title: Compression of self-assembled nano-objects: 2D/3D transitions in films of (perfluoroalkyl)alkanes--persistence of an organized array of surface micelles.
abstract: 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+1)C(m)H(2m+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, F8H18, F8H20, and F10H16 can actually be compressed far beyond the "collapse" of their monolayers at approximately 30 A(2). For molecular areas A between 30 and 10 A(2), 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 A(2), surface pressure increases again, reaching up to approximately 48 mN m(-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.
authors.affiliation: Systèmes Organisés Fluorés à Finalités Thérapeutiques (SOFFT), Université de Strasbourg, Institut Charles Sadron (CNRS), 23 rue du Loess, 67034 Strasbourg Cedex 2, France.
authors.forename: Caroline
authors.initials: C
authors.lastname: de Gracia Lux
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Gallani
authors.forename: Gilles
authors.initials: G
authors.lastname: Waton
authors.forename: Marie Pierre
authors.initials: MP
authors.lastname: Krafft
mesh_terms: D000473:Alkanes / Q000737:chemistry*; D005466:Fluorocarbons / Q000737:chemistry*; D008823:Micelles; D018625:Microscopy, Atomic Force; D009040:Motion Pictures; D049329:Nanostructures / Q000737:chemistry*; D055585:Physical Phenomena; D013499:Surface Properties
chemical_list: D000473:Alkanes; D005466:Fluorocarbons; D008823:Micelles