Orientational phase transition in molecular monolayer on an air–water interface

Incorporation of Molecular Reorientation into Modeling Surface Pressure-Area Isotherms of Langmuir Monolayers

Langmuir monolayers can be assembled from molecules that change from a low-energy orientation occupying a large cross-sectional area to a high-energy orientation of small cross-sectional area as the lateral pressure grows. Examples include cyclosporin A, amphotericin B, nystatin, certain alpha-helical peptides, cholesterol oxydation products, dumbbell-shaped amphiphiles, organic-inorganic nanoparticles and hybrid molecular films. The transition between the two orientations leads to a shoulder in the surface pressure-area isotherm. We propose a theoretical model that describes the shoulder and can be used to extract the energy cost per molecule for the reorientation. Our two-state model is based on a lattice-sublattice approximation that hosts the two orientations and a corresponding free energy expression which we minimize with respect to the orientational distribution. Inter-molecular interactions other than steric repulsion are ignored. We provide an analysis of the model, including an analytic solution for one specific lateral pressure near a point of inflection in the surface pressure-area isotherm, and an approximate solution for the entire range of the lateral pressures. We also use our model to estimate energy costs associated with orientational transitions from previously reported experimental surface pressure-area isotherms.

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 gas-liquid first-order phase transition (FOPT) and an orientational second-order phase transition (SOPT) in Langmuir monolayers. It is found that the coupled SOPT and FOPT takes place simultaneously if the uncoupled FOPT occurs prior to the SOPT with compression, and that in the opposite case, the coupling makes the FOPT take place at a lower pressure, but still behind the SOPT. Gas-liquid phase separation always leads to a decrease in the averaged order parameter of the orientational phase transition, which is qualitatively consistent with experimental data.