Lamellar phases or phospholipid bilayers are abundantly found in nature, e.g., in cell membranes, and integral part of well-established drug delivery systems like liposomes. Exposed to an aqueous medium, amphiphilic phospholipids assemble into a two-layered sheet with the hydrophobic tails pointing to the inner part and the polar heads to the outer part, resulting in a phospholipid bilayer. This self-assembly occurs when the size, i.e. the cross-sectional area, of the phospholipid's headgroup is approximately equal to the cross-section of its two alkyl chains.
The packing parameter P is between 0.5 and 1. This is the case for nearly all diacyl phosphatidylcholines (PC) and phosphatidylethanolamines (PE). To further minimize the hydrophobic region exposed to water, closed phospholipid vesicles can be formed. These vesicles are also termed liposomes. What makes liposomes so interesting for pharmaceutical applications is the fact that hydrophilic drugs can be encapsulated in the aqueous core of the vesicles. In addition, lipophilic active ingredients can also be "hidden" within the phospholipid layer.