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1.
J Microsc ; 265(2): 196-206, 2017 02.
Artigo em Inglês | MEDLINE | ID: mdl-27716931

RESUMO

In recent years, many researchers have investigated bitumen surface morphology, especially the so-called bee-like structures, in an attempt to relate the chemical composition and molecular conformation to bitumen micromechanics and ultimately performance properties. Even though recent studies related surface morphology and its evolution to stiffness and stress localization, the complex chemical nature of bitumen and its time- and temperature-dependent properties still engender significant questions about the nature and origin of the observed morphological features and how they evolve due to exposure to various environmental and loading conditions. One such question is whether the observed surface features are formed from wax or from the coprecipitation of wax and asphaltene. Our prior work was mainly theoretical; it used density functional theory and showed that the coprecipitation theory may not stand, mainly because wax-asphaltene interactions are not thermodynamically favourable compared to wax-wax interactions. This paper presents a comprehensive approach based on experiments to study surface morphology of bitumen and conduct compositional mapping to shed light on the origin of the bee-like surface morphological features. We used Atomic Force Microscopy (AFM), with the main focus being on single-pass detection and mapping of local electric properties, as a novel approach to enhance existing compositional mapping techniques. This method was found to be highly effective in differentiating various domains with respect to their polarity. The results of our study favour the hypothesis that the bee-like features are mainly composed of wax, including a variety of alkanes.

2.
Biomacromolecules ; 12(2): 370-9, 2011 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-21192636

RESUMO

We report an atomic force microscopy (AFM) study of fibrinogen molecules and fibrin fibers with resolution previously achieved only in few electron microscopy images. Not only are all objects triads, but the peripheral D regions are resolved into the two subdomains, apparently corresponding to the ßC and γC domains. The conformational analysis of a large population of fibrinogen molecules on mica revealed the two most energetically favorable conformations characterized by bending angles of ∼100 and 160 degrees. Computer modeling of the experimental images of fibrinogen molecules showed that the AFM patterns are in good agreement with the molecular dimensions and shapes detected by other methods. Imaging in different environments supports the expected hydration of the fibrinogen molecules in buffer, whereas imaging in humid air suggests the 2D spreading of fibrinogen on mica induced by an adsorbed water layer. Visualization of intact hydrated fibrin fibers showed cross-striations with an axial period of 24.0 ± 1.6 nm, in agreement with a pattern detected earlier with electron microscopy and small-angle X-ray diffraction. However, this order is clearly detected on the surface of thin fibers and becomes less discernible with the fiber's growth. This structural change is consistent with the proposal that thinner fibers are denser than thicker ones, that is, that the molecule packing decreases with the increasing of the fibers' diameter.


Assuntos
Fibrina/química , Fibrinogênio/química , Microscopia de Força Atômica , Tamanho da Partícula , Propriedades de Superfície
3.
Microsc Microanal ; 17(4): 587-97, 2011 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-21771386

RESUMO

We demonstrate that single-pass Kelvin force microscopy (KFM) and dC/dz measurements in different environments expand the compositional imaging with atomic force microscopy. The KFM and dC/dz studies were performed in the intermittent contact mode with force gradient detection of tip-sample electrostatic interactions. Both factors contribute to sensitive measurements of the surface potential and capacitance gradient with nanometer-scale spatial resolution as it was verified on a broad range of materials: metal alloys, polymers, organic layers, and liquid-like objects. For many samples the surface potential and dC/dz variations complement each other in identification of individual components of heterogeneous materials. In situ imaging in different humidity or vapors of various organic solvents further facilitate recognition of the constituents of multicomponent polymer samples due to selective swelling of components.

