Influence of the molecular structure of thermotropic liquid crystals on their ability to form monolayers at interface

Langmuir and Langmuir–Blodgett (LB) films of some thermotropic liquid crystals of rod-like shape have been studied. It has been found that the compounds with the terminal isothiocyanato (–NCS) group are not able to form a compressible monolayer at the air–water interface. Very short and very long alkyl or alkoxy chains attached to the rigid molecular core also hinder the creation of the stable films. The Langmuir films have been characterised by the surface pressure-area and surface potential-area isotherms and by Brewster angle microscopy (BAM). The analysis of the isotherms and the BAM images suggests that the organisation of the mesogenic molecules on the water surface is dependent on their structure and to some extent reflects their ability to form an appropriate mesophase in the bulk. For the LB films of the liquid crystals the electronic absorption spectra have been recorded. On the basis of these spectra the conclusions about the creation of the self-aggregates by mesogenic molecules at the air-solid substrate interface have been drawn. The polarised absorption spectra have allowed one to determine the alignment of molecules on the quartz surface.