Self-assembled surfactant structures

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. 

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. 

Figure 8 Examples of (a) discrete and (b) continuous surfactant self-assembled structures. The latter can be extended in one (cylinders), two (lamellae), or three (bicontinuous bilayer) dimensions. (From...

The success of NMR for studies of microemulsions has basically been that it can address one of the key questions, that of microheterogeneity. This is, for various reasons, very difficult to study by other physicochemical approaches and often informative regarding other types of complex formation, including certain aspects of surfactant self-assembly. To understand this we need to look at microemulsions in a broader context, and we start with micelles. The spherical micelle was the first firmly established surfactant self-assembly structure, and the spherical aggregate structure has penetrated thinking in this field of study to such an extent that almost every new phase or phenomenon in surfactant solutions has at some point been considered as based on spherical micellar units ... 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

Figure 6.4. Schematic phase diagram for a three-component (oil, water, surfactant) system showing some of the self-assembled structures which form in the various regions.

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. 

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

Micelles are spontaneously formed by most surfactants (especially single-chained ones) even at fairly low concentrations in water, whereas at higher surfactant concentrations, with or without the addition of an oil (e.g. octane) or co-surfactant (e.g. pentanol), a diverse range of structures can be formed. These various structures include micelles, multibilayers (liquid crystals), inverted micelles, emulsions (swollen micelles) and a range of microemulsions. In each case, the self-assembled structures are determined by the relative amounts of surfactant, hydrocarbon oil, co-surfactant (e.g. pentanol) and water, and the fundamental requirement that there be no molecular contact between hydrocarbon and water. 

As discussed in Section 2.2, surfactant has a tendency to adsorb at interfaces since the polar head group has a strong preference for remaining in water while the hydrocarbon tail prefers to avoid water. The surfactant concentration affects the adsorption of surfactants at interfaces. Surfactant molecules lie flat on the surface at very low concentration. Surfactant molecules on the surface increase with increasing surfactant concentration in the bulk and surfactant tails start to orient towards gas or non-polar liquid since there is not enough space for the surfactant molecules to lie flat on the surface. Surfactant molecules adsorb at the interface and form monolayer until the surface is occupied at which point surfactant molecules start forming self-assembled structures in the liquid (Section 2.3). 

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... 

Nanoparticle shape control can be easily effected by using self-assembled structures such as micelles (arising due to spontaneous assembly of suitable surfactants in water) as templates [4-6]. 

Biological systems provide numerous examples of self-assembled objects. Owing to the relatively weak interactions involved, a self-assembled structure is much more sensitive and responsive to its environment than a more rigid structure held together by covalent bonds. Unlike processes involving simple surfactants, polymers, and nanoparticles, self-assembly processes in biological systems are usually directional and functional and often lead to the formation of extremely complex structures. For example, the three-dimensional structure adopted by a protein in solution is critical to the protein s function, and this structure is determined by both strong (covalent) and weak... 

Mixtures of two homopolymers (A and B) and their corresponding diblock copolymer (A-B) are polymeric counterparts of mixtures of water, oil and surfactant. The immiscible nature between water and oil is also observed in polymer blends due to the fact that most polymers are immiscible in each other. The addition of diblock copolymers into blends of homopolymers has effects similar to adding surfactants into water-oil mixtures. The resulting reduction in interfacial tension and formation of the preferred interfacial curvature yield a variety of self-assembled structures. 

Finally, it needs to be mentioned that the mechanisms of supramolecular self-assembly apply to many systems more complex than simple surfactants and amphiphilic block copolymers. Supramolecular self-assembly and pattern formation is one of the crucial principles in nature and gives rise to hierarchical structure ranging from the length scale of a few nanometers to the macroscopic domain. The generation of intricate and very regular structures by templating supramolecularly self-assembled structures in the shape of LLC phases has proven extremely successful. Therefore, the utilization of the more complex systems found in nature (e.g. protein assemblies in viruses) is at the hands of mod-... 

---