Coupling between a first-order gas-liquid phase transition and a second-order orientational transition in Langmuir monolayers

The Ginzburg-Landau theory is developed to investigate coupling between a g as-liquid first-order phase transition (FOPT) and an orientational second-o rder phase transition (SOPT) in Langmuir monolayers. It is found that the c oupled SOFT and FOPT takes place simultaneously if the uncoupled FOPT occur s prior to the SOFT with compression, and that in the opposite case, the co upling makes the FOPT take place at a lower pressure, but still behind the SOFT. Gas-liquid phase separation always leads to a decrease in the average d order parameter of the orientational phase transition, which is qualitati vely consistent with experimental data. [S1063-651X(99)08411-1].