Symmetry, structural phase transitions and phase diagram of Langmuir monolayers

The Landau theory of phase transitions is developed for transitions between condensed phases of langmuir monolayers. The phase diagram is explained by the coupling between the order parameters describing collective tilt of the molecules and ordering of their backbone (zigzag) planes. if the latter ordering is considered as occurring between hexatic phases, then the predictions do not agree with X-ray data on the structure of phases of either fatty acid or long-chain monolayers on the water surface. However this transition can be explained as translational ordering with doubling of the unit cell realized either by herringbone ordering or by alternating orientations of the heads of the molecules in adjacent rows.