Miscibility and segregated structures in mixed Langmuir monolayers

The phase behavior and morphology of segregated structures are considered for mixed Langmuir monolayers, which comprise a type of supramolecular polymer having a complex internal structure mixed with a long chain fatty acid. We fabricated two different series of mixed monolayers from a polyglutamate (PG) copolymer having 30% octadecyl ester side chains and 70% methyl ester side chains and fatty acids. These mixed monolayers deposited on a solid substrate were studied by pressure-area diagram measurements, X-ray analysis, and atomic force microscopy. Stearic acid (STA) and hexacosanoic acid (HCA) with alkyl chain lengths of 17 and 25 carbon atoms, respectively, were used as low molecular weight components. For the mixture PG:STA, where the length of the STA molecules is comparable to the length of the PG side chains, we observed the formation of partially miscible monolayers. These mixtures exhibit a nanometer scale domain morphology formed by the STA molecules dissolved in the outer shell of the PG monolayer. In contrast, for the PG:HCA mixture we observed a strong tendency for microphase separation and the formation of well-defined submicron segregated structures in the monolayers. Lateral compression of the mixed monolayers to a point close to the collapse pressure promotes microphase separation in both types of mixed monolayers with the formation of anisotropic surface morphology and oriented domains.