Non-polar end groups

We now look more closely at the structure of casein. It is a long molecule with different ends one end is polar and the other is nonpolar. In milk, the polar group (ending with a phosphate group) is positioned to face the polar water, and the non-polar end faces the oil. In effect, each particle of oil has a double coating the inner layer is the non-polar end of the casein emulsifier, and the outer layer is a sheath of polar phosphate groups. 

Orientational Effects. Solute molecules which have distinct polar and non-polar parts take up specific orientations in micelles, such that their non-polar end stays in the non-polar interior of the micelle, the polar group residing at the water interface. Photocycloaddition of these molecules will therefore lead preferentially to the head-to-head dimer, even if the head-to-... 

Surfactants are those chemicals whose molecules have two parts of widely differing polarity and solubility. The soaps, for example, have an ionizing salt head to the molecule and a long non-polar hydrocarbon tail . A different type are the polyoxyethylene ethers of dodecyl alcohol, e.g. Ci2H25 0 (CH2 CH2 0)6 H. These contain the non-polar dodecyl group (C12H25-), from dodecyl alcohol, at one end of the molecule and the successive polar ether linkages at the other. Whatever the formula or type, all surfactant molecules have in common these polar and non-polar portions. 

At the non-polar end of the elution scheme, the elution of alkanes to ethers with perhaps only hexane or heptane would be expected. For elution of moderate-to-polar compounds, a polar modifier has to be added to the major non-polar component of the mobile phase. Much of the guesswork as to which solvent combinations to use has been simplified by the work of Snyder and Kirkland. Their work has grouped solvents of similar selectivities (i.e., alcohols give identical elution order sequences only the elution times change from one alcohol to another). As a result of their studies, the recommended solvents to be mixed with hexane or heptane to effect the greatest possible selectivity differences are diethyl ether, chloroform, and methylene dichloride. 

Optimization of the valence and dihedral angles yields planar cyclic structures for the 3- to 5-ring intermediates in contrast to a chair conformation for that of the 6-ring. In the cases of n = 4, 5, 6 the oxygen atom is placed almost in the plane of the three C-atoms directly bonded to it. Therefore, an intramolecular solvation of the cationic chain end by methoxy groups which are bonded to the polymer backbone is preferred in the gas phase. The calculations show that for a non-polar solvent such as CH2C12 a decrease in stability of the cyclic intermediates exists. But this decrease does not result in a total break of the intramolecular solvation (Table 13). An equilibrium between open chain and cyclic intermediates must only be taken into account in more polar solvents, due to the competition of intra- and intermolecular solvation. 

The presence of a large number of chain-ends in the fully synthesized dendrimer molecules makes them highly soluble and also readily miscible, for example with other dendrimer solutions. The solubility is controlled by the nature of the end-groups, so that dendrimers with hydrophilic groups, such as hydroxyl or carboxylic acid, at the ends of the branches are soluble in polar solvents, whereas dendrimers with hydrophobic end-groups are soluble in non-polar solvents. The density of the end-groups at the surface of the dendrimer molecule means that they have proportionately more influence on the solubility than in linear polymers. Hence a dendritic polyester has been shown to be more soluble in tetrahydrofuran than an equivalent linear polyester. 

An L-B film is formed by the dispersion of amphoteric molecules at an air-water surface (Figure 8.20). These molecules have a polar group at one end, something like a carboxy substituent (in this respect they resemble the surfactant molecules which make micelles), and a long non-polar aliphatic chain. The polar group stays in the polar water phase, and the aliphatic chain stays in the non-polar air environment. The L-B film at the water surface is then made by the controlled compression of these molecules by means of a floating barrier. The molecules then line up to form a mono-molecular layer on the water surface. 

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