Geometric boundary condition for the chain alignment in lipid monolayers

Langmuir monolayers consisting of phospholipids have been studied intensively in the past using gracing incidence X-ray diffraction (GIXD). Recent investigations focus on monolayers prepared on the surface of complex fluids like hydrogels or -sols (Phys. Rev. Lett. 88 (2002) 25502). These systems allow to modify the properties of the surfactant layer via the tuneable characteristics of the subphase. Using GIXD, the parameters of the lattice formed by the lipid chains are accessible. In addition, the alignment of the scatterer in the unit cell can be extracted from modelling the measured Bragg rods (Transitions in Soft-Condensed Matter, Plenum Press, New York, 1989, pp. 113-138). In this work, we present a boundary condition for the perpendicular wave vector transfer that has to be considered in the calculation of the chain alignment from Bragg rod scans. The theoretical part is illustrated with GIXD measurements of the in-plane structure of lipid monolayers in repulsive electrostatic interaction with charged nano-particles.