2018, Vol.91, No.5

After a brief reminder of the specific properties of fluorocarbons, fluorinated chains and molecular fluorocarbon-hydrocarbon diblocks (semifluorinated alkanes, CnF2n+1CmH2m+1, FnHm) that account for their exceptional aptitude for self-organization, we review recent advances on the self-assembled surface nanodomains that FnHm diblocks form on water and solid surfaces, their shape and size characteristics, and their hierarchical organization into structures of higher complexity. Remarkably indeed, FnHm diblocks, when spread as Langmuir monolayers on water, self-assemble into circular mesoscopic nanodomains that exist even in the absence of lateral pressure, and self-organize into regular hexagonal arrays upon compression. These surface domains can be transferred essentially unchanged onto solid surfaces. They can also be obtained by direct casting or spin coating of solutions of diblocks on solids, or by spontaneous formation on liquid crystals. The nanodomains retain their size, shape and organization upon compression and, amazingly, even beyond the collapse of their Langmuir monolayers. The domain-patterned films display uncommon rheology, with predominantly elastic monolayers, and two-dimensional gels were generated, including at zero surface pressure. The formation and behavior of surface domains from related tri- and tetrablocks have also been reported. A tetrablock afforded the first example of pressure-driven stacking of self-assembled nano-objects. The domain-patterned films constitute attractive templates for organizing nanoparticles in components of electronic devices and sensors, and for fabricating ordered mesoporous solids. Most recently, a novel aggregation mode was found for FnHm diblocks, namely their crystallization into micron-size polycrystalline two-dimensional radial and/or ring-banded spherulites. Applications in medicine and materials science are being investigated.

Little wonder that exchanging a hydrogen atom by a fluorine on a carbon skeleton changes a chemical’s properties so dramatically: fluorine has 9 electrons and 9 protons vs. only one electron and one proton for hydrogen. Organofluorine compounds have extensive applications in medicine and agriculture.17 Introducing a perfluoroalkyl chain in a molecule has critical consequences. Fluorocarbons, fluorinated polymers and fluorinated surfactants provide extreme performance materials, unattainable with hydrocarbon materials, and have found countless uses.810

Among the highly fluorinated compounds, (F-alkyl)alkanes (semifluorinated alkanes, CnF2n+1CmH2m+1, FnHm) consist of two immiscible, covalently linked moieties, a fluorocarbon block (CnF2n+1, Fn) and a hydrocarbon block (CmH2m+1, Hm), that display significantly distinct properties. Although they are devoid of hydrophilic function, and have very low solvent polarity indices,11 FnHm diblocks display surface activity, co-surfactant properties and a striking penchant for promoting self-assembly, nanocompartmentation, nanostructuration and nanoorganization.1216 They provide valuable tools for enhancing, directing and adjusting phase behavior, stability and properties of colloids and interfaces. They allow control of emulsion droplet size and stability, liposome stability and permeability, fiber and gel formation and rheology, etc.4,1722

The behavior at interfaces of the so simple FnHm diblocks turned out to be surprisingly singular and complex.13,14 FnHm diblocks are prone to formation of unexpectedly well-organized thin films. Thus, when compressed as Langmuir monolayers on water or deposited on solid substrates, FnHm diblocks were found to self-assemble into monodisperse, often circular surface domains that organize into ordered hexagonal arrays (two-dimensional crystals).23,24 These surface domains are much larger, and involve thousands of molecules, that is, orders of magnitude more than classical surfactant micelles. The formation and organization of such surface domains has been observed for numerous diblocks, directly on the surface of water, or after Langmuir-Blodgett (LB) transfer, spin-coating, dip-coating, or expulsion from mixed monolayers, on diverse substrates, including liquid crystals, thus establishing that surface domain formation and ordering are intrinsic properties of FnHm diblocks.14 This unanticipated behavior raised substantial attention in the last decade and significant efforts were devoted to answering a number of unresolved questions. These questions include whether the surface domains existed at large molecular areas, that is, when the molecules are not subjected to lateral pressure; what the conformation and ordering of the alkyl (Hm) moieties within the domains were; what the relationships between domain size and morphology and block lengths were; whether surface pressure influences domain shape, size and ordering; whether it is possible to control (and predict) domain shape and size. Still other questions addressed the occurrence of diblock self-assembly on water versus on solid substrates, including oriented substrates; Fn and Hm block orientations; the forces that drive domain formation (different from those inducing the phase separation of hydrocarbon and fluorinated surfactants that results in 2D nanomosaics;25) their rheological behavior; the fate of the domains after the films have collapsed upon compression; the potential of these surface domains with respect to providing templates for organizing nanomaterials, with possible applications in catalysis, nanoelectronics, sensor technology, mesoporous materials synthesis, etc. More recently, it was found that FnHm diblocks could generate two-dimensional polycrystalline spherulites having radial or ring-banded morphologies, raising new questions, including about their intimate structure, mechanism of and driving forces for formation, morphology control and customization.

