Typical hydrophilic groups

For example, the (> -hydroxyalkyl groups (R1 and/or R2 = OH), typical of many acetate dyes, were replaced by less hydrophilic groups such as CN, OCOR, and COOR, which enhanced the affinity for polyester fibers and, in many instances, the fastness to light and sublimation [9,15],... 

Lipids are far more diverse chemically than other typical biomolecules such as amino acids, carbohydrates, and nucleotides. The definition of lipids includes simple fatty acids and their glycerol esters, sterols such as cholesterol, and phospholipids, sphingolipids, and cerebrosides. Lipids are generally defined by their common hydrophobic character, which makes them soluble in organic solvents such as chloroform. Virtually all lipids also have a hydrophilic group, which makes them surface active. 

The hydrophobic portions are usually saturated or unsaturated hydrocarbon chains or, less commonly, heterocyclic or aromatic ring systems. The hydrophilic regions can be anionic, cationic, zwitterionic, or nonionic. Surfactants are generally classified according to the nature of the hydrophilic group. Typical examples are given in Box 6.1. 

The methods of preparing lipid vesicles are well known [2,5,9]. Vesicles are made predominantly from amphiphiles, a special class of surface-active molecules, which are characterized by having a hydrophilic (water soluble) and a hydrophobic (water insoluble) group on the same molecule. A typical vesicle-forming molecule, such as lecithin (see Fig. 7), has two hydrocarbon chains, also called hydrophobic or nonpolar tails, attached to a hydrophilic group, often named the polar head. In general, most of these molecules are not soluble in water however, instead of solutions they form colloidal dispersions [15]. 

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