Wedge-type molecules

As far as the criterion for sectioiung this review article is concerned, it is not straightforward. For conveiuence, we will review 1-D assemblies of molecular rods in terms of molecular architecture. We classify rod assemblers into six molecular categories (1) rod-coil molecifles, (2) macrocyclic molecules, (3) dendron-rod-coil molecules, (4) dumbbell-shaped molecules, (5) wedge-typed molecules, and (6) conjugate rods with lateral chains. We will describe biomimetic or bioconjugate amphiphiUc rods in the last section. 

In principle, amphipathic lipids can also form hilayers (Fig. 3-25), but some do so more readily than others this ability depends on the diameter of the head group relative to the cross-sectional area of the hydrocarbon chain(s). More wedge-shaped molecules tend to favor the formation of micelles while cylindrical molecules tend to form bilayers. The latter consist of two sheets of lipid with opposed hydrocarbon chains. An isolated bilayer cannot exist in water because exposed hydrocarbon tails would exist at the edges and ends of the sheet. However, this is obviated by the sheet cinwing to form a self-sealed, hollow sphere. This type of bilayered micelle is referred to as a vesicle (Fig. 3-25). 

The possibility that IPMS can occur in lipid systems as a structure of microemulsions or liquid-crystalline phases was first pointed out by Scriven (1976). The concept of IPMS and the possibilities of cubic phases consisting of lipid bilayers curved as an IPMS has attractive features that merit consideration. The lipid bilayer is the dominating structure type in lipid systems. In most cases there is an L -phase on one side of the cubic phase in the phase diagram. The IPMS is intersection free, so that the main difference compared to the infinite bilayer structure of the L -phase is the curvature, which can allow for wedge-shaped molecules (see further discussions in Section 8.2.8). The hydrostatic pressure is the same everywhere in an IPMS structure. All molecules can in fact have an identical close-packing environment, with lateral diffusion freedom similar to that of the L -phase. 

Fischer projection formulas can be used to represent molecules with several stereogenic centers and are commonly used for caibohydrates. For other types of structures, a more common practice is to draw the molecule in an extended conformation witii the main chain horizontal. In this arrangement, each tetrahedral caibon has two additional substituents, one facing out and one in. The orientation is specified widi solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

The formulation has been related with the type and properties of emulsions since Bancroft s rule of thumb (1913) and Langmuirs wedge theory (1917). The hydrophilic-lipophilic balance (HLB) was introduced by Griffin 60 years ago, probably as a selling argument for the (by the time) new non-ionic surfactants. It accounts for the relative importance of the hydrophilic and lipophilic parts of an amphiphilic molecule on a weight basis [19]. For decades there was no other numerical yardstick. The simplicity of the HLB concept was its main advantage in spite of very serious limitations, such as an inaccuracy sometimes over two units, and the fact that it does not take into account several variables which are known to alter the phase behaviour, independently of the surfactant. 

Methane (CH4) is a tetrahedral molecule its four hydrogens occupy the comers of a tetrahedron with carbon at its center. Several types of molecular models of methane are shown in Figure 1.6, and Table 1.7 recalls their tetrahedral geometry by way of a ball-and-spoke model. Table 1.7 also shows a common method of representing three-dimensionality through the use of different bond styles. A solid wedge ( -) stands for a... 

The steric requirements of surfactant molecules have historically been referred to in terms of an oriented wedge of surfactant molecules at the interface. The concept lead to the rule of thumb that if the hydrophilic head of the surfactant was larger than the tail, the result would be an o/w emulsion. If the relationship were reversed, the emulsion would be of the w/o type—a very neat and simple relationship which, due to numerous exceptions and lack of theoretical foundation, fell out of favor for some time. More recently, however, consideration of the critical role of the structure of the... 

Note that the four hydrogen atoms are distributed to the corners of an imaginary tetrahedron in three-dimensional space around the carbon of 4d, which is buried in the center of the tetrahedron. In addition, a three-dimensional molecular model of methane is shown where cylinders represent the bonds and spheres represent the atoms (4e), which is the so-called ball-and-stick model. The bonds represented by wedges and dashed lines in the two-dimensional model are marked in 4e. A so-called space-filling model, 4f, is also shown to illustrate the relative size of the atoms. Both types of molecular models (ball and stick or space filling) will be used for other molecules in various places in the book. 

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