Surfactant bilayer

An attractive way to overcome this problem is to use microheterogeneous photocatalytic systems based on lipid vesicles, i.e. microscopic spherical particles formed by closed lipid or surfactant bilayer membranes (Fig. 1) across which it is possible to perform vectorial photocatalytic electron transfer (PET). This leads to generation of energy-rich one-electron reductant A" and oxidant D, separated by the membrane and, thus, unable to recombine. As a result of such PET reactions, the energy of photons is converted to the chemical energy of spatially separated one electron reductant tmd oxidant. 

Rgure 2.30. Two adhesive emulsion droplets. A flat hquid fllm stabilized by the surfactant layers is located between the droplets. This fllm being very thin, it can be usually considered as a surfactant bilayer. Yf is the tension of the fllm and y nt the tension of single isolated interface. 

Lamellar focal conics show a fascinating highly-ordered structure when observed under the polarization microscope. This texture consists of surfactant bilayers that are shaped like ice cream cones and stuck inside of one another. These stacks of cones are quite densely packed in the solution and, under the polarization microscope, create extended regions of amazing regularity. Figure 3 shows a photograph of such a system, taken with a polarization microscope with a A mask to achieve color contrast. 

The process utilizing supramolecular organization involves pore expansion in silicas. A schematic view of such micelles built from the pure surfactant and those involving in addition n-alkane is shown in Figure 4.9. Another example of pore creation provides a cross-linking polymerization of monomers within the surfactant bilayer [30]. As a result vesicle-templated hollow spheres are created. Dendrimers like that shown in Figure 4.10 exhibit some similarity to micellar structures and can host smaller molecules inside themselves [2c]. Divers functionalized dendrimers that are thought to present numerous prospective applications will be presented in Section 7.6. 

The final surfactant structures we consider as models for biological membranes are vesicles. These are spherical or ellipsoidal particles formed by enclosing a volume of aqueous solution in a surfactant bilayer. When phospholipids are the surfactant, these are also known as liposomes, as we have already seen in Vignette 1.3 in Chapter 1. Vesicles may be formed from synthetic surfactants as well. Depending on the conditions of preparation, vesicle diameters may range from 20 nm to 10 pirn, and they may contain one or more enclosed compartments. A multicompartment vesicle has an onionlike structure with concentric bilayer surfaces enclosing smaller vesicles in larger aqueous compartments. 

Figure 26-24 Charge reversal created by a cationic surfactant bilayer coated on the capillary wall. The diffuse part of the double layer contains excess anions, and electroosmotic flow is in the direction opposite that shown in Figure 26-20. The surfactant is the didodecyldimethylammonium ion, (n-C,2H25)N(CH3)2, represented as in the illustration.

One of the several shapes that micelles can take is laminar. Since the ends of such micelles have their lyophobic portions exposed to the surrounding solvent, they can curve upwards to form spherical structures called vesicles. Vesicles are spherical and have one or more surfactant bilayers surrounding an internal pocket of liquid. Multi-lamellar vesicles have concentric spheres of uni-lamellar vesicles, each separated from one another by a layer of solvent [193,876] (Figure 14.1). The bilayers are quite thin (-10 nm) and are stabilized by molecules such as phospholipids, cholesterol, or other surfactants (Figure 14.2). Vesicles made from phospholipid bi-layers are called liposomes. Liposomes can be made by dispersing phospholipids (such as lecithin) into water and then agitating with ultrasound. 

In contrast to the measurements by McDermott et al [58], neutron reflectivity measurements for the Ci2E6/Ci6TAB mixture in 0.1 M NaBr at the air-water interface and SANS measurements of the mixed micelles show close to ideal mixing. Penfold et al. [60] has used neutron reflectivity to investigate this mixture at the solid-solution interface. For the hydrophilic silicon surface, the surface composition of the mixed surfactant bilayer adsorbed at the interface depended strongly upon the solution pH. At pH 2.4, the surface composition... 

The lamellar—.hexagonal transformation of Zr02 is likely to be initiated first by the removal of some of the surfactant species, followed by the curling of the surfactant bilayer in order to minimize the surface/interface energy as shown in Fig. 4(a) and (b).13,15 The curled bilayers transform to cylindrical rods to further minimize the surface energy as shown in Fig. 4(c) and the cylindrical rods assemble to give the ordered hexagonal structure shown in Fig. 4(d). In order to examine... 

Fujii H., Ohtaki M., Eguchi K., Synthesis and photocatalytic activity of lamellar titanium oxide formed by surfactant bilayer templating, J. Am. Chem. Soc. 120 (1998) pp. 6832-6833. 

In Region IV the surface is completely covered by a surfactant bilayer, with the solution again in contact with polar groups. Hence, the wettability again returns toward the hydrophilic condition exhibited at low surfactant concentration. 