This article reviews the recent progress of our knowledge about the formation and organization of segregated domains of FnHm diblocks and some related tri- and tetrablocks at interfaces. The connection between the Fn and Hm blocks is usually a C-C bond, but di- and tetrablocks with central phenyl groups have also been investigated. Section 2 is a brief recapitulation of the specific properties of fluorocarbons and fluorinated chains as compared to those of their hydrocarbon counterparts. The particular properties generated by covalently linking the antagonist fluorocarbon and hydrocarbon segments are highlighted. Section 3 discusses recent advances in our understanding of the formation and structural ordering of nanodomains of FnHm diblocks on water and on solid surfaces, and Section 4 the rheological properties of diblock films. Section 5 focuses on the spontaneous assembly of nanodomains on the surface of liquid crystals. Section 6 is devoted to nanodomains formed in mono- and multilayers of tri- and tetrablocks. Section 7 reports the self-organization of FnHm diblocks into two-dimensional radial and ring-banded spherulites. Finally, Section 8 broaches the potential for applications of FnHm diblock-containing organized systems.

The particular physical chemistry of fluorocarbons, fluorinated chains and fluorocarbon-hydrocarbon diblocks, their aptitude for boosting surface activity and outstanding capacity for inducing self-assembly and fostering organization have been extensively reviewed and discussed previously; the reader is referred to these papers for details and references.9,13,14 Only a brief reminder of those properties relevant to diblock organization is provided here.

Fluorine is more abundant by weight than carbon in the Earth’s crust; it is, however, not the cheapest to mine and isolate of the chemical elements. Compared to hydrogen, it has a higher ionization potential (1676 vs. 1312 kJ mol−1), electron affinity (328 vs. 73 kJ mol−1) and electronegativity (3.98 vs. 2.20). Its covalent radius is larger (0.57 vs. 0.31 Å), as well as its van der Waals radius (around 1.47 Å vs. 1.20 Å). Fluorine has a tightly packed, dense electron cloud that is less polarizable than that of hydrogen (α = 0.557 vs. 0.667 10−24 cm3) and of all the other elements of the Periodic Table, except helium and neon. It is the most electronegative of all elements and forms very strong covalent bonds with many of them, including the “inert” noble gases.26 It is the only element besides hydrogen that is capable of forming extended structures with carbon skeletons, namely fluorocarbons and perfluoroalkylated chains, the direct fluorinated counterparts of hydrocarbons and alkyl chains.

Fluorocarbons, due to fluorine’s lower polarizability than that of hydrogen, display significantly lower cohesive energy densities in their condensed states than hydrocarbons. They also have lower surface energies (and hence, surface tensions), refractive indexes, and dielectric constants, but higher densities, compressibilities, viscosities, critical temperatures, and gas-dissolving capacities than their hydrocarbon counterparts. Their vapor pressures are also much higher, relative to molecular weight. These specificities basically reflect the stronger intramolecular bonds and weaker intermolecular interactions displayed by fluorocarbons relative to the corresponding hydrocarbons. C–C bonds become stronger and usually less reactive when the carbon atoms bear fluorines, resulting in outstanding thermal stability and chemical inertness. The latter actually raise some environmental issues that restrict large tonnage uses and commands use of closed-loop systems.8

Linear perfluoroalkyl chains (CnF2n+1, F-chains) differ from hydrocarbon chains (correspondingly noted H-chains) in many ways. They are much bulkier, with a cross-section in the 27–30 Å2 range for the former as compared to 18–21 Å2 for the latter. F-chains consequently display larger surface areas than H-chains, which is a chief contributor to their extreme hydrophobicity and enhanced surface activity. F-chains are indeed considerably more hydrophobic than H-chains and are markedly lipophobic as well, which prompts phase separation and compartmentation within colloidal systems. Due to the larger steric needs of fluorine, F-chains are substantially stiffer than H-chains. They commonly adopt an all-trans helical (rather than planar) conformation in standard conditions, thus relieving the 1,3 repulsive fluorine-fluorine interactions caused by the larger van der Waals radius of fluorine. The pitch of the helix depends to some extent on the number of CF2 units; it increases with chain length and decreases when temperature or pressure increases. A planar zigzag conformation may still be advantageous for short F-chains and could be fostered by linkage to an H-chain. The conformational freedom of linear F-chains is substantially reduced as compared to that of H-chains, with at least 25% larger trans/gauche interchange enthalpies than for their hydrocarbon counterpart.27 The dense, “lubricating” electron sheath that surrounds F-chains, and their helical conformation, also provide a smooth, “streamlined” molecular shape that could facilitate rotation and translation (slipping) of an F-chain as a whole along its long molecular axis. Disorder arising from such movements occurs at lower temperatures in F-chains than in H-chains. The impeded internal reorientation about C-C bonds and reduced occurrence of gauche defects contribute to facilitate F-chain ordering, stacking and crystallization. It should be reiterated that fluorine atoms in F-chains usually do not engage in hydrogen bonding.28

