Phase separation in mixed monolayers of arachidic acid and a phthalocyanine of zinc

Mixed Langmuir and Langmuir-Blodgett films of dissimilar components, arachidic acid and a phthalocyanine of zinc, have been investigated. These compounds form mixed films with phase separation, with domains at the sub-micro scale which are detected by AFM in LB films but not observed by BAM. These domains present different contrast in the surface potential images obtained with SP-AFM and in the friction images. The analysis of the isotherms and of the AFM images, at several compositions, reveals that the components are immiscible. The difference in height between the arachidic acid and the phthalocyanine of zinc phases is in accordance with the heights of the molecules, and points to the presence of a monolayer in the zinc phthalocyanine phase. AFM has revealed as a necessary technique to elucidate the miscibility in mixed films at the nanometric scale. UV-Vis spectroscopy shows changes in the spectra bands of LB films in respect to those in solution, due to the organization of the molecules in the LB film.