Layering transitions and schlieren textures in Langmuir films of two organic radicals

Two paramagnetic radicals have been investigated in terms of their film-forming properties at the air-water interface. Although the radicals failed to display any mesomorphic behavior in the bulk, they were found prone to built-up multilayer films on the Langmuir trough. The molecules seem to dimerize in the upper layers of the films that exhibit striking Schlieren textures when observed with Brewster angle microscopy. These Schlieren textures, together with the ability to form multilayers, indicate that the molecules came close to displaying smectic mesomorphism. A tentative model of the layers' structure is proposed, and a suggestion for synthesizing new molecules with actual mesomorphism is offered. The presented results show that the study of the behavior of molecules at the air-water interface can shed a new light on their behavior in the bulk and help in the design of new magnetic mesogens.