As a consequence of the disparity in cohesive energy densities, the mixing of liquid fluorocarbons and hydrocarbons, and likewise of F-chains and H-chains, is highly nonideal.2932 When mixed with F-hexane, hexane undergoes coiling, thus increasing the proportion of gauche conformations.33 F-chains and H-chains tend to phase separate, inducing the formation of discrete micro- and nanosize domains in solutions, micelles, monolayers, membranes and other colloids. F-chains exhibit a powerful tendency to self-assemble and collect at interfaces, thus fostering surface activity, self-association and molecular organization.12,3438 Bilayered self-assemblies of F-chains were reported to form in aprotic hydrocarbon solvents.39,40

F-chains manifest a much more pronounced tendency than H-chains to generate layered structures with longer-range ordering. The ∼30% smaller cross-sectional area of hexagonally packed H-chains, as compared to similarly packed F-chains, can facilitate conformational disordering of the H-chains both in the bulk and in discrete self-assemblies. The activation energy for dynamic processes such as conformational changes and melting is usually lower for an H-chain than for an F-chain of similar length, and the onset of these processes occurs accordingly, first in the H-chain when temperature is raised. On the other hand, translations and rotations within a collection of molecules may be facilitated for F-chains due to their more streamlined shape.

Fluorocarbon-hydrocarbon diblocks (or (perfluoroalkyl)alkanes or semifluorinated alkanes) are produced by covalently yoking together an F-chain and an H-chain (Scheme 1).13,41 This engenders considerable energetic and steric frustrations and produces specific properties, and hence, behaviors that are markedly different from those of the two parent moieties.13 Despite their structural simplicity, FnHm diblocks are not only amphiphilic (the Fn and Hm moieties display unlike affinities), but also amphisteric (the two moieties have different cross-sections, conformations and space requirements) and amphidynamic (one is rigid, rod-like, and prone to crystallization, yet “slippery”; the other is more flexible and more disposed to kinks and defects).

The carbon-carbon bond in CF3–CH3 is significantly shorter and has a higher dissociation energy than in the symmetrical molecules C2F6 and C2H6; it also displays a strong dipole moment that profoundly influences molecular packing and related properties. The length and volume of a fully stretched diblock can be calculated using standard bond lengths, bond angles, covalent radii, and a specific mean contribution for the CF2-CH2 junction.42 FnHm diblocks form stable Langmuir films on water in spite of the absence of polar heads.43 They also form dense films at the hydrocarbon/fluorocarbon interface (surface freezing).44 In their bulk solid state they can form fibers, liquid crystals and gels.4547

3.1. Formation, Size, Ordering and Hierarchical Organization.

The long range organization of Langmuir monolayers of F8Hm (m = 14, 16, 18, 20) diblocks was investigated using surface pressure (π) vs. area (A) per molecule isotherms, Brewster angle microscopy (BAM), grazing incidence X-ray diffraction (GIXD) and small-angle grazing incidence X-ray scattering (GISAXS).48 The F8Hm monolayers undergo a phase transition from a low-density expanded phase to a condensed phase upon compression. The condensed phase that appears at π > 0 mN m−1 is crystalline and presents two hexagonal lattices at two distinct scales (Figure 1).

A first hexagonal lattice is associated with the circular nanodomains. It displays an unusually large number of peaks for soft matter and a high coherence length (~200 nm), which is the signature of a crystalline organization. The second hexagonal lattice is associated with the hexagonal packing of the F8 blocks and has a typical lattice parameter of ~0.6 nm and a much shorter coherence length (~2 nm) that is limited by the dimension of the domains. The lattice parameter increases with the length of the Hm block for each surface pressure. The number of diblocks per unit cell increases from ~2200 for F8H14 to ~4800 for F8H20. This study also revealed that the F8Hm diblocks investigated form circular domains on the surface of water, suggesting that the elongated domains observed on transferred films of F8H18 and F8H2049 likely result from relaxation occurring during the transfer of the monolayer. Spirals, ribbons, toroids were also observed in films of FnHm diblocks spin-coated on silicon, mica and graphite substrates.50,51

Also noticeable is that the nanodomains were compressed when π was increased, but not the F8 blocks within the domains, as assessed by the fact that the diffraction peak associated with the F8 block organization remains unchanged with respect to the surface pressure. This indicates the presence of disordered diblocks at all π values, supporting the coexistence of a vertical ordered phase (FnHm nanodomains) surrounded by a disordered phase of diblocks lying on the surface, as described in a theoretical study that showed that the nanodomains’ limiting diameter was determined by the electrostatic activation energy (energy barrier for nanodomain fusion).52 The predicted, kinetically controlled nanodomain size was in the 24–32 nm range, in agreement with the experimental results.24

