The heterogeneity of biological membranes and, in particular, the formation of lipid domains is the motivation for this work. Lipid domains or ‘rafts' in phospholipid membranes are nanometer- to micrometer-sized domains of laterally phase-separated lipids. Nature uses rafts to locally confine and cluster membrane proteins and other macromolecules on the surface of cells with the aim to change the local multivalency and, therefore, the biological activities ranging from signal transduction, to cellular trafficking and to viral infection mechanisms.

In this work we study the phase separation of lipids in model membranes in the form of liposomes. These membranes contain lipids with ionized titrable head groups that phase separate from zwitterionic lipids of shorter acyl-chain lengths, and form domains in the plane of the liposome membrane. Phase-separation occurs upon protonation of the ionized titrable lipids that is controlled by pH. The kinetics of phase-separation and domain formation was studied by various techniques such as differential scanning calorimetry, and fluorescence energy transfer among pyrene-labeled acyl-chain lipids. The studies were performed for lipid combinations of various acyl-chain lengths and fractions of ionized titrable lipids in solutions with different ionic strength as a function of pH. These studies coupled with results on changes of liposome membrane permeability during lipid phase-separation provide better understanding of the molecular mechanisms involved in domain formation, help us project on the potential role of lipid rafts in biological membranes, and enables us to design liposomes as drug delivery carriers for the advancement of human health.