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O-27 PHASE TRANSITIONS AND ORDERING PHENOMENA INAMPHIPHILICMONOLAYERS AT THE AIR/WATER INTERFACEWillem MulderDepartment of Chemistry, MonaThe surface activity of molecules such as lipids, proteins and fatty acids is associated with their dualcharacter; that is, one part is hydrophobic (“water-fearing”) and the other part is hydrophilic(“water-loving”). Such molecules are also called “amphiphiles”. The primary requirement for anamphiphile to be surface-active is that the balance between hydrophilic and hydrophobic parts besuch that the molecule has its lowest free energy when it is adsorbed at an interface between twoimmiscible bulk phases (water/oil or water/air). The amphiphile will then be present as amonomolecular film. Examples include the surfactant films in microemulsions, Langmuir filmsformed by spreading amphiphilic molecules on a water surface, the dense (Langmuir-Blodgett) filmsof chain molecules on solid supports, or the two lipid monolayers facing each other in biologicalmembranes.The most common experiment performed on Langmuir monolayers is the determination of surfacepressure vs. area isotherms using a film balance 1 . The surface pressure Π is defined as the differencebetween the surface tension γ0 of pure water and the surface tension γof the surface covered by themonolayerΠ = γ0 − γ (1)A barrier can be slid across the water surface to change the area accessible to a fixed amount ofsurface-active material. The surface pressure can be measured by determining the force on a floatseparating the monolayer from a clean water surface. A typical Π-A isotherm for a pure fatty acid orphospholipid is shown in the figure, where two regions can be discerned in which different surfacephases coexist. At large areas the amphiphile behaves like a two-dimensional gas. Upon compressiona plateau is reached, marking the onset of 2D condensation of the gas into a so-called liquidexpanded(LE) phase. At the end of the plateau the entire monolayer is in the LE phase. On furthercompression the lateral pressure again increases steeper until the start of a second transition regionwhere the LE phase condenses into a liquid-condensed (LC) phase which is associated withincreased chain ordering. At the end of the LE/LC coexistence region there is another steep increasein pressure until the collapse pressure is reached, which marks the transformation to more stablethree-dimensional (micellar) phases.The formation of LC from LE in the second coexistence region (sometimes called the “maintransition”) can be followed by fluorescence microscopy if one adds a small amount of surface activefluorescent dye which is less soluble in the LC compared to the LE phase. Alternatively, one can usethe more recently developed non-invasive technique known as Brewster angle microscopy. Themicrographs obtained with both techniques reveal that the LC phase exists in the form of isolateddomains (sizes ~10µm) which, in the case of chiral amphiphiles, change their shape from circles viaellipses to stripes, spirals, triskelions, etc. These have been established to represent equilibriumshapes and the result of a competition between lateral electrostatic repulsion between the polar (orcharged) head groups of surfactants and (anisotropic) line tension.45