The above GISAXS and GIXD study48 relied solely on the position of the diffraction peaks, and used the lattice parameters to determine the size of the domains, which might cause an overestimation of the latter if the domains are not closely packed in a hexagonal lattice. A more recent GISAXS study53 provided full calculation of the structure factor S(qy) and form factor F(qy) for a series of F8Hm (m = 14, 16, 18, 20) and FnH16 (n = 8, 10, 12) nanodomains at the air/water interface. These factors allowed direct and quantitative determination of the diameter and height of the nanodomains, as well as the extent of their lateral inter-correlation. The diameter of the domains, as determined from the form factors, was found to increase monotonically with the length of both blocks (Figure 2), which can be ascribed to an increase in the line tension that results from the increase in attractive van der Waals interactions between longer Fn or Hm blocks. The lateral correlation between domains can reach distances 10 to 26 times longer than the diameter of a single domain. This means that the domains are strongly correlated and have, in particular, a higher normalized correlation length than, for example, that of the domains of a perfluoroalkylated surfactant found in a phospholipid matrix.54 The study by Veschgini et al. demonstrates that the GISAXS calculation approach can provide an effective tool for finely monitoring the effect of molecular structure on surface domain structure and interactions.

Langmuir monolayers of F12H12 and F12H20 were investigated by neutron reflectivity directly at the air/water interface and by scanning force microscopy (SFM) coupled with Kelvin probe force microscopy (KPFM, or surface potential microscopy) experiments after transfer onto silicon wafers.55 It was found that F12H12 forms surface domains of ~30 nm in diameter and that these domains are composed of 10 nm-size circular or “spherical cap” sub-structures, the assembly of which is controlled by free energy minimization (Figure 3). Based on results from neutron reflectivity and KPFM, a model with the Fn blocks oriented toward air was confirmed.

F12H20 was shown to form similar, although larger (diameter ~50 nm) hexagonally shaped surface domains that are densely packed in hexagonal arrays, which coexist with elongated, tightly interlocked elongated structures,55 in line with49 (Figure 4). The symmetry of the molecule does not seem to play a significant role on domain morphology.

The GISAXS studies of FnHm diblocks allowed unambiguous determination of the existence, size and organization of surface domains for molecular areas below 30 Å2, that is, in the molecular area range for which nanodomains are in close contact and form a hexagonal array.24,48,53 However, GISAXS signals were not detected for the higher molecular areas, which means either that the nanodomains do not exist or that they are disordered on the surface of water. This question of the existence of the nanodomains at large molecular area is basic because it is relevant to the mechanisms of their formation. It was found that, after transfer on silicon wafers of monolayers at low surface pressure, the surface domains already exist,56 but this does not imply that they already existed on the surface of water prior to transfer.

A recent infrared reflection absorption spectroscopy (IRRAS) study has conclusively resolved this issue by showing that nanodomains are formed when F8H16 is spread on water as a Langmuir film at very large surface areas (zero surface pressure).57 The infrared absorption positions below 2920 cm−1 and 2851 cm−1 for νas(CH2) and νs(CH2), respectively, are characteristic of alkyl chains in fully stretched all-trans conformation.58 F8H16 is seen to vibrate consistently at the same low wavenumbers, independently of surface pressure and of the available surface area (Figure 5). This is in contrast with dipalmitoylphosphatidylcholine monolayers, for example, which exhibit a liquid expended to liquid condensed phase transition upon compression. It establishes that the H16 segments always retain a fully stretched all-trans conformation that is, remain highly organized throughout compression. This experiment provides decisive evidence for self-aggregation of the diblock into organized structures at the air/water interface, including at high molecular areas. Furthermore, in order to compensate for the larger space requirement of the F8 blocks, which are standing normal to the surface of water, the H16 blocks adopt a tilted orientation, by 30°, relative to the film’s normal. It is also noteworthy that the internal structure of the surface domains is not affected by compression.

The common structural assumption had up to then been that the inner core Hm segments within the nanodomains were in a disordered liquid state in order to compensate for the difference in cross-sectional area between Fn and Hm blocks. The IRRAS study decisively established that, contrary to these earlier assumptions, the conformation of the alkyl moieties is not liquid-like.57 The H16 moieties are actually stretched in an all-trans configuration and tilted by 30°. This twist angle shows that the alkyl chains are densely packed and that rotation about their long axis is impeded. This means that the alkyl core of the surface domains is truly crystalline and not disordered, and this independently of molecular area. The nanodomains can thus be envisioned as solid particles spread at the air/water interface.

The adsorption and arrangement at the air/water interface of the shorter-chain F6H10 and F6H16 diblocks have been investigated using molecular dynamics simulations.59 Starting from a random mixture of orientations, the spontaneous formation of elongated domains was determined for both diblocks for molecular area A larger than ∼50 Å2. For areas between 50 and 30 Å2, two pseudophases, one rich in H-chains and the other in F-chains, are formed. For molecular areas lower than ∼30 Å2 multilayers are predicted.