4.
Biochemistry ; 49(1): 68-77, 2010 Jan 12.
Artigo em Inglês | MEDLINE | ID: mdl-19929007

RESUMO

The physical properties of substrates are known to control cell adhesion via integrin-mediated signaling. Fibrin and fibrinogen, the principal components of hemostatic and pathological thrombi, may represent biologically relevant substrates whose variable physical properties control adhesion of leukocytes and platelets. In our previous work, we have shown that binding of fibrinogen to the surface of fibrin clot prevents cell adhesion by creating an antiadhesive fibrinogen layer. Furthermore, fibrinogen immobilized on various surfaces at high density supports weak cell adhesion whereas at low density it is highly adhesive. To explore the mechanism underlying differential cell adhesion, we examined the structural and physical properties of surfaces prepared by deposition of various concentrations of fibrinogen using atomic force microscopy and force spectroscopy. Fibrinogen deposition at high density resulted in an aggregated multilayered material characterized by low adhesion forces. In contrast, immobilization of fibrinogen at low density produced a single layer in which molecules were directly attached to the solid surface, resulting in higher adhesion forces. Consistent with their distinct physical properties, low- but not high-density fibrinogen induced strong alpha(IIb)beta(3)-mediated outside-in signaling in platelets, resulting in their spreading. Moreover, while intact fibrin gels induced strong signaling in platelets, deposition of fibrinogen on the surface of fibrin resulted in diminished cell signaling. The data suggest that deposition of a multilayered fibrinogen matrix prevents stable cell adhesion by modifying the physical properties of surfaces, which results in reduced force generation and insufficient signaling. The mechanism whereby circulating fibrinogen alters adhesive properties of fibrin clots may have important implications for control of thrombus formation and thrombogenicity of biomaterials.


Assuntos
Fibrina/química , Fibrinogênio/química , Adesividade Plaquetária/fisiologia , Complexo Glicoproteico GPIIb-IIIa de Plaquetas/metabolismo , Transdução de Sinais , Sítios de Ligação , Plaquetas/metabolismo , Plaquetas/ultraestrutura , Adesão Celular , Citoplasma , Fibrina/metabolismo , Fibrinogênio/metabolismo , Humanos , Microscopia de Força Atômica , Microscopia Eletrônica de Varredura , Propriedades de Superfície
5.
Langmuir ; 26(22): 17269-77, 2010 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-20883009

RESUMO

The deposition of a multilayered fibrinogen matrix on various surfaces results in a dramatic reduction of integrin-mediated cell adhesion and outside-in signaling in platelets and leukocytes. The conversion of a highly adhesive, low-density fibrinogen substrate to the nonadhesive high-density fibrinogen matrix occurs within a very narrow range of fibrinogen coating concentrations. The molecular events responsible for this transition are not well understood. Herein, single-cell and molecular force spectroscopy were used to determine the early steps in the formation of nonadhesive fibrinogen substrates. We show that the adsorption of fibrinogen in the form of a molecular bilayer coincides with a several-fold reduction in the adhesion forces generated between the AFM tip and the substrate as well as between a cell and the substrate. The subsequent deposition of new layers at higher coating concentrations of fibrinogen results in a small additional decrease in adhesion forces. The poorly adhesive fibrinogen bilayer is more extensible under an applied tensile force than is the surface-bound fibrinogen monolayer. Following chemical cross-linking, the stabilized bilayer displays the mechanical and adhesive properties characteristic of a more adhesive fibrinogen monolayer. We propose that a greater compliance of the bi- and multilayer fibrinogen matrices has its origin in the interaction between the molecules forming the adjacent layers. Understanding the mechanical properties of nonadhesive fibrinogen matrices should be of importance in the therapeutic control of pathological thrombosis and in biomaterials science.


Assuntos
Fibrinogênio/química , Fibrinogênio/metabolismo , Microscopia de Força Atômica , Adesivos/química , Adesivos/metabolismo , Células HEK293 , Humanos
6.
Nature ; 430(7001): 764-8, 2004 Aug 12.
Artigo em Inglês | MEDLINE | ID: mdl-15306805

RESUMO

Natural pore-forming proteins act as viral helical coats and transmembrane channels, exhibit antibacterial activity and are used in synthetic systems, such as for reversible encapsulation or stochastic sensing. These diverse functions are intimately linked to protein structure. The close link between protein structure and protein function makes the design of synthetic mimics a formidable challenge, given that structure formation needs to be carefully controlled on all hierarchy levels, in solution and in the bulk. In fact, with few exceptions, synthetic pore structures capable of assembling into periodically ordered assemblies that are stable in solution and in the solid state have not yet been realized. In the case of dendrimers, covalent and non-covalent coating and assembly of a range of different structures has only yielded closed columns. Here we describe a library of amphiphilic dendritic dipeptides that self-assemble in solution and in bulk through a complex recognition process into helical pores. We find that the molecular recognition and self-assembly process is sufficiently robust to tolerate a range of modifications to the amphiphile structure, while preliminary proton transport measurements establish that the pores are functional. We expect that this class of self-assembling dendrimers will allow the design of a variety of biologically inspired systems with functional properties arising from their porous structure.


