Study of binary mixtures of liquid crystals with terminal –CN and –NCS group at the air–water interface

Langmuir films of two liquid crystal materials, 4-octyl-4′-cyanobiphenyl (8CB) and 4-octyl-4′-isothiocyanatobiphenyl (8BT), and of their mixtures have been studied by recording surface pressure–area isotherms and by Brewster angle microscopy (BAM) images. It has been found that 8BT is not able to create compressible monolayers at the air–water interface. However, if this liquid crystal is mixed with 8CB, the formation of a Langmuir film is possible, although, as revealed by the BAM images, it is not homogeneous. The surface pressure–area isotherms of films formed from 8BT/8CB mixtures reveal that both components are at least partially miscible when the molar fraction of 8BT is not higher than 0.5. The analysis of the isotherms and BAM images suggests that the organization of the liquid crystal molecules at the water surface can be connected to certain degree with their ability to form the appropriate mesophase in the bulk.