3.2. Organization Beyond Collapse. Stacking of Surface Domains.

It was found that Langmuir films of F8Hm (m = 16, 18, 20) and F10H16 can actually be compressed far beyond the “collapse” of their monolayers at ~30 Å2.60 For 10 < A < 30 Å2, a partially reversible 2D/3D transition occurs between a monolayer of nanodomains and a multilayer that coexist on a large plateau. AFM of F8H20 films established that the initial monolayer of surface domains is progressively covered by one, and eventually two bilayers (Figure 6). Compression of films of the more rigid F10H16 diblock results in crystalline inflorescences. For both diblocks, a hexagonal array of nanodomains is consistently seen, even when the 3D structures eventually disrupt, which means that the self-assembled FnHm domains are robust enough to persist throughout the compression experiments and collapse upheaval.

A subsequent GISAXS study determined that in the collapsed films of F8H18 at the air/water interface the first layer in contact with the water sub-phase exhibits the same hexagonal structure than that observed in the monolayer before its collapse (Figure 7).61 The domains never coalesce, contrary to the upper layers (bilayers), which do not exhibit a nanodomain organization, as shown by the AFM study done on transferred monolayers.60 These results confirm the importance of the interactions between FnHm diblocks and the water sub-phase in the formation of this robust organization of diblocks in nanodomains. The collapses of Langmuir monolayers of F8H16 and F10H14 were studied by BAM and the results were modeled using the nucleation–growth–collision theory.62

3.3. Diblocks with a Central Phenyl Linkage: Formation of Elongated Domains.

The impact of the introduction of a central phenyl linkage in the diblock’s molecular structure on the morphology of the surface domains has been investigated.55 Langmuir monolayers of 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene (C11F23CH2O(C6H2Br2)OC12H25) have been examined directly at the air/water interface using neutron reflectivity and, after transfer onto silicon wafers, by SFM coupled KPFM. Introduction of the phenyl ring changes substantially the shape of the domains. C11F23CH2O(C6H2Br2)OC12H25 was found to produce linear rows of branched elongated surface domains with a width of ~10 nm and an average length of ~400 nm, that would result from antiparallel molecular packing (Figure 8). This illustrates how the molecular structure of the diblocks can be used to tailor their self-assembly, and possibly their hierarchical organization. The neutron reflectivity results suggested that the most probable structural model consists of molecules with mixed/alternating orientations (Figure 8d).

The Langmuir films of 1-perfluorododecyl-4-(dodecyl)benzene (C12F25C6H4C12H25) and 1-perfluorododecyloxy-4-(dodecyloxy)benzene (C12F25OC6H4OC12H25) were heterogeneous, with some local organization in the form of dense packing,63 and showed no evidence of long-range order.

Films of mesoscopic domains self-assembled from FnHm diblocks (n = 8; m = 14, 16, 18, 20) and (n = 8, 10, 12; m = 16) spread at the air/water interface were found to display highly elastic behavior.64 The interfacial viscoelasticity of domain-patterned FnHm Langmuir monolayers was investigated by applying periodic shear stresses. The frequency-dependent dynamic surface elastic (storage) modulus G′ and viscous (loss) modulus G′′ were measured simultaneously at different surface pressures without disrupting the mesoscopic FnHm domains. Remarkably, physical two-dimensional gels are generated, even at zero surface pressure.64 This behavior is unprecedented for surfactants, for which gelation is usually only observed at high surface pressures. The fact that such gels were not observed for F12H1263,65 could be due to the presence of the elongated surface domains that are seen in addition to the circular ones. Systematic variation of the Hm (n = 8; m = 14, 16, 18, 20) and Fn (n = 8, 10, 12; m = 16) block lengths demonstrates that small changes in block length ratio can help adjusting the gel mechanics, with an increase in storage moduli by over one order of magnitude (Figure 9). These findings open new perspectives for the fabrication of 2D gels with tunable viscoelasticity via self-assembly of low molecular weight molecules into rigid mesoscale domains.

