Partially Fluorinated Thioethers at the Water/Air Interface. Langmuir Monolayer Characterization and X-ray Scattering Studies

A series of partially fluorinated aliphatic thioethers of the following formula: CF(3)-(CF(2))(7)-(CH(2))(10)-S-C(n)H(2n+1), where n = 1, 2, 4, 8, 12, and 18, were synthesized and spread from chloroform solution onto the water/air interface in a Langmuir trough. Upon compression of the film, the surface pressure-mean molecular area isotherms were recorded. It turned out that all the investigated compounds are capable of stable Langmuir monolayers formation. The electrical properties of the studied monolayers have been investigated by monitoring the change in the surface potential (ΔV) upon film compression. The performed experiments proved that regardless of the length of the C(n)H(2n+1) chain, the orientation of the film molecules is very similar. The investigated monolayers have been visualized applying Brewster angle microscopy. The monolayers of the shorter homologues turned out to be homogeneous until their collapse, whereas for longer homologues, the formation of multilayer aggregates was observed. The methods of grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR) have also been applied for the characterization of the organization of thioether molecules in their monolayers. The GIXD experiments proved that the films of the studied thioethers possess a periodical in-plane structure. They are collectively tilted toward the next neighbor (NN tilt), and their packing within the monolayer (xy) plane can be described by the 2-dimentional rectangular centered lattice. Interpretation of the XR results indicate that only the partially fluorinated chain of the thioether molecule is periodically organized at the water/air interface, whereas the hydrocarbon chain is disordered and has a liquid character.