From nano-seggregation to mesophases: probing the liquid structure of perfluoroalkylalkanes with 129Xe NMR spectroscopy

Origin of the central pit in hemimicelles of semifluorinated alkanes: How molecular dipoles and substrate deformation can determine supra-molecular morphology

HYPOTHESIS

Semifluorinated alkanes amphiphiles spontaneously form highly monodispersed hemimicelles at the surface of water. The origin of the formation and complex structure of these surprising supramolecular aggregates were only recently clarified using molecular dynamics simulations (MD). The existence of a pit at the center of these aggregates made up of almost 3000 molecules was indeed reproduced by the MD simulations, but not understood.

METHOD

A careful strategy of atomistic MD simulations comparing non-electrostatic force fields with force fields that include electrostatic forces, thus bearing an implicit or explicit dipole, allowed demonstrating the roles of dipolar interactions and interactions with the liquid subphase on the morphology of the aggregates.

FINDINGS

The simulation results clearly show that within the hemimicelles the strong molecular dipoles located at the CH2-CF2 junctions tend to align, leading to a collective shift of the PFAA molecules relatively to each other. This shift is responsible for the curvature of the hemimicelles and originates the central pit, provided the possibility of deforming the surface of the water sub-phase. Comparisons with non-electrostatic force field results further contribute to understand the origin of the self-assembling process. The results directly connect for the first time a molecular property with a mesoscopic structural feature. Given the molecular simplicity of these "primitive" amphiphiles compared to the common hydrophilic/hydrophobic surfactants, the results contribute to understand self-assembly in general.

Towards compartmentalized micelles: Mixed perfluorinated and hydrogenated ionic surfactants in aqueous solution

HYPOTHESIS

Aqueous solutions of mixtures of hydrogenated and perfluorinated ionic surfactants are known to display anomalous aggregation behavior due to the mutual phobicity between hydrogenated and perfluorinated chains. Despite all efforts, different experimental limitations prevented so far a definite interpretation of the existing experimental results: both intermicellar and intramicellar segregation remain acceptable possibilities.

METHOD

The potential for segregation of mixtures of fluorinated and hydrogenated ionic surfactants in water was assessed using atomistic molecular dynamics simulations. Aqueous mixtures of hydrogenated and perfluorinated ionic surfactants were studied: mixtures of anionic surfactants (sodium dodecylsulfate (SDS) + sodium perfluoro-octanoate (SPFO)) and catanionic surfactants (decyltrimethylammonium bromide (DeTAB) + SPFO) were simulated.

FINDINGS

The mixture of anionic surfactants, SDS + SPFO, exhibits clear intramicellar segregation between fluorinated and hydrogenated chains, displaying hydrogenous-rich and fluorous-rich regions. Compartmentalized micelles are thus clearly formed. The simulation results also suggest the possibility of intermicellar segregation. Conversely, catanionic mixtures of DeTAB and SPFO in water solution assemble into a large oblate structure, containing all available molecules in the simulation box, resembling a double layer membrane or a vesicle wall. In this case mixing between fluorinated and hydrogenated surfactants is dictated by charge alternation.

129Xe: A Wide-Ranging NMR Probe for Multiscale Structures

Porous materials are ubiquitous systems with a large variety of applications from catalysis to polymer science, from soil to life science, from separation to building materials. Many relevant systems of biological or synthetic origin exhibit a hierarchy, defined as spatial organization over several length scales. Their characterization is often elusive, since many techniques can only be employed to probe a single length scale, like the nanometric or the micrometric levels. Moreover, some multiscale systems lack tridimensional order, further reducing the possibilities of investigation. 129Xe nuclear magnetic resonance (NMR) provides a unique and comprehensive description of multiscale porous materials by exploiting the adsorption and diffusion of xenon atoms. NMR parameters like chemical shift, relaxation times, and diffusion coefficient allow the probing of structures from a few angstroms to microns at the same time. Xenon can evaluate the size and shape of a variety of accessible volumes such as pores, layers, and tunnels, and the chemical nature of their surface. The dynamic nature of the probe provides a simultaneous exploration of different scales, informing on complex features such as the relative accessibility of different populations of pores. In this review, the basic principles of this technique will be presented along with some selected applications, focusing on its ability to characterize multiscale materials.

Breaking the Structure of Liquid Hydrogenated Alcohols Using Perfluorinated tert-Butanol: A Multitechnique Approach (Infrared, Raman, and X-ray Scattering) Analyzed by DFT and Molecular Dynamics Calculations

The state of aggregation at room temperature of tert-butanol (TBH) and perfluoro tert-butanol (TBF) liquid mixtures is assessed by vibrational spectroscopy (Raman and infrared) and X-ray diffraction and analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. It is shown that larger clusters (mostly tetramers) of TBH are destroyed upon dilution with TBF. Small oligomers, monomers, and mainly heterodimers are present at the equimolar concentration. At variance with slightly interacting solvents, the signature of hetero-oligomers is shown by the appearance of a new broad band detected in the infrared region. The same spectral observation is detected for mixtures of other hydrogenated alcohols (methanol and 1-butanol). The new infrared feature is unaffected by dilution in a polar solvent (CDCl₃) in a high-concentration domain, allowing us to assign it to the signature of small hetero-oligomers. MD simulations are used to assess the nature of the species present in the mixture (monomers and small hetero-oligomers) and to follow the evolution of their population upon the dilution. Combining MD simulations with DFT calculations, the infrared spectral profile is successfully analyzed in equimolecular mixtures. This study shows that TBF is a structure breaker of hydrogen-bonded alcohol networks and that the TBF (donor)-TBH (acceptor) heterodimer is the dominant species in an extended range of concentration, centered in the vicinity of the equimolar fraction.

Langmuir Films of Perfluorinated Fatty Alcohols: Evidence of Spontaneous Formation of Solid Aggregates at Zero Surface Pressure and Very Low Surface Density

In this work, Langmuir films of two highly fluorinated fatty alcohols, CF3(CF2)12CH2OH (F14OH) and CF3(CF2)16CH2OH (F18OH), were studied. Atomic Force Microscopy (AFM) images of the films transferred at zero surface pressure and low surface density onto the surface of silicon wafers by the Langmuir-Blodgett technique revealed, for the first time, the existence of solid-like domains with well-defined mostly hexagonal (starry) shapes in the case of F18OH, and with an entangled structure of threads in the case of F14OH. A (20:80) molar mixture of the two alcohols displayed a surprising combination of the two patterns: hexagonal domains surrounded by zigzagging threads, clearly demonstrating that the two alcohols segregate during the 2D crystallization process. Grazing Incidence X-Ray Diffraction (GIXD) measurements confirmed that the molecules of both alcohols organize in 2D hexagonal lattices. Atomistic Molecular Dynamics (MD) simulations provide a visualization of the structure of the domains and allow a molecular-level interpretation of the experimental observations. The simulation results clearly showed that perfluorinated alcohols have an intrinsic tendency to aggregate, even at very low surface density. The formed domains are highly organized compared to those of hydrogenated alcohols with similar chain length. Very probably, this tendency is a consequence of the characteristic stiffness of the perfluorinated chains. The diffraction spectrum calculated from the simulation trajectories compares favorably with the experimental spectra, fully validating the simulations and the proposed interpretation. The present results highlight for the first time an inherent tendency of perfluorinated chains to aggregate, even at very low surface density, forming highly organized 2D structures. We believe these findings are important to fully understand related phenomena, such as the formation of hemi-micelles of semifluorinated alkanes at the surface of water and the 2D segregation in mixed Langmuir films of hydrogenated and fluorinated fatty acids.