The viscoelasticity of monolayers of F12H12, F12H20 and C11F23CH2O(C6H2Br2)OC12H25 has been investigated using an interface stress rheometer based on a gliding magnetic needle under oscillating magnetic fields (Figure 10).63,65 Films of F12H12, which forms nearly circular nanodomains on water, exhibit frequency-independent surface moduli at all pressures and rheological properties reminiscent of those of colloidal glasses. By contrast, the response of F12H20 films depends markedly on surface pressure. At low π (under 4 mN m−1) the rheological response was qualitatively similar to that of F12H12. At the highest π (10 mN m−1), the storage and loss moduli at high frequencies exceed those of F12H12 by almost an order of magnitude. Such rheological behavior is typical of a gel-like response. This notable change in viscoelastic behavior between F12H12 and F12H20 was assigned to a change in morphology from circular surface domains for the former to elongated and interlocked domains for the latter, a behavior that is reminiscent of that of interconnected structures. C11F23CH2O(C6H2Br2)OC12H25, which forms long, interconnected domains exhibits gel-like rheological properties, that were assigned to classical ππ–driven entanglement of these elongated domains. C12F25OC6H4OC12H25 forms an amorphous viscoelastic liquid interfacial film.63 These findings show that changes in molecular structures can cause large differences in the linear viscoelastic response to shear stress of Langmuir films of diblocks, and hence, offer the possibility to molecularly tune the rheology of fluid interfaces.63

F12H18 had been shown to form Gibbs films on liquid alkanes that exhibit a sharp transition from a dilute state at the higher temperatures to a dense state at lower temperatures.66 It was later discovered that F12H18 also forms Gibbs films on the surface of cyanobiphenyl liquid crystals (4′-n-octyl-4-cyanobiphenyl, 8CB, and 4′-n-dodecyl-4-cyanobiphenyl, 12CB) and exhibits a similar transition at a temperature that can be tuned by adjusting the concentration of the diblock initially added to the liquid crystal bulk phase.67 This transition modifies the anchoring of the liquid crystal from homeotropic at higher temperatures to planar at lower temperatures. When the transition temperature of the Gibbs film is tuned so that it occurs above the nematic–isotropic or smectic–isotropic transition of the liquid crystals, addition of the diblock significantly modifies the nematic or smectic surface order.

A further study revealed that the Gibbs films of F12H18 self-organize in hexagonal arrays of surface domains on the surface of thermotropic smectic liquid crystals (Figure 11).68 The shape and lateral dimensions of these domains are similar to those formed on aqueous and solid surfaces. It should be noted that their long-range order is exceptional, and reminiscent of the organization of F8H16 spread on water24 or of F8H18 spin-coated on a silicon wafer.69 F8H16 nanodomains were also found to coexist on water next to a dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayer at low surface pressures.70,71 When pressure is increased, the nanodomains are ejected on the top of the DPPC monolayer.

The symmetrical triblock F6H6F6 was found to form stable films at the surface of water.72 Its surface pressure/area isotherm shows a monotonous pressure rise, followed by a pseudo-plateau region. BAM and AFM proved the presence of circular micron-size domains in the pseudo-plateau region. The calculated domain thickness (20–21 Å) agrees with the experimental value estimated from AFM (20.3 ± 1.4 Å).

Fluorophilic/lipophilic tetrablock amphiphiles di(FnHm) 1 (Chart 1) with n = 8, 10; m = 6, 12, 14, 16, 18, 20, were synthesized73 and investigated on the surface of water and on solid substrates using compression isotherms, Brewster angle microscopy, and atomic force microscopy.74 At low pressures, the tetrablocks form monolayers of closely packed surface domains. Further compression initiates a 2D/3D transition. At the end of the transition plateau, about half of the material deposited is expelled and forms a second monolayer on top of the initially formed one. The second, upper monolayer also consists of closely packed surface domains, thus providing the first example of compression-driven stacking of self-assembled nano-objects. Spontaneous formation of stacked layers of surface domains are also seen when the tetrablocks are spin-coated on silicon or mica (Figure 12).

Dynamic frequency sweep measurements on Langmuir monolayers of bis(C11F23CH2O(C6H2Br2)OC12H25) 2 and of the fluorene-centered tetrablock 3 (Chart 2) were performed at various surface pressures (Figure 13).63

For the fluorene-centered tetrablock, a solid-like behavior was observed (G′ > G′′, nearly independent of frequency) at all surface pressures. The dynamic moduli are, respectively, two and one orders of magnitude larger than those of F12H12 and bis(C11F23CH2O(C6H2Br2)OC12H25) measured at the same surface pressures. Different G′ vs π dependences were observed for the three compounds F12H12, bis(C11F23CH2O(C6H2Br2)OC12H25) and fluorene-centered tetrablock, with power-law dependences with exponents of 0.75, 2, and 0.42, respectively. Further dynamic strain amplitude sweep tests suggest that the interfacial films of the latter tetrablock exhibit a brittle-like viscoelastic response similar to that of the layers of bis(C11F23CH2O(C6H2Br2)OC12H25).

