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The lateral organization of biomembranes has gained significant interest when the fluid mosaic model was challenged by the model of "lipid rafts". Several lipid classes like cholesterol and sphingolipids are considered to be essential for their formation. Here we investigate the lateral domain formation in binary mixtures of sphingomyelin and phosphatidylcholine. Both are major lipid components of lipoproteins and mammalian cell membranes at various molar ratios. Surface pressure-area isotherms and surface potential-area isotherms of monolayers composed of these lipids clearly indicated non-ideal mixing. In addition, Brewster angle microscopy provided a well-suited approach to image the formation of lateral domains. These images demonstrated that pure sphingomyelin forms very stable finger-like domains that exhibit a distinct internal organization suggesting an anisotropic orientation of the acyl side chains. Similar behavior was found for mixtures containing more than 60 mol% sphingomyelin. With increasing content of phosphatidylcholine the domain size decreased and the surface pressure, where domain formation occurred, increased. At lower sphingomyelin content (30-60 mol%) rather round-shaped, smaller domains were observed. Thus, the potential of sphingomyelin domains as potentially important building blocks for actual domains that could be building blocks for raft formation is suggested, even without the presence of cholesterol. In addition, these observations may suggest a role for the distinct molar ratio of these key lipids frequently found in physiologically relevant particles such as low and high density lipoproteins or the outer leaflet of the human erythrocyte membrane.

In the intermediate temperature and the upper pressure regions, the phase diagram of an eicosanol monolayer consists of four phases: (1) one normal condensed high-pressure phase, A', (2) and (3) two high-pressure superliquids, Rot I and Rot II, and (4) one low-pressure phase, C. The phase transitions A'/C and Rot II/C are of second order and correlated with a collective tilting of the molecules. The phase transition C/Rot I, however, is of first order and correlated with formation and growth of nuclei consisting of tilted or normally oriented molecules, depending on the direction of the transition. The kinetics of the