Disorder in Langmuir monolayers. 1. Disordered packing of alkyl chains

Grazing incidence synchrotron X-ray diffraction (GID) studies of Langmuir monolayers of several substances such as 2-monopalmitoyl-glycerol, 2-hexadecanol, and 2-hydroxypalmitic acid reveal diffraction patterns that indicate a disordered packing of alkyl chains. The diffracted intensity is distributed along a characteristic are in reciprocal space. A relation between in-plane lattice and two parameters, polar tilt angle and tilt azimuth, is derived in order to describe the observed distribution that results from the superposition of the diffracted intensities of different lattices. it is possible to ascribe the diffraction patterns unambiguously to a variation of the tilt azimuth. Variation of the polar tilt angle was ruled out. A detailed analysis of the distributed intensity allows one to obtain at least the polar tilt angle and the lattice spacing perpendicular to the tilt azimuth in the limit of a lattice with nearest-neighbor tilt.