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

Long-Range Lateral Correlation between Self-Assembled Domains of Fluorocarbon-Hydrocarbon Tetrablocks by Quantitative GISAXS

The structure and lateral correlation of fluorocarbon-hydrocarbon tetrablock di(F10Hm) domains at the air/water interface have been determined by quantitative analysis of grazing incidence small-angle X-ray scattering (GISAXS) data. The measured GISAXS signals can be well represented by the full calculation of the form and structure factors. The form factor suggests that di(F10Hm) domains take a hemiellipsoid shape. Both major and minor axes of the hemiellipsoids monotonically increased in response to the elongation of the hydrocarbon blocks, which can be explained by the concominant increase in van der Waals interaction. The structure factor calculated from the GISAXS signals suggests that the domains take an orthorhombic lattice. Remarkably, the lateral correlation can reach over a distance that is more than 14 times longer than the distance to the nearest neighbors. Our data suggest that quantitative GISAXS enables the optimal design of mesoscopic self-assemblies at the air/water interface by fine-tuning of the structures of molecular building blocks.

Emergence of Strong Nonlinear Viscoelastic Response of Semifluorinated Alkane Monolayers

Viscoelasticity of monolayers of fluorocarbon/hydrocarbon tetrablock amphiphiles di(FnHm) ((C_nF_2n+1CH₂)(C_m-2H_2m-3)CH-CH(C_nF_2n+1CH₂)(C_m-2H_2m-3)) was characterized by interfacial dilational rheology under periodic oscillation of the moving barriers at the air/water interface. Because the frequency dispersion of the response function indicated that di(FnHm) form two-dimensional gels at the interface, the viscosity and elasticity of di(FnHm) were first analyzed with the classical Kelvin-Voigt model. However, the global shape of stress response functions clearly indicated the emergence of a nonlinearity even at very low surface pressures (π ≈ 5 mN/m) and small strain amplitudes (u₀ = 1%). The Fourier-transformed response function of higher harmonics exhibited a clear increase in the intensity only from odd modes, corresponding to the nonlinear elastic component under reflection because of mirror symmetry. The emergence of strong nonlinear viscoelasticity of di(FnHm) at low surface pressures and strain amplitudes is highly unique compared to the nonlinear viscoelasticity of other surfactant systems reported previously, suggesting a large potential of such fluorocarbon/hydrocarbon molecules to modulate the mechanics of interfaces using the self-assembled domains of small molecules.

Thermal behavior and high- and low-temperature phase structures of gemini fluorocarbon/hydrocarbon diblocks

The thermal behavior and phase structure of two series of gemini fluorocarbon/hydrocarbon diblock amphiphiles with the general formula (CnF2n+1CH2)(Cm - 2H2m - 3)CH-CH(CnF2n+1CH2)(Cm - 2H2m - 3), with n = 8, 10 and m = 6, 12, 14, 16, 18, 20 (abbreviated as di(FnHm)), have been investigated by differential scanning calorimetry, polarized optical and freeze-fracture transmission electron microscopies, dilatometry, and small-angle X-ray scattering (SAXS). The various terms of the series exhibit the same thermal behavior, essentially composed of two exothermal transitions, a low-temperature event that corresponds to the melting of the hydrocarbon chains at TH and a high-temperature transition associated with the melting of the fluorocarbon chains at TF. Below TH, a disordered plastic rotator phase, MLT, and above TH, a lamellar phase, MHT, were determined by SAXS experiments. Above TF, the compounds eventually clear into the isotropic liquid. In the MHT phase, both the Fn and Hm blocks are segregated from each other, forming sublayers with sharp interfaces, as revealed by the five lamellar orders and remarkable sharpness of the SAXS peaks. In the MLT phase, the partial crystallization of the aliphatic blocks when the temperature is lowered leads to the disruption of the aliphatic sublayers into rows of ribbons arranged according to pseudohexagonal and/or rectangular arrangements with different lattice sizes (p2gg symmetry). The Fn segments form the fluorinated continuum. In support of SAXS, molecular packing models of the tetrablocks are proposed on the basis of the temperature/volume variations of di(F10H20) and di(F10H16) in both high- and low-temperature phases, as determined by dilatometry. It is notable that the arrangements found for di(FnHm) are completely different from those previously reported for FnHm diblocks, revealing the influence of the linker unit on the solid-state behavior of the tetrablocks.

Large organized surface domains self-assembled from nonpolar amphiphiles

For years, researchers had presumed that Langmuir monolayers of small C(n)F(2n+1)C(m)H(2m+1) (FnHm) diblock molecules (such as F8H16) consisted of continuous, featureless films. Recently we have discovered that they instead form ordered arrays of unusually large (~30-60 nm), discrete self-assembled surface domains or hemimicelles both at the surface of water and on solid substrates. These surface micelles differ in several essential ways from all previously reported or predicted molecular surface aggregates. They self-assemble spontaneously, even at zero surface pressure, depending solely on a critical surface concentration. They are very large (~100 times the length of the diblock) and involve thousands of molecules (orders of magnitude more than classical micelles). At the same time, the surface micelles are highly monodisperse and self-organize in close-packed hexagonal patterns (two-dimensional crystals). Their size is essentially independent from pressure, and they do not coalesce and are unexpectedly sturdy for soft matter (persisting even beyond surface film collapse). We and other researchers have observed large surface micelles for numerous diblocks, using Langmuir-Blodgett (LB) transfer, spin-coating and dip-coating techniques, or expulsion from mixed monolayers, and on diverse supports, establishing that hemimicelle formation and ordering are intrinsic properties of (perfluoroalkyl)alkanes. Notably, they involve "incomplete" surfactants with limited amphiphilic character, which further illustrates the outstanding capacity for perfluoroalkyl chains to promote self-assembly and interfacial film structuring. Using X-ray reflectivity, we determined a perfluoroalkyl-chain-up orientation. Theoretical investigations assigned self-assembly and hemimicelle stability to electrostatic dipole-dipole interactions at the interface between Fn- and Hm-sublayers. Grazing-incidence small-angle X-ray scattering (GISAXS) data collected directly on the surface of water unambiguously demonstrated the presence of surface micelles in monolayers of diblocks prior to LB transfer for atomic force microscopy imaging. We characterized an almost perfect two-dimensional crystal, with 12 assignable diffraction peaks, which established that self-assembly and regular nanopatterning were not caused by transfer or induced by the solid support. These experiments also provide the first direct identification of surface micelles on water, and the first identification of such large-size domains using GISAXS. Revisiting Langmuir film compression behavior after we realized that it actually was a compression of nanometric objects led to further unanticipated observations. These films could be compressed far beyond the documented film "collapse", eventually leading to the buildup of two superimposed, less-organized bilayers of diblocks on top of the initially formed monolayer of hemimicelles. Remarkably, the latter withstood the final, irreversible collapse of the composite films. "Gemini" tetrablocks, di(FnHm), with two Fn-chains and two Hm-chains, provided two superposed layers of discrete micelles, apparently the first example of thin films made of stacked discrete self-assembled nanoobjects. Decoration of solid surfaces with domains of predetermined size of these small "nonpolar" molecules is straightforward. Initial examples of applications include deposition of metal dots and catalytic oxidation of CO, and nanopatterning of SiO(2) films.