PARTIAL ORDER IN PHOSPHOLIPID MONOLAYERS

Phospholipid monolayers at the air/water interface are studied by fluorescence microscopy and by X-ray scattering techniques. Due to long-range electrostatic forces superlattices of domains of ordered lipid are observed. For lateral densities between an isotropic fluid (LE) and a nearly incompressible (S) phase, an additinal phase is detected and characterized. This phase (LC) exhibits long-range bond orientational and short-range positional order as expected for a hexatic phase. Yet it has lower than sixfold symmetry due to uniform tilt of the aliphatic tails with tilt angle decreasing on compression. Reflectivity data are in agreement with the view that the transition to the S phase involves head group ordering. At lower temperature, the LE phase is suppressed but the LC phase exhibits the same features as at room temperature with respect to lattice spacings, positional coherence length and tilt angle. These features appear to be rather general for many phospholipids and for single chain surfactants like fatty acids. © 1990.