Liquid spherical micelle phase

Fig. 4.1 Top schematic illustration of micellar phases formed by the Pluronic copolymer P85 (PE 026PP0i9 PEO,6) with increasing temperature. Bottom small-angle neutron scattering patterns from sheared solutions in D20 of this copolymer (25wt%). The three columns (left-right) correspond to a liquid spherical micelle phase at 25 °C, a cubic phase of spherical micelles at 27 °C and a hexagonal phase of rod-like micelles at 68 °C (Mortensen 1993a).

In contrast to the above-described kinetic stability, colloids may also be thermodynamically stable. A stable macromolecular solution is an example we have already discussed. Formation of micelles beyond the critical micelle concentration is another example of the formation of a thermodynamically stable colloidal phase. However, when the concentration of the (say, initially spherical) micelles increases with addition of surfactants to the system, the spherical micelles may become thermodynamically unstable and may form other forms of (thermodynamically stable) surfactant assemblies of more complex shapes (such as cylindrical micelles, liquid-crystalline phases, bilayers, etc.). 

The crystallinity of liquid crystal phases refers to the large assortment of ways these micellar structures can be organized within a bulk phase. For example, spherical micelles of... 

Recent work has supported early observations (e.g. Aggarwal 1976 Hashimoto et al. 1983) of a liquid micellar phase between the BCC micelle phase and the disordered phase. A representative TEM image from a spherical micellar liquid phase is shown in Fig. 2.18. Kinning and Thomas (1984) analysed SANS data obtained by Berney et al. (1982) on PS-PB diblocks and PS/PS-PB blends where the minority (PB) component formed spherical micelles with only liquid-like ordering. The Percus-Yevick model for liquids of hard spheres was used to obtain the interparticle contribution to the scattered intensity (Kinning and Thomas 1984). The ordering of an asymmetric PS-PI diblock was observed by Harkless... 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

Typically, micelles tend to be approximately spherical over a fairly wide range of concentration above the c.m.c., but often there are marked transitions to larger, non-spherical liquid-crystal structures at high concentrations. Systems containing spherical micelles tend to have low viscosities, whereas liquid-crystal phases tend to have high viscosities. The free energies of transition between micellar phases tend to be small and, consequently, the phase diagrams for these systems tend to be quite complicated and sensitive to additives. 

Figure 16.2 Possible mechanisms of the formation of mesoporous silica materials, by generation of hexagonal silica-surfactant mesophases horn the interaction of silica precursor with (a) spherical micelles, (b) rod-like micelles, or (c) a preformed liquid crystalline phase of the surfactant.

Micellar solutions are isotropic microstructured fluids which form under certain conditions. At other conditions, liquid crystals periodic in at least one dimension can form. The lamellar liquid crystal phase consists of periodically stacked bilayers (a pair of opposed monolayers). The sheetlike surfactant structures can curl into long rods (closing on either the head or tail side) with parallel axes arrayed in a periodic hexagonal or rectangular spacing to form a hexagonal or a rectangular liquid crystal. Spherical micelles or inverted micelles whose centers are periodically distributed on a lattice of cubic symmetry form a cubic liquid crystal. 

Other surfactants show the similar lyotropic liquid-crystal phase behavior and follow the same succession of phases, but not all of the phases are always present. Figure 8.7 shows a phase diagram for the CTAB-water binary system. CMC can also be classified CMC1 (spherical micelle) and CMC2 (rod-shaped micelle). 

SBA-1 and SBA-6 were synthesized by using different surfactants and from acidic and basic synthesis media, respectively. They have the same structure and show the similar XRD patterns. Their structure is similar with cubic Ij phase, spherical micelles packed in Pm3n symmetry, in lyotropic liquid-crystal phase diagram for surfactant-water systems. 

The common structure models for Iniim symmetry are shown in Figure 8.28. Among the well known lyotropic liquid-crystal mesophases, these are at least two mesostructures with Im3m symmetry one locates near the Ii region in the phase diagram, with a possible spherical micelle packed structure. Another one is close to the Vi region, and its most probable structure can be described by a P surface. 

We begin with a relatively simple model, which was suggested some years ago by Mukeqee and which provides considerable insight on solubilization of small amounts of solutes in spherical micelles. Suppose that an aqueous micellar solution has reached its solubilization limit and is in equilibrium with an excess liquid phase of a pure hydrocarbon or some other compound of low polarity. Equating the chemical potentials j,g and of the solute in the bulk organic phase and in the micelles, we have... 

Self-assembled microstructures of water and surfactant with or without oil have been the subject of intense research for several decades because of their rich structural variety. Microstructures ranging from spherical micelles, rod-like micelles, bicontinuous micro emulsions and liquid crystalline phases have broad commercial and scientific applications including nanomaterial synthesis, controlled delivery, coatings and detergents among many others. 

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