Assuntos
Biopolímeros/química , Biopolímeros/metabolismo , Dipeptídeos/química , Dipeptídeos/metabolismo , Transporte Biológico , Varredura Diferencial de Calorimetria , Dicroísmo Circular , Ligação de Hidrogênio , Espectroscopia de Ressonância Magnética , Microscopia Eletrônica , Modelos Moleculares , Porosidade , Estrutura Quaternária de Proteína , Prótons , Estereoisomerismo
7.
J Nanosci Nanotechnol ; 10(11): 7060-4, 2010 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-21137865

RESUMO

High-resolution studies of self-assemblies of semifluorinated alkanes molecules F12H8 and F14H20 [FnHm = CF3(CF2)n(CH2)mCH3], and CdTe particles were performed with single-pass Kelvin force microscopy. Surface potential contrast, which is related to the strength and orientation of molecular dipoles, empowers the characterization of self-organized structures. Lamellar structures, ribbons and toroids of F14H20 and F12H8 were observed on graphite and the differences of surface potential were interpreted in terms orientation of -CH2-CF2- dipoles. A gradual sublimation of F12H8 molecules allowed a visualization of top and bottom molecular layers on the substrate. Prior to the sublimation a part of lamellae of the bottom layers was transformed into the ribbons. The surface potential data suggest that this transition proceeds with the reorientation of the molecular chains from the horizontal to vertical direction. Self-assembly of CdTe nanoparticles into nanowires was monitored upon drying on mica. The process is accompanied by drastic changes of surface potential. The formed nanowires exhibit strong positive surface potential that assumes a structural transition with a formation of strong dipole moment in these self-assemblies.

8.
Methods Mol Biol ; 1814: 99-127, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-29956229

RESUMO

Capabilities of atomic force microscopy (AFM) for characterization of local electrical properties of materials are presented in this chapter. At the beginning the probe-sample force interactions, which are employed for detection of surface topography and materials properties, are described theoretically in their application in different AFM modes and electrical techniques. The electrical techniques, which are based on detection of electrostatic probe-sample forces, are outlined in AFM contact and oscillatory resonant modes. The basic features of the detection of surface potential and capacitance gradients are explained. The applications of these techniques are illustrated on metals, surfactant compounds, semiconductors, and different polymers. Practical recommendations on use of the AFM-based electrical methods and the related challenges are given in the last section.


Assuntos
Microscopia de Força Atômica/métodos , Polímeros/química , Eletricidade Estática
9.
Sci Adv ; 4(2): e1700682, 2018 02.
Artigo em Inglês | MEDLINE | ID: mdl-29487900

RESUMO

The symmetry of metallic nanocolloids, typically envisaged as simple geometrical shapes, is rarely questioned. However, the symmetry considerations are so essential for understanding their electronic structure, optical properties, and biological effects that it is important to reexamine these foundational assumptions for nanocolloids. Gold nanorods (AuNRs) are generally presumed to have nearly perfect geometry of a cylinder and therefore are centrosymmetric. We show that AuNRs, in fact, have a built-in electrostatic potential gradient on their surface and behave as noncentrosymmetric particles. The electrostatic potential gradient of 0.11 to 0.07 V/nm along the long axes of nanorods is observed by off-axis electron holography. Kelvin probe microscopy, secondary electron imaging, energy-filtered transmission electron microscopy, and plasmon mapping reveal that the axial asymmetry is associated with a consistently unequal number of cetyltrimethylammonium bromide moieties capping the two ends of the AuNRs. Electrostatic field maps simulated for the AuNR surface reproduce the holography images. The dipole-like surface potential gradient explains previously puzzling discrepancies in nonlinear optical effects originating from the noncentrosymmetric nature of AuNRs. Similar considerations of symmetry breaking are applicable to other nanoscale structures for which the property-governing symmetry of the organic shell may differ from the apparent symmetry of inorganic core observed in standard electron microscopy images.

