Liposomes are vesicular colloidal particles composed of self-assembled amphiphilic molecules. Amphiphiles are molecules that contain two groups with different solubility. The hydrophilic group, often referred to as the polar head, is “water loving,” while the hydrophobic part, the so-called nonpolar tail, is “water hating.” Therefore, these molecules self-assemble and form ordered structures in aqueous solutions. Single-chain amphiphiles, such as soaps and detergents, form micelles. These are small spherical structures in which surface polar heads shield the nonpolar interior against water. Many natural amphiphiles, such as lecithin (diacyl phosphatidylcholine), have two nonpolar tails and due to a bulky nonpolar part cannot be packed into micelles. These molecules normally self-assemble into lipid bilayers in which two polar surfaces shield the nonpolar interior. Bilayered lamellae have their edges exposed to water, therefore, at lower concentrations they self-close into spherical structures to eliminate this unfavorable exposure, and lipid vesicles or liposomes are formed. Figure 6-1 shows the structure of micelles and lipid bilayers schematically. Single-chain surfactants self-aggregate into micelles (top) while double-chain surfactants form bilayers. Because bilayers have exposed edges they self-close in aqueous suspensions into spherical structures called lipid vesicles or liposomes. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780138748807/72610e4e-d49d-4118-b247-391907d64545/content/fig6_1.tif"/>