[Show abstract][Hide abstract] ABSTRACT: Semifluorinated alkyl-azobenzene derivatives (SFAB) can form stable Langmuir layers at the air-water interface. These systems combine the amphiphobic character of the semifluorinated alkyl units as structure-directing motif with a photochromic behavior based on the well-known, reversible cis-trans isomerization upon irradiation with UV and visible light. Here, we report on our investigations of the structural and dynamic tunability in these SFAB layers at the air-water interface in response to an external light stimulus. The monolayer structures and properties are studied for [4-(heptadecafluorooctyl)phenyl](4-octylphenyl)diazene (F8-azo-H8) and bis(4-octylphenyl)diazene (H8-azo-H8) by neutron reflectivity, surface pressure-area isotherms with compression-expansion cycles, and interfacial rheology. We find that UV irradiation reversibly influences the packing behavior of the azobenzene molecules, and interpret it as a transition from organized layer structures with the molecular main axis vertically oriented in the trans form to random packing of the cis isomer. Interestigly, this trans-cis isomerization leads to an increase of surface pressure, which is accompanied by a decrease of viscoelastic moduli. These results suggest ways for tailoring the properties of responsive fluid interfaces.
Physical Chemistry Chemical Physics 09/2015; DOI:10.1039/C5CP04242A · 4.49 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Planar polyelectrolyte brushes are prepared by Langmuir-Schaefer based grafting of perdeuterated (styrene)49-b-(acrylic acid)222 block copolymer (dPS-b-PAA) to dPS pre-coated silicon supports with grafting density σPAA from 0.07 to 0.11 nm-2. The structure of the solvent-swollen brushes, i. e. the volume fraction profile of polymer segments, φPAA, as a function of altitude z from the grafting plane into the liquid phase is extracted from neutron reflectivity measurements. We find that for all cases investigated φPAA(z) resembles a Gaussian profile. Although very condensed, the PAA brushes can be loaded with bovine serum albumin (BSA). The integral amount of adsorbed BSA scales linearly with grafting density. We compare our z-resolved volume fraction profile φBSA(z) of BSA on PAA brushes with existing literature on that system. It is found that a cross-over takes place in the adsorption scheme from ternary compressive, where proteins can approach the grafting surface only by compressing the brush, to ternary insertive, where proteins enter the brush with only local perturbation of the concentration profile, as a function of RP/Hmax, where RP is the Stokes-Radius of the protein, and Hmax is the experimentally determined maximum height of the brush.
Zeitschrift für Physikalische Chemie 01/2015; DOI:10.1515/zpch-2014-0540 · 1.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Adsorption of a natural glycoside surfactant Quillaja bark saponin ("QBS", Sigma Aldrich 84510) was studied at the air/water and Si/water interfaces using a combination of surface pressure (SP), surface dilatational rheology, neutron reflectivity (NR), Infra-Red Attenuated Total Reflection Spectroscopy (IR ATR) and Quartz Crystal Microbalance (QCM). The adsorbed layers formed at the air/water interface are predominantly elastic, with the dilatational surface storage modulus reaching the maximum value of E' =184 mN/m. The NR results point to a strong hydration of the adsorbed layers (about 65% hydration, corresponding to about 60 molecules of water per one QBS molecule), most likely related to the presence of multiple sugar groups constituting the glycone part of the QBS molecules. With a layer thickness of 19 angstrom, the adsorbed amount obtained from NR seems largely underestimated in comparison to the value obtained from the surface tension isotherm. While this high extent of hydration does not prevent formation of dense and highly elastic layers at the air-water surface, QBS adsorption at the Si/water interface is much weaker. The adsorption isotherm of QBS on Si obtained-from the QCM study reflects much lower affinity of highly hydrated and negatively charged saponin molecules to the Si/water interface. We postulate that at the air/water interface, QBS adsorbs through the triterpene aglycone moiety. In contrast, weak hydrogen bonding between the glycone part and the surface silanol groups of Si is responsible for QBS adsorption on more polar Si/water interface.
[Show abstract][Hide abstract] ABSTRACT: The interactions between a model phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a biosurfactant Quillaja Bark Saponin (QBS) obtained from the bark of Quillaja saponaria Molina were studied using simple models of biological membranes. QBS is known to interact strongly with the latter, exerting a number of haemolytic, cytotoxic and anti-microbial actions. The interaction of QBS dissolved in the subphase with DPPC monolayers and silicon-supported bilayers was studied above the cmc (10(-3)M). Surface pressure relaxation and surface dilatational rheology combined with quartz crystal microbalance (QCM) and neutron reflectivity (NR) were employed for this purpose. The DPPC-penetrating abilities of QBS are compared with those of typical synthetic surfactants (SDS, CTAB and Triton X-100). We show that the penetration studies using high surface activity (bio)surfactants should be performed by a subphase exchange, not by spreading onto the surfactant solution. In contrast to the synthetic surfactants of similar surface activity, QBS does not collapse DPPC mono- and bilayers, but penetrates them, improving their surface dilatational elastic properties even in the highly compressed solid state. The dilatational viscoelasticity modulus increases from 204 mN/m for pure DPPC up to 310 mN/m for the QBS-penetrated layers, while it drops to near zero values in the case of the synthetic surfactants. The estimated maximum insertion pressure of QBS into DPPC monolayers exceeds the maximum surface pressure achievable in our setup, in agreement with the surface rheological response of the penetrated layers.
