Inverted cylindrical micelle

Fig. C5.2 Possible structures formed in solution of amphiphilic molecules spherical micelles (a), cylindrical micelles (b), lamellas, or layers (c), inverted cylindrical micelles (cf), inverted spherical micelles (e). In three-dimensional space these structures will organize pretty much in the same way as shown in Figure C4.9. The figure is courtesy of P.G. Khalatur.

FIGURE 15.3. Of the many theoretically possible hquid crystal structures, five are most commonly encountered in surfactant systems. The lamellar phase (a) is simply alternating layers of surfactant molecules. The hexagonal phases (fe,c) are infinite hexagonal close-packed structures of normal and inverted cylindrical micelles. The most complicated, and difficult to visualize and shown schematically here, are the cubic bicontinuous (or interpenetrating) network d) and the cubic close packed ellipsoidal or finite cylindrical arrays (e). 

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. 

Fig. 1 Association colloids (A) spherical micelle (B) cylindrical micelle (C) flattened disc-shaped micelle (D) microtubular micelle (E) inverted micelle and (F) micelle swollen by the presence of solubilized lipid soluble drug.

The tail volume, Vt, which is the volume of hydrocarbon liquid per hydrocarbon molecule. It is assumed that there is an incompressible hydrocarbon liquid in the core of the micelle due to the presence of hydrocarbon tail molecules. If we cut a uniform conic volume per surfactant from a spherical micelle, as shown in Figure 5.9, we can find the value of Vt, from simple geometry. However, if the micelle is not spherical, V, does not represent a uniform cone instead it shows the form of a truncated cone to produce a cylindrical micelle, vesicle or liposome a cylinder to form a bilayer micelle, or an inverted cone to form an inverted micelle. 

To tell the truth, you cannot usually get all five successive stages (Figure C5.2 a-e) with the same substance. Either the tails are too thick to form spherical or even cylindrical micelles, or, on the contrary, they may be too thin to construct inverted micelles. Normally one substance can exhibit only two or three of the structures in Figure C5.2. 

When P < 1/3, individual molecules are conically shaped, as shown in Figure 10. This results in the formation of spherical micelles in solution. As the volume of the hydrophobic tail is increased, P increases. For 1/3 < P < 1/2, nonspherical (cylindrical) micelles are formed. As P increases further, bilayer structures are formed. At P > 1, inverted structures are formed. This packing parameter can be used to rationalize why sodium dodecyl sulfate (SDS) forms spherical micelles in solution, while lysolecithin forms wormlike micelles. 

The major lyotropic phases encountered with double-chain phospholipids are lamellar, inverted hexagonal, and cubic phases. Single chain lipids have surfactant properties and can also fonn spherical and cylindrical micelles. Figure 5 shows some of the possible aggregation stnictures. Phospholipids not only show lyotropic mesomorphism, i. e. different phases as a ftmetion of water content, but also thennotropic mesomorphism, i. e. transitions between different phases can be induced by varying the temperature. 

Fig. 7. Schematic diagrams of different phases of amphiphiles. Amphiphiles, such as detergents with large head groups (gray) containing only one small hydrophobic chain (tail), can be thought of as cones that pack together to form spherical micelles. Bilayerforming lipids are more cylindrical in shape and pack together into planes. The head groups of nonbilayer lipids occupy less area than the tails and therefore the lipids form inverted phases.

As discussed by Israelachvili (1992), the shapes of surfactant aggregates can, to a first approximation, be anticipated based on the packing of simple molecular shapes (Tanford 1980). Figure 12-1 from Israelachvili illustrates this principle Conical molecules with bulky head groups attached to slender tails form spherical micelles cylindrical molecules with heads and tails of equal buUdness form bilayers and wedge-shaped molecules with tails bulkier than their heads form inverted micelles containing the heads in their interiors. A simple dimensionless molecular parameter that controls the shape of the aggregates is the molecular shape parameter here v is the volume occupied by the hydrocarbon... 

Possible candidates for aggregates can now be examined. For surfactant-water systems these have been restricted in the past to spherical micelles, non-spherical micelles (globular, cylindrical), vesicles, liposomes, bilayers, and for oil-water-surfactant systems spherical drops, normal or inverted (water in oil) or (oil in water). 

Phospholipids are the major lipid building blocks most membranes and their molecules comprise of a hydrophobic (acyl chain) and a hydrophilic (polar) head group. The relative size of the hydrophobic tails and hydrophilic head of the molecule characterizes the molecular shape and determines the structure of the molecular assemblies in contact with w ater. Molecules with polar and non-polar regions (PC, PS, PI, Sphm) of equal size have a cylindrical shape and form lipid bilayers. Molecules that have a larger non-polar region are cone-shaped (PE, PA, Choi, Car), and form reversed micelles, in contact to water. When the polar region is larger (lysophospholipids) the molecule assembles an inverted cone and form micelles. Fig. (8). 

Organic polymers are comparable to the above catalysts, having microenvironments different from those of the surrounding media and being swellable (see 14.2.4.1). Micelles, which are colloidal species produced by aggregation of ca. 20 to thousands of surfactant molecules or ions with both polar and nonpolar portions, also have these characteristics. In a typical aggregate, the hydrophobic ends of the molecules are clustered in the core of the micelle, and the polar ends are located at the interface to the aqueous phase. Micelles may be spherical or (in highly concentrated solutions) cylindrical or lamellar. Inverted micelles may form in a hydrocarbon solvent. 

Although the evidence was not fully compelling, it was suggested that DNA-lipid interactions involved DNA encapsulation within a cylindrical inverted micelle, included in the lipid membrane (Figure 16.13). 

II wt%. The structure is formed by cylindrical water channels (1.8-3.5 nm diameter cylindrical inverted micelles) lined with hydrophilic side groups (Fig. 6.4a, b). A cross section through the cylindrical water channels is shown in Fig. 6.4c, where water channels (white) and Nafion crystallites (black) in the non-crystalline Nafion matrix (grey) are schematized. 

FIGURE 15.8. The most important micelle shapes include (a) normal spherical, (b) lamellar, (c) inverted spherical, (d) oblate ellipsoidal, and (e) prolate cylindrical or rod-shaped. 

Figure 2. Five proposed shapes of micelle (a) spherieal (b) inverted (or reversed) (e) lamellar (d) disk (e) cylindrical or rodlike.

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