Spherulites are polycrystalline aggregates with a quasi-spherical outer boundary. They have been observed to form from a large variety of compounds, including organic compounds and minerals.75 Two-dimensional spherulites have been obtained in films of such materials cast on solid substrates. Lately, sustained interest has focused on non-birefringent ring-banded spherulites.76 The latter have, however, only been observed for a few polymers and seldom for small molecules. Unpredictably, the F10H16 diblock was recently found to form non-birefringent 2D spherulites when deposited as thin films on solid surfaces such as glass or silicon wafers (Figure 14).77

This is the first example of spherulite formed from a simple FnHm diblock. Remarkably, depending on experimental conditions, we could obtain radial-only, or ring-banded-only spherulites at will (Figure 14). Both radial and ring-banded topographies could also be obtained concurrently within a same spherulite. Control of spherulite morphology was achieved by adjusting such key parameters that govern crystallization as melting and cooling temperatures and rates. Two crystallization events were actually observed that comprise first an initial outwards crystallization step that starts from a nucleus and spreads radially, producing fibrous spherulites, followed by a rhythmical precipitation step that generates unusually high concentric rings, or ridges (width ~22 µm, height ~2–3 µm, spacing ~25 µm). Improving control over the morphology of 2D spherulites and understanding the mechanisms of their formation, which can involve a twisting of the crystal or a diffusion-controlled rhythmic crystallization process, both of which being able to act in interplay, are required in order to customize new materials. In depth AFM analysis specified that the ring bands consist of lamellae (thickness ~5.5 nm; width ~300 nm) regularly stacked in plates. Off-specular neutron scattering experiments disclosed a third and a fourth Bragg sheets besides the first Bragg sheet, confirming that the diblocks arrange in stacked lamellae (repeating distance ~5.7 nm). The lamellae consist of bilayers of interdigitated or tilted diblock molecules.

FnHm diblocks have found diverse applications and have potential for development, mostly in the biomedical and materials sciences. Medical uses were found in ophthalmology (e.g. tamponades for retina detachment and vitreous substitutes).7880 Eye drops of F6H8 are commercialized for the treatment of dry eye (Meibomian gland disease) patients.81 F4H5 was identified as a fitting carrier for cyclosporine A, a calcineurin inhibitor that increases tear secretion, goblet cell density and visual acuity, and decreases epithelial damage.82 FnHm diblocks are highly effective for stabilizing fluorocarbon emulsions destined for intravascular oxygen delivery,17,83 diagnosis,84 and biomedical research.85,86 In combination with normobaric oxygenation, an emulsion of F6H8 improved oxygen supply, decreasing tissue hypoxia and tissue damage following transient focal cerebral ischemia.87 F4H5 and F6H8 provided suitable excipients in aerosolized formulations for pulmonary delivery of ibuprofen.88 FnHm diblocks were investigated as drug carriers in acute respiratory failure.89 As a reminder, FnHm diblocks proved effective for stabilizing and reducing permeability of liposomes and more generally to control phospholipid self-assembly and film properties.18,22,90 and in various types of emulsions involving a fluorocarbon phase.12,91

The possibility of decorating surfaces with regular arrays of stable highly hydrophobic nanometric surface domains of predetermined size using simple small “nonpolar”, easily removable FnHm diblock molecules could provide a valuable approach to organic 2D templates of tunable periodicity for controlled elaboration of arrays of metal or polymer nanoparticles. The development of nanoelectronics and “smart” materials relies on suitably sized functional building blocks. By filling the ten-to-hundred nanometer gap between supramolecules and bio(macro)molecules, and patterns achieved by cutting-edge lithographic procedures, nanoparticles offer a route to a range of electronic and sensor components. Control of nanoparticle organization on surfaces is key to the development of such advanced devices because it rules their collective properties.92 Thus, for example, self-assembly of FnHm diblocks allowed patterning of square-centimeter-large surfaces with silver nanoparticles arranged in a high-density hexagonal network.93 Catalyzed oxidation of CO into CO2 could be achieved with this system at low temperature. Nanodomains of (F-alkyl)acids have provided templates for producing patterned thin films of SiO2.94 Surfaces nanopatterned with highly hydrophobic, low adhesion material could modify cell behavior (e.g., allow cell adhesion control). Porous and mesoporous silicon carbide materials were obtained using a soft templating method in which molecular silicon-based precursors are templated on a solid network of semi-fluorinated alkanes.95

Control of the anchoring of liquid crystals by a nanopatterned F12H18 surface film could be of interest for designing electrically tunable optical microresonators.67,68 The latter consist of nematic liquid crystal droplets and represent a new concept for the manipulation of light using soft matter with extended possible applications in voltage-tunable optical microdevices, surface-sensitive sensors, tunable microcavity lasers and soft matter photonic circuits.96 It is envisioned that the dilute/condensed phase transition occurring in the FnHm diblock film could trigger the switching of the director field of nematic liquid crystal droplets covered by a film of diblocks, thus controlling the optical properties of the droplets.

