Self-Assembly and Phase Behavior

The primary characteristic of surfactant molecules is their ability to locate at the interface between two different fluids. So far, we have only discussed the effects of this property at low surfactant concentrations, but at high surfactant concentrations, it is equally important. 

When there is a much larger number of surfactant molecules in the solvent than it would take just to form a monolayer at the surface, the surfactants in the bulk solution move randomly and adopt the most energetically favorable configuration. This self-assembly mechanism leads to the spontaneous formation of a variety of different surfactant phases driven by the hydrophobic effect (also known as lyotropic phases because their structure depends on concentration in the solvent). In the following sections, we examine some of the different concentration-dependent phases that can form in surfactant systems. 

FIGURE 3.5 Idealized geometry of a micelle. The alkyl chains on the surfactant molecule are directed toward the center of the micelle the head groups arrange to coat the outside, shielding the hydrophobic core from the surrounding water molecules. 

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Minich E.A., Nowak A.P., Deming T.J., and Pochan, D.J. Rod-rod and rod-coil self-assembly and phase behavior of pol3fpeptide diblock copolymers. Polymer, 45, 1951, 2004. 

FTSs are widdy used to investigate self-assembly and phase behavior of heterogeneous polymeric materials, particularly block copolymers. Fredrickson and coworkers have shown that this approach can be extended in various ways, for example, for systems with external stresses and for charged polymers. One of the main advantages of the field-theoretic methods is that any arbitrary morphology can be described... 

Minich EA, Nowak AP, Deming TJ, Pochan DJ (2004) Rod-rod and rod-coU self-assembly and phase behavior of polypeptide diblock copolymers. Polymer 45 1951-1957... 

Balazs, A.C. (2007) Modeling self-assembly and phase behavior in complex mixtures. Anna. Rev. Phys. Chem., 58, 211-233. 

The final example, which we discuss in some detail, is a molecular dynamics study of the self-assembly and phase behavior of reversibly aggregating linear model polymers. Here we consider the coupling between the polymer length distribution and orientational ordering as a function of monomer concentration including the influence of aggregate flexibility (cf. above). In this model, a detailed description is given elsewhere [87] monomers interact via a Lennard-Jones-like potential with anisotropic attraction i.e.. 

Elastin-like polypeptides have been explored as hydrogels due to their capacity for self-assembly and phase transition behavior they also mimic many features of the extracellular matrk and have the potential to guide the migration, growth and organization of... 

Tailoring block copolymers with three or more distinct type of blocks creates more exciting possibilities of exquisite self-assembly. The possible combination of block sequence, composition, and block molecular weight provides an enormous space for the creation of new morphologies. In multiblock copolymer with selective solvents, the dramatic expansion of parameter space poses both experimental and theoretical challenges. However, there has been very limited systematic research on the phase behavior of triblock copolymers and triblock copolymer-containing selective solvents. In the future an important aspect in the fabrication of nanomaterials by bottom-up approach would be to understand, control, and manipulate the self-assembly of phase-segregated system and to know how the selective solvent present affects the phase behavior and structure offered by amphiphilic block copolymers. 

Although a wide range of polymers have been investigated with various cyclodex-trins, these studies mainly focused on the IC preparation techniques and characterization of solid phases. The solution properties, such as the self - assembly behavior, dissociation, particle size an surface activity, were not commonly reported. These solution properties, especially the assembly and surface behavior, are vital for the potential applications of such systems in biomedical science, such as in controlled drug delivery. 

The following is a summary, based on Ref. 1, of the types of molecular interactions that are important in understanding the structure and phase behavior of surfaces and interfaces. Because they are multicomponent, the interactions in systems with surfaces and interfaces are often related to the interactions between molecules in a particular type of medium. This is particularly important for self-assembling systems composed of surfactants or polymers, where the interactions and the subsequent equilibrium structures are strongly influenced by the type of solvent. 

Much progress in the theoretical understanding of self-assembly, structure, and phase behavior of amphiphilic systems has been made since (for reviews, see Refs. 6-8). To a large extent, this is due to the careful analysis of statistical-mechanical models of microemuisions. Here, two different classes of models are used. 

Surfactant self-assembly is a delicate balance between hydrophobic and hydrophilic interactions, and the interactions between the headgroups and the solvent are decisive both for the onset of self-assembly and for the curvature of the surfactant films and thus for aggregate shape and phase behavior. H, H, and NMR have been used successfully to study the hydration of surfactant aggregates. The three by far most used approaches are H (or H) self-diffusion, O quadrupole relaxation, and quadrupole splittings. We stress at the outset that a division into free and bound water molecules on which the concepts of hydration and hydration number are based is far from unambiguous, and furthermore this division is dependent on the physicochemical parameter monitored. 

FLUOROPOLYMERS IN SUPERCRITICAL CARBON DIOXIDE PHASE BEHAVIOR, SELF-ASSEMBLY, AND STABILIZATION OF WATER/CO2 EMULSIONS... 

R Alexandridis, U. Olsson, P, Linse, B. Lindman, Structural polymorphism of amphiphilic block copolymers in mixtures with water and oil Comparison with solvent-free block copolymers and surfactant systems. In Amphiphilic Block Copolymers. Self-Assembly and Applications, Ed. P. Alexandridis, U. Olsson, B. Lindman, p. 169, Elsevier, Amsterdam (2000) (overview over the phase behavior of PEO-PPO block copolymers). 

Self-assembly and morphology of block copolymers depend on their architecture and composition [3]. Several equilibrium phases like lamellae, gyroid, hexagonal-packed cylinders, and body-centered cubic phases were observed in melts. In thin films, microphase separation resulted in formation of lamellae, stripes, and circular domains. Various types of micellar structures and arrangements were seen in dilute solutions [4], These phase behaviors were dictated by Flory-Huggins interaction parameter (/), copolymer degree of polymerization N), and composition (/) in melts and thin films. In addition to these parameters, amphiphilicity was the most important property of block copolymers enabling them to self-assemble into various stmctures in dilute solutions [3]. 

Phase behavior, self-assembly, and rheology properties of 3-amino-propyltriethojq silane (APTES) and nonionic surfactant C12E04 in water were studied. The self-assembled structures were detected by means of small angle X-ray scattering (SAXS), cryo-transmission electron microscopy (ctyo-TEM), freeze-fracture TEM (FF-TEM), and H NMR measurements and the properties of self-assembled mixtures were obtained by rheology measurements. With a variation of the compn., different self-assembled structures were obtained. ... 

Su et al. (Xiao et al., 2008) fabricated a new type of thermotropic liquid-crystalline photosensitive supramolecular ionic self-assembly of polyelectrolyte and functional unit azobenzene IL crystal (azo-ILC), where the thermal and phase behaviors can be modulated by changing the spacer length (methylene units in azo). Ma et al. (Ma et al., 2008) found that the addition of very small amounts of an alcohol or water into tri-n-decylmethylphosphonium chloride and bromide salts (IPlOX) induced the formation of liquid crystalline, where strong association between the hydroxyl groups of alcohol or water and the head groups of IPlOX is indicated. 

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