The Effect of the Reduction of the Available Surface Area on the Hemicyanine Aggregation in Laterally Organized Langmuir Monolayers

This paper analyzes the effect of the reduction of the available area on the aggregation of an hemicyanine dye, 4-[4-dimethylamino)styryl]-1-docosylpyridinium bromide (SP), at the air-water interface. Mixed films of SP and stearic acid (SA) in 1:1 molar ratio have been studied, and compared with the previously studied films of SP and dimyristoyl-phosphatidic acid (DMPA) in 1:1 molar ratio. With regard to the SP:DMPA films, the replacement of DMPA by SA involves reducing the area available for the hemicyanine aggregation, since the SA molecule provides only an alkyl chain to the set, whereas the DMPA molecule provides two alkyl chains to the set. The SP:SA mixed films have been studied by Grazing Incidence X-ray Diffraction (GIXD), Brewster angle microscopy (BAM), and reflection spectroscopy at the air-water interface. Langmuir-Schaeffer films have been studied by UV-vis transmission spectroscopy. The SP:SA mixed monolayer forms star-shaped domains with inner textures, indicating anisotropy. Circular domains were observed for the SP:DMPA system. GIXD experiments relate the star-shaped domains with an orthorhombic phase, as for circular domains observed for the SP:DMPA system. The results obtained by reflection and transmission polarized spectroscopy for the SP:SA system showed the splitting of the absorption band of the aggregate. The splitting has been related with a twisting in the hemicyanine groups. The reduction on the available area accounts for the aggregation of the hemicyanine group. The domain shape at the mesoscopic level mainly originated from the interactions between the polar groups and the reduced surface area. The interactions between the alkyl chains are essential to maintain the crystalline structure, although these interactions did not play the main role in the final shape of the domain.