Surfactant Self-Assembly

The power of optical spectroscopies is that they are often much better developed than their electron-, ion- and atom-based counterparts, and therefore provide results that are easier to interpret. Furtlienuore, photon-based teclmiques are uniquely poised to help in the characterization of liquid-liquid, liquid-solid and even solid-solid interfaces generally inaccessible by other means. There has certainly been a renewed interest in the use of optical spectroscopies for the study of more realistic systems such as catalysts, adsorbates, emulsions, surfactants, self-assembled layers, etc. 

For obvious reasons, we need to introduce surface contributions in the thermodynamic framework. Typically, in interface thermodynamics, the area in the system, e.g. the area of an air-water interface, is a state variable that can be adjusted by the observer while keeping the intensive variables (such as the temperature, pressure and chemical potentials) fixed. The unique feature in selfassembling systems is that the observer cannot adjust the area of a membrane in the same way, unless the membrane is put in a frame. Systems that have self-assembly characteristics are conveniently handled in a setting of thermodynamics of small systems, developed by Hill [12], and applied to surfactant self-assembly by Hall and Pethica [13]. In this approach, it is not necessary to make assumptions about the structure of the aggregates in order to define exactly the equilibrium conditions. However, for the present purpose, it is convenient to take the bilayer as an example. 

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

Describe how the critical packing parameter for surfactant self-assembly can be used to describe the structure of typical biological lipid membranes. 

In terms of particle growth, some analogies between surfactant self assemblies and natural media can be proposed. In both cases, this growth needs a supersaturated medium where the nucleation can take place. 

Biologically friendly ionic surfactants can be added to the wastewater at concentrations above the threshold value beyond which the surfactants self-assemble to form micelles. The resulting micelles can trap the hydrocarbon wastes since the hydrocarbon solutes prefer the hydrocarbon interior of the micelle over the aqueous environment outside. In addition, ionic wastes in the water adsorb to the polar heads of the surfactants (see Fig. 8.1). The resulting waste-laden micelles can then be removed more easily using ultrafiltration methods. Such a process, known as micellar-enhanced ultrafiltration (MEUF), can be made continuous, scalable, cost effective, and environmentally friendly (through the use of biodegradable surfactants). 

Finally, a discussion of surfactant self-assembly will not be complete without a mention of surfactant assemblies in biological systems. Although they are outside the scope of our book, we have already drawn attention to such biological applications of colloid science in Chapters 1 and 7 and above in this chapter. Some additional discussion is provided in the last section of this chapter (Section 8.11). 

Nagarajan, R., and E. Ruckenstein. 1991. Theory of surfactant self-assembly A predictive molecular thermodynamic approachangmuir7 2934-2969. 

I. Sodeberg, C. J. Drummond, D. N. Furlong, S. Godkin, and B. Matthews, Non-ionic sugar-based surfactants Self assembly and air/water interfacial activity, Colloids Surf. A, 102 (1995) 91-97. 

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

Nagarajan, R. Ruckenstein, E. Theory of surfactant self-assembly a predictive molecular thermodynamics. Langmuir 1991, 7, 2934. 

Two theoretical techniques worthy of serious review here, perturbation and Green function methods, can be considered complementary. Perturbation methods can be employed in systems which deviate only slightly from regular shape (mostly from planar geometry, but also from other geometries). However, they can be used to treat both linear and nonlinear PB problems. Green function methods on the other hand are applicable to systems of arbitrary irregularity but are limited to low surface potential surfaces for which the use of the linear PB equation is permitted. Both methods are discussed here with reference to surfactant solutions which are a potentially rich source of nonideal surfaces whether these be solid-liquid interfaces with adsorbed surfactants or whether surfactant self-assembly itself creates the interface. 

Expanding the synthesis tool box beyond surfactant self-assembly 2.6.1 Block co-polymers templates... 

Comparable to the binary systems (water-surfactant or oil-surfactant), self-assembled structures of different morphologies can be obtained ranging from (inverted) spherical and cylindrical micelles to lamellar phases and bicontin-uous structures. To map out these regions, a phase diagram is most useful. 

A central issue in the field of surfactant self-assembly is the structure of the liquid crystalline mesophases denoted bicontinuous cubic, and "intermediate" phases (i.e. rhombohedral, monoclinic and tetragonal phases). Cubic phases were detected by Luzzati et al. and Fontell in the 1960 s, although they were believed to be rare in comparison with the classical lamellar, hexagonal and micellar mesophases. It is now clear that these phases are ubiquitous in surfactant and Upid systems. Further, a number of cubic phases can occur within the same system, as the temperature or concentration is varied. Luzzati s group also discovered a number of crystalline mesophases in soaps and lipids, of tetragonal and rhombohedral symmetries (the so-called "T" and "R" phases). More recently, Tiddy et al. have detected systematic replacement of cubic mesophases by "intermediate" T and R phases as the surfactant architecture is varied [22-24]. The most detailed mesophase study to date has revealed the presence of monoclinic. 

Quaternary structure is akin to the mesostructure of lipid or surfactant self-assemblies, such as the aggregates characteristic of mesh-structures in bacterial protein coats (described in Chapter 4), or the cholesteric liquid crystals found in... 

Many reports propose that clay has played an important role, both as a catalyst for condensation of amino acids into peptides [26] and even for information storage. Recall that clays may well e)Aibit a hyperbolic architecture at a mesostructural level (Chapter 2). Just as zeolites can be templated by hyperbolic surfactant self-assemblies, lipid assemblies can be templated by clays. 

Leontidis, E. Hofmeister anion effects on surfactant self-assembly and the formation of mesoporous solids. Curr. Opin. Colloid Interface Sci. 2002, 7 (1-2), 81-91. 

In the case of simple ionic surfactants, such aggregates have also been called hemimi-celles, admicelles, and surfactant self-assemblies. 

As already mentioned, for a long period of time, the microstructure of microemulsions was considered to be that of droplets of one liquid dispersed in another, i.e. either water-in-oil (w/o-) or oil-in-water (o/w-) microemulsions. While this picture was easy to understand for water-rich or oil-rich systems, it became problematic for microemulsions with similar volume fractions of the two solvents. Even more intriguing from a microstructural point of view was the discovery by Friberg and Shinoda of systems with a continuous transition from water-rich to oil-rich systems. Suggestions of a coexistence of oil and water droplets were made by others. However, contradicting our general understanding of surfactant self-assembly structures, they were immediately rejected. 

It is interesting to note that while the cnc for cationic polyelectrolytes is lower on glass than on mica, the opposite is true for cationic surfactants [11]. The reason for this difference is that both electrostatic forces and hydrophobic interactions between neighboring surfactant tails drive the surfactant self-assembly on the surface. Since the lattice sites are closer to each other on mica than on glass, the self-assembly of the surfactant is more favorable on the former surface. 

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