10.
Science ; 359(6383): 1509-1513, 2018 03 30.
Artigo em Inglês | MEDLINE | ID: mdl-29599240

RESUMO

Active camouflage is widely recognized as a soft-tissue feature, and yet the ability to integrate adaptive coloration and tissuelike mechanical properties into synthetic materials remains elusive. We provide a solution to this problem by uniting these functions in moldable elastomers through the self-assembly of linear-bottlebrush-linear triblock copolymers. Microphase separation of the architecturally distinct blocks results in physically cross-linked networks that display vibrant color, extreme softness, and intense strain stiffening on par with that of skin tissue. Each of these functional properties is regulated by the structure of one macromolecule, without the need for chemical cross-linking or additives. These materials remain stable under conditions characteristic of internal bodily environments and under ambient conditions, neither swelling in bodily fluids nor drying when exposed to air.

11.
Nanotechnology ; 18(4): 44032, 2007 May 09.
Artigo em Inglês | MEDLINE | ID: mdl-20721301

RESUMO

Established techniques for global gene expression profiling, such as microarrays, face fundamental sensitivity constraints. Due to greatly increasing interest in examining minute samples from micro-dissected tissues, including single cells, unorthodox approaches, including molecular nanotechnologies, are being explored in this application. Here, we examine the use of single molecule, ordered restriction mapping, combined with AFM, to measure gene transcription levels from very low abundance samples. We frame the problem mathematically, using coding theory, and present an analysis of the critical error sources that may serve as a guide to designing future studies. We follow with experiments detailing the construction of high density, single molecule, ordered restriction maps from plasmids and from cDNA molecules, using two different enzymes, a result not previously reported. We discuss these results in the context of our calculations.

12.
Beilstein J Nanotechnol ; 2: 15-27, 2011.
Artigo em Inglês | MEDLINE | ID: mdl-21977411

RESUMO

We demonstrate that single-pass Kelvin force microscopy (KFM) and capacitance gradient (dC/dZ) measurements with force gradient detection of tip-sample electrostatic interactions can be performed in the intermittent contact regime in different environments. Such combination provides sensitive detection of the surface potential and capacitance gradient with nanometer-scale spatial resolution as it was verified on self-assemblies of fluoroalkanes and a metal alloy. The KFM and dC/dZ applications to several heterogeneous polymer materials demonstrate the compositional mapping of these samples in dry and humid air as well as in organic vapors. In situ imaging in different environments facilitates recognition of the constituents of multi-component polymer systems due to selective swelling of components.

13.
Langmuir ; 25(2): 1091-6, 2009 Jan 20.
Artigo em Inglês | MEDLINE | ID: mdl-19177652

RESUMO

We describe the self-assembly of multilayer hexagonal DNA arrays through highly regular interlayer packing. Slow cooling of a mixture of three single-stranded DNA sequences with various Mg2+ concentrations leads to the self-assembly of diverse multilayer architectures. The self-assembled aggregates were deposited onto mica surfaces and examined with atomic force microscopy. The size of the two-dimensional arrays and subsequent stacking to form multilayer structures are highly dependent on Mg2+ concentration. DNA bilayers and multilayers of defined shape are favored in 2-5 mM Mg2+ with an average lateral size of 700 nm. Arrays are much larger (up to 20 microm across) in 10-15 mM Mg2+, although multiple layers still make up 20-60% of the observed structures. Domains within single layer architectures were identified using Moiré pattern analysis. Distinct structural phases within the multilayer assemblies include two layers translated by 17.5 nm and interlayer rotations of 20 degrees and 30 degrees. Three layer assemblies have cubic close packing and taller multilayer architectures of 2D DNA sheets were also identified.


Assuntos
DNA/química , Análise de Sequência com Séries de Oligonucleotídeos/métodos , Silicatos de Alumínio/química , Bicamadas Lipídicas/química , Magnésio/química , Microscopia de Força Atômica , Tamanho da Partícula , Propriedades de Superfície
14.
Nat Nanotechnol ; 2(8): 507-14, 2007 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-18654349

RESUMO

Tapping-mode atomic force microscopy (AFM), in which the vibrating tip periodically approaches, interacts and retracts from the sample surface, is the most common AFM imaging method. The tip experiences attractive and repulsive forces that depend on the chemical and mechanical properties of the sample, yet conventional AFM tips are limited in their ability to resolve these time-varying forces. We have created a specially designed cantilever tip that allows these interaction forces to be measured with good (sub-microsecond) temporal resolution and material properties to be determined and mapped in detail with nanoscale spatial resolution. Mechanical measurements based on these force waveforms are provided at a rate of 4 kHz. The forces and contact areas encountered in these measurements are orders of magnitude smaller than conventional indentation and AFM-based indentation techniques that typically provide data rates around 1 Hz. We use this tool to quantify and map nanomechanical changes in a binary polymer blend in the vicinity of its glass transition.


