Liquid crystal anchoring transformations induced by phase transitions of a photoisomerizable surfactant at the nematic/aqueous interface

A photoisomerizable fatty acid (containing an azobenzene unit) was dissolved in a nematic liquid crystal (LC), and adsorbed at the LC/water interface, influencing the anchoring of the LC material. Reorientation of the LC phase was observed upon exposure of the system to different irradiation wavelengths. Homeotropic, tilted, and planar anchoring were all observed under various thermodynamic and illumination conditions. Lateral coexistence between regions displaying different anchoring was often observed, as were dynamic and reversible transitions between the various states. These observations suggest that adsorbed monolayers of the azobenzene-fatty acid (probably in equilibrium with the same surfactant in solution) display a rich binary phase diagram, where the relative fractions of cis and trans isomers are determined by the photostationary state associated with the illumination wavelength. The morphology of these phase diagrams has been determined as a function of three variables—bulk concentration, temperature, and composition (cis–trans ratio) of the dissolved surfactant—and six distinct interfacial phases were identified, including five condensed phases and a dilute interfacial vapor phase. This phase diagram is distinct from the behavior of Langmuir monolayers of the same amphiphile at the air/water interface, which exhibits only two condensed phases.