The adsorption of binary mixtures of anionic surfactants of a homologous series (sodium octyl sulfate and sodium dodecyl sulfate) on alpha aluminum oxide was measured. A thermodynamic model was developed to describe ideal mixed admicelle (adsorbed surfactant bilayer) formation, for concentrations between the critical admicelle concentration and the critical micelle concentration. Specific... 

Figure 1. A. Computer graphic portion of a periodic surface of constant mean curvature, having the same space group and topological type as the Schwarz D minimal surfhce. This surbce, together with an identical displaced copy, would represent the polar/apolar dividing surface in a cubic phase with space group 224 (Pn3m). The two graphs shown would thread the two aqueous subspaces. B. Computer graphic of a portion of the Schwarz D minimal sur ce (mean curvature identically zero). In the 224 cubic phase structure, this sur ce would bisect the surfactant bilayer.

There are three main liquid crystalline phases with hexagonal (Hj), cubic (Vj) and lamellar (L) structures (Figure 3). Ihe Hj phase is the result of a hexagonal packing of cylindrical micelles, while the L phase corresponds to the formation of surfactant bilayers. 

Another force [57, 58] occurs in a multilayered system, like a swollen lamellar phase of surfactant bilayers or phospholipid vesicles. Shape fluctuations in the bilayers can give rise to steric effects that are supposed to stabilise such systems where the van der Waals and double-layer forces are very weak, as they often are. The magnitude of such fluctuations depends on the "stiffness" of die bilayer. The status of these forces is the subject of an active debate and imclear. 

Figure 4.7 Images of (left) a portion of a surfactant bilayer wrapped onto the P-surface, a triply-periodic minimal surface, with two interwoven polar labyrinths and (right) a reversed bilayer on the P-surface, with interwoven lipophilic labyrinths.

In the previous section, it has been shown that a surfactant bilayer is constrained to adopt a hyperbolic (or planar) geometry if the constituent monolayers have identical molecular shape (characterised by the surfactant parameter). In the case of monolayers, all three geometries - elliptic. 

In the intensification of chromate removal from water, a double-chain cationic surfactant, dioctadecyl-dimethylammonium chloride (DODDMAC), was used as a carrier and a cross-flow electrofiltration was used, in which both the transient and the steady-state fluxes and the rejection of metal ions and surfactant were measured.Dioctadecyldimethy-lammonium chloride in water forms multilamellar droplets, even at very low concentrations. This structure is shown in Fig. 10. Metal ions are entrapped within the water layers and organic toxins can be immobilized within the surfactant bilayers. Under an electric field. 

Fig. 10 Schematic representation of surfactant in multilamellar droplet phase with entrapped metal ions (M ) in the aqueous phase (W) layers, which are separated by surfactant bilayers. The number of layers (hence the size of the surfactant droplet) is dependent on temperature and concentration. When the surfactant head group is positively charged thus encapsulating oppositely charged metal ions, under an electric field, surfactant lamellar droplets migrate to the anode and form a highly stable viscous gel in which the positively charged metal ions are concentrated at the anode only separated by the surfactant bilayer. (From...

Evans, E. and Needham, D. (1987) Physical properties of surfactant bilayer membranes-thermal transitions, elasticity, rigidity, cohesion, and colloidal interactions. Journal of Physical Chemstry, 91 (16), 4219 228. 

More lipophilic surfactants form larger, nonspherical micelles, vesicles, or lyotropic liquid crystalline phases at rather low concentrations in water. For example, at temperatures above those where the chains form crystalline structures, phospholipids and other surfactants with two relatively long hydrocarbon chains typically form the lamellar liquid crystalline phase consisting of many parallel surfactant bilayers separated by water layers. The hydrocarbon interiors of the bilayers are rather fluid as in micelles. Of course, in this case a true phase separation occurs beginning at a definite surfactant concentration. 

Monoglycerides form an inverse hexagonal phase with glycerol, as in water [112], Mixtures of triethanolamine and oleic acid form a nonaqueous lamellar liquid crystal with a surfactant bilayer of soap and acid with intercalated ionized and unionized alkanolamine as solvent [113,114], Lamellar liquid crystals form analogously with dodecylbenzenesulfonic acid and triethanolamine [115]. 

Fluorescence lifetimes of diphenylhexatriene in molecules located in both flat and bent bilayer liquid membranes show the effect of changes both in exposure to water and burial within the nonpolar membrane . The effect of hydrostatic pressure on the system confirms the interpretation put forward to account for these effects. Photochemical electron transfer across surfactant bilayers has been shown to be mediated by the presence of 2,l,3-benzothiadiazole-4,7-dicarbonitrile . 

---