Phospholipid molecules can be described as composed of two parts:

a multipolar headgroup and two carbohydrate (CH2)n-CH3 tails.

The two tails may be of different lengths and may also contain

double bonds between the carbon atoms (for a thorough description

of different phospholipid molecules, (see Cevc and Marsh, 1987;

Israelachvili, 1997). When mixed with water or electrolyte solution,

above a certain threshold concentration the phospholipid molecules

self-assemble into different lipid structures/aggregates (Fig. 6.1)

so that the tails are hidden from the water, while the hydrophilic

parts of the molecules (phospholipid headgroups) are in contact

with the water or electrolyte solutions (Cevc and Marsh, 1987;

Israelachvili, 1997). In this way the least number of hydrogen

bonds between the water molecules are broken due to the presence

of phospholipid molecules. Among the possible objects that may

be formed by aggregation of lipid molecules, spherical aggregates

of single-chained lipids (surfactants) (Fig. 6.1A) are the smallest.

Double-chained lipids can aggregate into unilammellar lipid bilayers

(Fig. 6.1C) that may be closed into vesicles (Fig. 6.6) and separate

Figure 6.1 Schematically depicted polymorphism of phospholipid aggregates. Aggregated forms with the corresponding isotropic and anisotropic

shapes of phospholipid molecules: a spherical micelle (A), a cylindrical

micelle (B), a bilayer (C), an inverted cylinder (D), and an inverted spherical

micelle (E) (Perutkova´ et al., 2009).