ABSTRACT
Phospholipid vesicles may be defined as lipid particles surrounded by one or several closed phospholipid bilayers. The space between bilayers and the internal cavity of the vesicle are filled with aqueous solvent. Phospholipids are amphiphilic, surface-active molecules that have a high tendency to form aggregates both in the dry and in the fully hydrated state. The different kinds of aggregates are termed phases. Generally, lipids are much smaller in weight and size than macromolecules such as proteins, polysaccharides, and ribonucleic acids; yet they are sufficiently large to have distinct regions of greatly different polarity: the hydrophobic region and the hydrophilic region. A prerequisite for the preparation of well-defined phospholipid vesicles is the use of pure, chemically well-defined lipids. Frequently, commercial sources of phospholipids are used. These lipids can vary widely in purity from firm to firm and even from batch to batch within the same firm.
