Viscoelasticity of semifluorinated alkanes at the air/water interface

Abstract

As semifluorinated alkanes (SFA) present an unusual class of mesogenic units for supramolecular structure formation, the study of their physical properties and self-assembly at fluid interfaces is of substantial relevance. To this end, the two-dimensional phase behavior and the viscoelastic properties of semifluorinated alkanes with symmetric number of carbon atoms in the fluorinated and hydrogenated side chains (F(CF₂)₁₂–(CH₂)₁₂H and F(CF₂)₁₁CH₂–core–(CH₂)₁₂H with dibromophenyl core were investigated at the air/water interface and compared to the asymmetric F(CF₂)₁₂(CH₂)₂₀H system. Surface pressure/area isotherms and compression–expansion cycles were recorded at 20 °C, 40 °C, and 50 °C. Depending on the molecular details, an unexpected phase transition and hysteresis in compression–expansion cycles could be observed. Interfacial rheology revealed for all monolayer systems a solid-like viscoelastic response over the investigated range of surface pressures. The interfacial storage modulus G′_i and interfacial loss modulus G′′_i increased with increasing surface pressure. More importantly, at constant surface pressure the simple semifluorinated alkanes, forming nearly circular 2D micellar structures at the interface, exhibited rheological properties reminiscent of glass-like colloid systems. On the other hand, the SFA with the dibromophenyl core formed 2D interfacial micelles of highly elongated shape with dendritic domains and exhibited gel-like rheological properties, similar to those of the asymmetric alkane. Moreover, with increasing surface pressure the response of the asymmetric system alters from glassy to gel-like. These findings suggest that fine differences in the molecular structure are reflected in the linear viscoelastic response of the Langmuir films, and hence offer the possibility to molecularly tune the rheology of fluid interfaces.