Miscibility of Perylene Derivatives with Fatty Acid in Two-Dimensional Binary Mixture

Two component mixtures of a fluorescent perylene dye (derivative of 3,4,9,10-perylenetetracarboxylic acid) with arachidic acid in the whole range of mole fraction have been investigated in form of molecular monolayers at the air-water interface at different pH of the water subphase. Miscibility of the components in the Langmuir monolayers have been examined for pH 3 and pH 13 of the subphase. Using the Langmuir-Blodgett technique the monolayers were transferred onto freshly cleaved mica sheets and morphology of the films was analyzed using atomic force microscopy working in tapping mode. Investigation of the components miscibility in monolayers, determined by mean molecular area and surface pressure analysis reveals different behavior of the mixtures depending on pH of the subphase. Atomic force microscopy morphology images of the Langmuir-Blodgett films strongly support conclusion about the components mixing and coverage of the mica surface. It has been found that onto acidic subphases components are only miscible in vestigial form, whereas on alkaline the miscibility is strongly affected by the dye concentration.