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.

Assembling structures and dynamics properties of perfluorooctane sulfonate (PFOS) at water–titanium oxide interfaces

“…How aggregation is modified by the presence of solid surfaces is a key issue of surfactant chemistry and surface science [15][16][17]. The surfaces of natural and synthetic minerals commonly consist of different crystal planes.…”

Evidence for Interaction with the Water Subphase As the Origin and Stabilization of Nano-Domain in Semi-Fluorinated Alkanes Monolayer at the Air/Water Interface

“…AFM images of the collapsed film show thick domains surrounded by regions of monomolecular thickness. 15 These regions of remaining monolayers still exhibit domains, similar to those observed on the monolayer transferred at non-zero surface pressure before collapse, although they appear dispersed and non-organized. The bulk phase resulting from the collapse is proposed to be a continuous bilayer without in-plane structuration.…”

“…The deduced lattice parameter at this surface pressure is 38.5 nm 2 . This result demonstrates the presence of nanodomains that remain organized and monodispersed on the liquid surface during the 2D/3D transition, although they appear disorganized on the AFM images of ref 15. However, because the film is in a coexistence region, no information about the first monolayer of the bulk phase and interaction with the water subphase can be deduced, but this result also demonstrates the first-order character of the transition.…”

“…F n H m monolayers collapse at a rather low surface pressure, typically 10−20 mN/m. 15 The fact that the bottom layer remains nanostructured independent of its upper surface being in contact with air or with other SFA molecules indicates that the interaction of this layer with the upper medium is not the driving force for the self-organization of the layer. It could thus result from in-plane interactions, as proposed by Semenov et al, and/or from the interaction with the underlying substrate.…”

Nonpolar Gemini Amphiphiles Self‐Assemble into Stacked Layers of Nano‐Objects

“…[1][2][3] Further compression can induce a transition between a highly organized monolayer of surface micelles and less-organized composite multilayers. [4] Thus, F8H20 forms an upper bilayer of diblocks in coexistence with a lower carpet of surface micelles. F10H16 forms crystalline-like inflorescences that also coexist with a carpet of surface micelles.…”