Compression of self-assembled nano-objects: 2D/3D transitions in films of (perflnoroalkyl)alkanes-persistence of an organized array of surface micelles

Understanding and controlling the molecular organization of amphiphilic molecules at interfaces is essential for materials and biological sciences. When spread on water, the model amphiphiles constituted by C_n,F _2n+1C_mH2m+1 (FnHm) diblocks spontaneously self-assemble into surface hemimicelles. Therefore, compression of monolayers of FnHm diblocks is actually a compression of nanometric objects. Langmuir films of F8H16, FSH18, F8H20, and F1QH16 can actually be compressed far beyond the "collapse" of their monolayers at ∼30 Ų. For molecular areas A between 30 and 10 Ų, a partially reversible, 2D/ 3D transition occurs between a monolayer of surface micelles and a multilayer that coexist on a large plateau. For A < 10 Ų, surface pressure increases again, reaching up to ∼48mNm^-1 before the film eventually collapses. Brewster angle microscopy and AFM indicate a several-fold increase in film thickness when scanning through the 2D/3D coexistence plateau. Compression beyond the plateau leads to a further increase in film thickness and, eventually, to film disruption. Reversibility was assessed by using compression-expansion cycles. AFM of F8H20 films shows that the initial monolayer of micelles is progressively covered by one (and eventually two) bilayers, which leads to a hitherto unknown organized composite arrangement. Compression of films of the more rigid F10H16 results in crystalline-like inflorescences. For both diblocks, a hexagonal array of surface micelles is consistently seen, even when the 3D structures eventually disrupt, which means that this monolayer persists throughout the compression experiments. Two examples of pressure-driven transformations of films of self-assembled objects are thus provided. These observations further illustrate the powerful self-assembling capacity of perfluoroalkyl chains.