Thermodynamics and mesoscopic organisation in Langmuir monolayers of an azobenzene derivative

We have carried out the analysis of liquid crystalline Langmuir monolayers at the air-water interface composed of the amphiphilic azobenzene derivative 8Az5COOH. By varying the temperature and the isomeric (trans-cis) composition, the monolayer behaviour has been studied in comparison with a shorter homologue, 8Az3COOH, by measuring the surface pressure-area isotherms along with Brewster angle microscopy (BAM). Our data with the pure trans isomer enable a posterior thermodynamic analysis, which was not feasible with the shorter homologue. For the mixed trans-cis monolayers, BAM observations reveal a phase segregation with trans enriched domains surrounded by a cis enriched matrix. Line tension between the two phases is lower than in the shorter homologue. The organisation of the rodlike molecules inside the trans domains results in highly symmetric textures that make the quantitative analysis of the BAM images possible, and a better understanding of the microscopic structure of the monolayer can be achieved.