Coupling of phase transitions in Langmuir monolayers

A Ginzburg-Landau theory is presented to study coupled first-order gas-liqu id phase transition and second-order orientational phase transition in Lang muir monolayers. We show that the coupling between the two phase transition s has a strong influence on the orientational phase transition, making the polar order phase separated from the isotropic one. Such a phase separation will substantially suppress the onset of the transition and the growth of the polar order parameter, which accords with the existing experiments. (C) 1999 American Institute of Physics. [S0021-9606(99)52501-4].