An emerging concept in science and technology, “reconfigurable” soft matter aims at enabling the switch of the chemical, physical and functional properties of materials between well-defined states by external stimulation.97 Azobenzene derivatives reversibly undergo cistrans isomerization upon irradiation; the bent cis isomer is formed by exposure to short-wavelength UV and reconfiguration into the trans isomer is triggered by visible light. A recent study investigated the behavior under light of a Langmuir monolayer of a semifluorinated alkane possessing a central azobenzene linkage.98 It revealed that irradiation, by inducing cis-trans isomerization of the diblocks, significantly influences the interfacial organization and viscoelastic properties of the semifluorinated alkylazobenzene, whereas it has a much weaker effect on the hydrocarbon counterpart. This suggests the possibility of photoswitching the mechanical properties of Langmuir monolayers of these diblocks. Another study demonstrated the control of polymerization of 10,12-pentacosadiynoic acid in mixed monolayers of the dyinoic fatty acid and F8H16 by exploiting the vertical separation induced by the diblock.99

Finally, several studies have shown that FnHm diblocks and related compounds could also be useful for designing solid materials having a controlled compartmented internal structure.100102

Recent investigations have uncovered an amazing diversity of self-assembling and hierarchical organization behavior for the structurally so rudimentary fluorocarbon-hydrocarbon diblocks. This behavior highlights the irrepressible gregarious instinct of perfluoroalkylated chains. Numerous studies involving a variety of diblocks, a free water interface and solid supports (including liquid crystals), various spreading techniques and experimental conditions have determined that surface domain formation and ordering, even in the absence of lateral pressure, are intrinsic properties of semifluorinated alkanes. These molecular surface domains do not coalesce when compressed and are sturdy enough to survive beyond monolayer collapse. The rheology of monolayers of FnHm surface domains is also noteworthy: they are predominantly elastic at all pressures, which is unprecedented for surfactant films, and can form physical two-dimensional gels, even at zero surface pressure. Control of the rheological and other properties of the air/water interface by adjunction of diblocks could play a role in the differentiation of cells, as found for myoblasts cultured at the fluorocarbon/water interface.103 Modification of the diblocks’ and multiblocks’ structure allows adjustments of surface domain morphology and behavior. These studies illustrate magnificently the value of these unique chemicals as tools for inducing organization in 2D colloidal systems and for controlling their properties, thus opening a broad field of applications such as in nanoelectronics, photonics and sensing technologies.

The recently discovered diblock-based polycrystalline 2D spherulites, and the possibility of orienting their crystallization toward radial-only or ring-banded-only morphologies certainly warrant further investigation. FnHm diblocks may help develop applications for spherulites. It is surprising indeed that after a century of investigation very few applications have been suggested for these spectacular crystalline self-assemblies.

We thank the French Research Agency (ANR-14-CE35-0028-01; PhD grant for X-H. Liu), NanoTransMed, a project co-funded by the European Regional Development Fund in the framework of the INTERREG V Upper Rhine program, and GIS Fluor for a travel grant.

Xianhe Liu

Xianhe Liu was born in Yangzhou, China in 1991. She earned an Engineer diploma in chemistry and processes at the National Institute of Applied Sciences (INSA) of Rouen and a Master degree of materials science at the University of Rouen, France. Since 2015, she has been pursuing Ph.D. studies at the Institut Charles Sadron (University of Strasbourg, France), focusing on fluorinated compounds and their self-assembly at interfaces under the supervision of Dr. Marie Pierre Krafft.

Jean G. Riess

Trained as a chemical engineer, Jean G. Riess got his PhD from the University of Strasbourg, France, with Professor Guy Ourisson, post-doctored with Prof. John Van Wazer, became a Professor at the University of Nice, and founded the Unité de Chimie Moléculaire (CNRS). He later joined a Californian start-up as a VP of Research & Development, and held a Research Associate position at the University of California, San Diego. He retired in France as a consultant. His research involved phosphorus chemistry, organometallics, transition metals, and eventually perfluorochemicals (fluorocarbons, fluorinated amphiphiles, their colloidal chemistry, self-assemblies and biomedical uses, including “blood substitutes”, contrast agents, drug delivery systems, and environmental issues). Riess has published ~380 papers, filed ~25 patents. He also worked with the Stockholm Peace Research Institute and the Pugwash Conferences on chemical weapon control. He won awards from the Académie des Sciences, French Chemical Society, Alexander von Humboldt Stifftung, City of Nice, as well as Alliance’s first Distinguished Contribution Award.

Marie Pierre Krafft

Marie Pierre Krafft’s current research centers on the design, engineering and investigation of nano-compartmented, fluorocarbon-promoted molecular assemblies, fluorocarbon-based therapeutics, and active soft matter. She has published ~130 papers, holds 10 patents, has given ~75 invited lectures in International Meetings, is Editor-in-Chief of Curr. Opin. Colloid Interface Sci., received Awards from the Chemical Society of Japan and French Académie des Sciences, is a member of the International Comity for the Henri Moissan Prize, of the European Academy of Sciences and of the Légion d’honneur. She was an Invited Professor at Doshisha University (Kyoto, Japan).