Phospholipid Monolayer Density Distribution Perpendicular to the Water Surface. A Synchrotron X-Ray Reflectivity Study

Phospholipid monolayers at the air-water interface have been studied by synchrotron x-ray reflection methods. The range of momentum transfers (perpendicular to the surface) exceeded 2π/(layer thickness) thus yielding information about the density distribution across the layer. Data for a monolayer of L-α-dipalmitoylphosphatidylcholine (DPPC) in the solid phase (surface pressure 40 mN/m) are interpreted in terms of a simple model of the density with adjustable values for the densities and the projected lengths of the aliphatic tail and the polar head as well as an overall Gaussian smearing of the densities. We find that the tails are close-packed but uniformly tilted 30° relative to the interface normal.