Assuntos
Testes de Dureza/instrumentação , Teste de Materiais/instrumentação , Microscopia de Força Atômica/instrumentação , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nanotecnologia/instrumentação , Transdutores , Desenho de Equipamento , Análise de Falha de Equipamento , Dureza , Testes de Dureza/métodos , Teste de Materiais/métodos , Microscopia de Força Atômica/métodos , Nanotecnologia/métodos , Estresse Mecânico
15.
Langmuir ; 23(10): 5459-65, 2007 May 08.
Artigo em Inglês | MEDLINE | ID: mdl-17408295

RESUMO

Practical protocols are presented to reproducibly prepare micrometer-sized Au(111) substrates. Au(111) terraces of micrometer dimensions and atomic smoothness were crystallized by flame-annealing vacuum-deposited gold films on glass and on millimetric amorphous gold shots. Gold films and shots that were slowly cooled in a moderately applied stream of nitrogen gas exhibited large and stable crystal surfaces with Au(111) morphologies. Similarly, flame-annealed gold samples cooled with another protocol--in much rougher streams of nitrogen gas--produced morphologically unstable and highly mobile Au(111) layers. Within the first hour after preparation, however, rapid microscale restructuring in the layers produced complex morphologies of hexagonal channel networks and islands that were predominantly triangular. These channeled gold layers fused slowly in the following hours, with velocities of 0.01-0.2 A/s, as quantified by digital image correlation (DIC). Atomically smooth, stable, and predominantly triangular Au(111) terraces on the scale of micrometers were observed approximately 24 h after the sample preparations.


Assuntos
Ouro/química , Microscopia de Varredura por Sonda , Temperatura Alta , Propriedades de Superfície , Fatores de Tempo
16.
J Phys Condens Matter ; 18(14): S145-S159, 2006 Apr 12.
Artigo em Inglês | MEDLINE | ID: mdl-19337583

RESUMO

Using λ-Int recombination in E. coli, we have generated milligram quantities of supercoiled minicircle DNA. Intramolecular Int recombination was efficient down to lengths ~254 bp. When nicked and religated in the presence of ethidium bromide, 339 bp minicircles adopted at least seven unique topoisomers that presumably correspond to ΔLk ranging from 0 to -6, which we purified individually. We used these minicircles, with unique ΔLk, to address the partition into twist and writhe as a function of ΔLk. Gel electrophoresis and atomic force microscopy revealed progressively higher writhe conformations in the presence of 10 mM CaCl(2) or MgCl(2). From simplistic calculations of the bending and twisting energies, we predict the elastic free energy of supercoiling for these minicircles to be lower than if the supercoiling was partitioned mainly into twist. The predicted writhe corresponds closely with that which we observed experimentally in the presence of divalent metal ions. However, in the absence of divalent metal ions only limited writhe was observed, demonstrating the importance of electrostatic effects on DNA structure, when the screening of charges on the DNA is weak. This study represents a unique insight into the supercoiling of minicircle DNA, with implications for DNA structure in general.

17.
J Am Chem Soc ; 128(33): 10707-15, 2006 Aug 23.
Artigo em Inglês | MEDLINE | ID: mdl-16910665

RESUMO

Acid-base switchable supramolecular dendronized polyacetylenes (DPAs) with increasing steric bulk on going from generation one [G1] to three [G3], were constructed using multiple self-assembly processes between Fréchet-type [G1]-[G3]-dendritic dialkylammonium salts and a dibenzo[24]crown-8-containing polymer. The formation of the supramolecular systems is acid-base switchable to either an ON (rodlike dendronized polymers) or an OFF (flexible polymers) state. Thus, by controlling the superstructures of the supramolecular polymers with the [G1]-[G3] dendrons, it is possible to induce conformational changes within the polymer backbones. The supramolecular dendronized polymers, as well as their threading-dethreading properties, were characterized by (1)H NMR and UV absorption spectroscopies, gel permeation chromatography (GPC) and light scattering (LS). Independent measures of molecular weight (GPC, LS) indicate that DPAs behave as increasingly rigid macromolecules with each generation in solution. Molecular dynamics simulations of each DPA suggest that the lengths of the polymer backbones increase accordingly. Atomic force microscopy of the [G3]-dendronized polystyrene (DPS), as well as the DPAs, reveal surface morphologies indicative of aggregated superstructures.

