Occurrence, Shape, and Dimensions of Large Surface Hemimicelles Made of Semifluorinated Alkanes. Elongated versus Circular Hemimicelles. Pit- and Tip-Centered Hemimicelles

Abstract

The formation of large (approximately 20-35 nm) surface hemimicelles in monolayers of semifluorinated alkanes, C(n)F(2)(n)(+1)C(m)H(2)(m)(+1) (FnHm), observed after transfer onto silicon wafers, is a general phenomenon. F6H16 and F8H14 exclusively form highly monodisperse circular hemimicelles, organized in a hexagonal array. The other FnHm investigated form both circular and elongated hemimicelles. The longer FnHm is, the larger the area fraction of elongated micelles; both the hydrocarbon block (H-block) and the fluorocarbon block (F-block) affect this area fraction. The length of the elongated micelles increases with the total length of the diblocks. The diameter of the circular micelles increases with the length of the H-block but, unexpectedly, not with that of the F-block. Model calculations account for these observations. Close examination of the circular micelles showed that they generally present a pit or a tip at their center. The width of the elongated micelles is comparable to the radius of the circular micelles, suggesting that the latter arise from a partition of elongated micelles, followed by coalescence of the edges of the resulting fragments. The elongated micelles become shorter and fewer when surface pressure increases, further suggesting a conversion of elongated into circular micelles. This conversion is reversible. The surface pressure-molecular area isotherms do not present any feature that forebears the existence of hemimicelles. The obtaining of stable surface patterns from simple, "nonpolar" molecular fluorocarbon/hydrocarbon diblocks opens a new approach for producing featured nanostructures from organic templates.

Size, Shape, and Lateral Correlation of Highly Uniform, Mesoscopic, Self‐Assembled Domains of Fluorocarbon–Hydrocarbon Diblocks at the Air/Water Interface: A GISAXS Study

“…[24] The fractiona nd length of these elongatedm icelles werer eported to increasew ith the molecular length and to decrease with increasing surfacep ressure. This suggests that the correlation length should decrease with increasing block length which is in contrastt oo ur findings.…”

Compression of Self‐Assembled Nano‐Objects: 2D/3D Transitions in Films of (Perfluoroalkyl)Alkanes—Persistence of an Organized Array of Surface Micelles

“…The micelles are, however, packed more closely than in Figure 10, and are somewhat deformed from circular to faceted (inset of Figure 11 a' h ); the number of elongated micelles has increased, as reported in ref. [30]. The new observation is that some higher aggregates are now seen, which were not present at lower surface pressures.…”

“…[29,30,33,37,40] These surface micelles are highly monodisperse, start forming at very low surface pressures, following a concentration-rather than pressure-dependent mechanism, [41] and, upon compression, organize in regular hexagonal arrays without significant change in micelle size and morphology. The aptitude to form monolayers patterned by surface micelles was shown to originate in the dipole moment of the diblocks [42] and already covers a wide range of compounds (n = 8-14, m = 14-20).…”

“…The aptitude to form monolayers patterned by surface micelles was shown to originate in the dipole moment of the diblocks [42] and already covers a wide range of compounds (n = 8-14, m = 14-20). [30,33] Therefore, Langmuir film studies of these FnHm compounds actually concern the compression of arrays of nanometer-sized objects, or supramolecular assemblies, rather than of individual molecules, which provides a unique opportunity for investigating the behavior of films of self-assembled objects. The Langmuir-Blodgett technique is a versatile method that has been widely used to organize nanoscale materials of different morphologies and sizes, including nanoparticles, nanorods, nanowires, carbon nanotubes, [43] and dendrimers.…”

“…AFM of transferred FnHm diblock films: It has previously been established that FnHm diblocks self-assemble into regular arrays of surface micelles when spread as monolayers at the air/water interface, [39] or when deposited on a solid surface using Langmuir-Blodgett [30] or spin-coating techniques. [33] AFM monitoring was essential for achieving the prime objective of this work, namely to investigate the 3D structures that build up upon compression of the monolayer of surface hemimicelles beyond its collapse.…”