Tilting phase transition of Langmuir monolayers: a competition between steric repulsion and interaction of dipolar molecules with liquid surface

Monolayers of amphiphilic molecules at the liquid–air interface are investigated, taking into account the steric repulsion between molecules and the Coulomb attractive interaction of dipoles with the liquid surface. It is shown that the former favors a perpendicular alignment of molecules at the surface (SmAp phase) and the latter gives a tilted orientation (SmCp phase). As the temperature decreases, the competition between these two interactions leads to a second-order phase transition from SmAp to SmCp. The large pretransitional fluctuation of the tilt angle at a critical molecular area is predicted.