread than micelle formation. Indeed, an understanding and knowledge of liquid crystals can provide a comprehensive guide to the application of surfactants. This is because the size and shape of the surfactant molecules determine the structure of the self-assembled aggregates, which in turn, controls the liquid crystal... 

Consider the complexity involved in modeling steric stabilization with a diblock copolymer. The reservoir bulk solution of copolymer is usually dilute (<1 wt % polymer) and the copolymer and solvent equilibrate between the bulk and surface regions. However, as solvent quality is decreased to the LCST phase boundary, the bulk solution will also separate into polymer-rich and polymer-lean phases. In addition, many diblock copolymers form self-assembled aggregates such as micelles and lamellae, if the concentration is above the critical micelle concentration. Thus, stabilizer can partition among up to four phases as solvent quality or polymer concentration is changed. The unique density dependence of supercritical fluids adds another dimension to the complex phase behavior possible. In the theoretical studies discussed below, surfactant adsorption energy, solubility, and concentration are chosen carefully to avoid micelle formation or bulk phase separation, in order to focus primarily on adsorption and colloid stability. 

Figure 4 Progression of aggregate microstructures for single ionic surfactants beyond the cac. (Reprinted from Coll. Surf. A, 167, R. A. Johnson and R. Nagarajan, Modeling self-assembly of surfactants at solid liquid interfaces. I. Hydrophobic surfaces, 31 2000, with permission from Elsevier.)...

Wormlike micelles are elongated and semiflexible aggregates resulting from the self-assembly of surfactant molecules in aqueous solutions. Wormlike micellar solutions have received considerable attention during the past few decades because of their remarkable structural and rheological properties. 

Polymer brushes are a recurrent structural motif in self-assembled aggregates of polymeric surfactants. The theory of polymer brushes has been reviewed exten-... 

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