Models for the phase transition in a monolayer of amphiphilic bipolar molecules on an aqueous substrate

Models are proposed to explain the mechanism of the phase transition observed in a monolayer of amphiphilic dipolar molecules of which one of the two polar groups may, under lateral compression, leave the interface between the monolayer and the aqueous substrate. Good agreement with the experimental results are obtained when the binding energy of the polar group leaving the substrate, the molecular size effects, and the intramolecular free energies are taken into consideration. In this framework we propose the following interpretation for the first-order phase transition. The monolayer goes from a low surface density phase, where both polar groups of the molecule are anchored to the substrate, to a condensed phase where a large fraction of the molecules have only one polar group attached to the aqueous substrate.