Symmetry and phase transitions in Langmuir monolayers: The Landau theory

Condensed phases of Langmuir monolayers are treated in the framework of the Landau theory of phase transitions as a result of successive ordering of the hexatic phase. All types of ordering allowed by symmetry and the couplings between them are described. The phase diagrams and x-ray structural data on monolayers of various long-chain amphiphilic molecules can be explained by using three coupled order parameters. One of the parameters governs the collective tilt of the molecules, while the other two describe one-dimensional ``weak crystallization'' involving herringbone ordering of the molecular backbone planes along the bond direction and normal to it. Possible manifestations of other order parameters are discussed.