What X-rays tell us about the ordering of molecular backbones in Langmuir monolayers

Fatty acid Langmuir monolayer phases are commonly described in terms of the lattice, molecular tilt angle and tilt direction. However, the molecules are not cylinders with circular cross-sections, and so the orientation of the molecular ‘backbones’ about their long axes is also relevant. This article surveys some of the currently available experimental evidence regarding backbone ordering and order–disorder transitions in fatty acid monolayers.