18.
Chemistry ; 12(22): 5731-46, 2006 Jul 24.
Artigo em Inglês | MEDLINE | ID: mdl-16703658

RESUMO

A library of eleven high cis-content cis-transoidal polyphenylacetylenes (PPAs) dendronized with self-assembling dendrons was prepared from a library of fifteen convergently synthesized macromonomers. Using [Rh(C triple bond CPh)(nbd)(PPh(3))(2)] (nbd=2,5-norbornadiene) in the presence of 10 equiv of N,N-dimethylaminopyridine, predictive control over molecular weight and narrow molecular weight distribution are obtained. The PPA backbone serves as a helical scaffold for the self-assembling dendrons. The dendron primary structure dictates the diameter of the cylindrical PPAs in bulk, both in the self-organized hexagonal columnar (Phi(h)) lattice determined by X-ray diffraction (XRD) and in monolayers on highly ordered pyrolytic graphite (HOPG) and mica visualized by atomic force microscopy (AFM). Thermal and bulk phase characteristics of the cylindrical PPAs reinforces the generality that flexible polymer backbones adopt a helical conformation within the cylindrical macromolecules generated by polymers jacketed with self-assembling dendrons.

19.
J Am Chem Soc ; 127(34): 11886-7, 2005 Aug 31.
Artigo em Inglês | MEDLINE | ID: mdl-16117496

RESUMO

Using atomic force microscopy, we describe the linear arrangement of cationic gold nanoparticles directed by programmable self-assembling RNA ladders and demonstrate that the regular spacing of nanoparticles is controlled by the RNA architecture acting as nanocrown scaffoldings. Thus, precise positioning of molecular components can be accomplished with RNA not only through electrostatic but also via size and shape recognitions.


Assuntos
Coronantes/química , Ouro/química , Nanoestruturas/química , Nanotecnologia , RNA/química , Cátions , Microscopia Eletrônica de Transmissão , Modelos Químicos , Tamanho da Partícula , Proteínas/química , Tiocolina/química
20.
Langmuir ; 21(6): 2308-16, 2005 Mar 15.
Artigo em Inglês | MEDLINE | ID: mdl-15752020

RESUMO

Scanning force microscopy on monomolecular films of eicosylperfluorotetradecane, F(CF(2))(14)(CH(2))(20)H, on mica, silicon oxide, or water revealed spontaneous organization to well-defined nanoscopic ribbon and spiral or toroidal superstructures. Whether ribbons or nanospirals were formed depended on the solvent from which the molecular monofilm was cast. Ribbons were observed when a hydrocarbon or a perfluorocarbon solvent was used, e.g., decalin or perfluorodecalin. When the compound, however, was deposited from nonselective hexafluoroxylene, the molecules assembled into spirals of defined size. The spirals/toroids transformed to ribbons when exposed either to decalin or perfluorodecalin vapor, and the ribbons transformed to toroids when exposed to hexafluoroxylene vapor. These changes could be observed in situ. Scanning force microscopy yielded an identical height and width for the bands forming the spirals and for the parallel flat ribbons. X-ray reflectivity yielded a height of 3.61 +/- 0.05 nm, again identical for both morphologies. Yet, the length of the extended F(CF(2))(14)(CH(2))(20)H molecule, i.e., 4.65 nm, exceeds the layer thickness obtained from X-ray reflectometry. It is, however, consistent with an arrangement where the fluorinated chains are oriented normal to the surface layer and where the alkyl segments are tilted with a 122 degrees angle between the two segments. Within the plane defined by the tilt, this angle allows a dense packing of the alkyl segments compensating for the larger cross-section of the fluorocarbon segment. The tilt plane defines an "easy" direction along which the monolayer structure can preserve order. In the plane perpendicular to this axis, long-range ordered dense packing of the alkyl chains is not possible. Incommensurable packing can in principle explain the finite and regular width of the ribbons and the stepwise turn in the spirals.

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