Chiral Discrimination and Phase Transitions in Langmuir Monolayers

Chiral discrimination is investigated theoretically for chiral molecules that form an insoluble Langmuir monolayer at the water/air interface. For particular tripodal shaped molecules, we calculate the chiral discrimination for various types of intermolecular interactions: van der Waals, dipoles, charges, etc. The calculation, based on Boltzmann-weighted averaging of molecular orientations, predicts a preferred heterochiral behavior for van der Waals interactions and homochiral behavior for electrostatic ones. Other interactions are also discussed. To understand monolayer phase diagrams, we draw the analogy with sublimation experiments in bulk systems and propose a three-component thermodynamic model. The variable area per molecule and also the chiral discrimination parameter enter as important parameters in the model. Phase diagrams for conglomerates and racemic compounds are calculated in qualitative agreement with experiments. Possible connections and interpretation of existing experimental data are discussed, and some new experiments for chiral monolayers are proposed.