[Show abstract][Hide abstract] ABSTRACT: The application of diamond-like carbon (DLC) coatings on the contacts of mechanical systems is becoming widespread thanks to their excellent tribological properties. Numerous studies of DLC coatings have been reported over the past decade and, as a result, the understanding of their lubrication has improved. The tribological properties of boundary-lubricated contacts are drastically affected by adsorbed layers; however, due to the variety of lubricant additives and coating structures, no general adsorption mechanisms for DLC coatings have been put forward until now. This has, unfortunately, hindered improvements in their lubrication performance. Many of the essential physical properties of the adsorbed layers also remain undefined. In this work, we used neutron reflectometry to determine the thickness and the density of the adsorbed layers of fatty acid molecules on coatings of a-C, a-C:H, a-C:H:F and a-C:H:Si. The results showed that a 0.9-nm-thick layer adsorbed onto the a-C and a-C:H coatings. In contrast, both doped coatings, i.e. the a-C:H:F and a-C:H:Si, showed a poorer adsorption ability towards the fatty acid molecules than the a-C and a-C:H. Continuous adsorption layers were not detected on the a-C:H:F and a-C:H:Si; however, the possibility of adsorption in lower quantities cannot be ruled out.
[Show abstract][Hide abstract] ABSTRACT: Diamond-like carbon (DLC) coatings are notable for their excellent
tribological properties. Our understanding of the lubrication of DLC
coatings has improved drastically over the past decade. However, only a
few details are known about the properties of the adsorbed layers on
DLC, which crucially affect their tribological properties under
lubricated conditions. In this work we used neutron reflectometry to
determine the thickness and the density of adsorbed layers of alcohol
molecules on several different types of DLC coatings, i.e.,
non-hydrogenated (a-C) and hydrogenated, of which both non-doped (a-C:H)
and doped (a-C:H:F and a-C:H:Si) coatings were used. The results showed
that a 0.9-nm-thick and relatively dense (≈45%) layer of alcohol
adsorbed on the a-C coating. In contrast, no adsorption layer was found
on the a-C:H, confirming the important role of hydrogen, which
predominantly acts as a dangling-bond passivation source and affects the
reactivity and tribochemistry of DLC coatings. The incorporation of F
into a DLC coating also did not cause an increase in the adsorption
ability with respect to alcohol molecules. On the contrary, the
incorporation of Si increased the reactivity of the DLC coating so that
a 1.3-nm-thick alcohol layer with a 35% bulk density was detected on the
surface. We also discuss the very good agreement of the current results
with the surface energy of selected coatings found in these experiments.
[Show abstract][Hide abstract] ABSTRACT: Cover: Neutron reflectivity provides means to access buried interfaces of solution cast pressure sensitive adhesives. From the extracted scattering length density profiles the inner composition at the adhesive‐adherent interface can be obtained which can allow predictions about the durability of bonds. Further details can be found in the article by M. Schindler, S. Pröller, T. Geue, P. Müller‐Buschbaum* on page 549.
[Show abstract][Hide abstract] ABSTRACT: The adhesive–adherent interface of a model system for pressure sensitive adhesives (PSAs) is probed with neutron reflectivity to reveal its inner composition. The statistical copolymer poly(ethylhexyl-stat-d-methylmethacrylate) P(EHA-stat-dMMA), consisting of 80 wt% ethylhexylacrylate (EHA) and 20 wt% deuterated methylmethacrylate (dMMA), is used as model PSA. Its interface with silicon is investigated. The surface cleaning treatments applied to silicon strongly affect the inner near interface structure of the solution cast P(EHA-stat-dMMA). Two opposite behaviors are observed. On acid cleaned silicon the soft and sticky majority component EHA enriches at the interface to the substrate. In contrast, the glassy minority component dMMA forms an enrichment layer at the interface to the basic cleaned silicon surface.
[Show abstract][Hide abstract] ABSTRACT: The importance of surfactant self-assemblies in foam stabilization is well-known. The aim of the current study was to investigate the self-assemblies of the nonionic surfactant polyglycerol ester (PGE) in bulk solutions, at the interface and within foams, using a combined approach of small-angle neutron scattering, neutron reflectivity, and electron microscopy. PGE bulk solutions contain vesicles as well as open lamellar structures. Upon heating of the solutions the lamellar spacing increases, with significant differences in the presence of NaCl or CaCl2 as compared to the standard solution. The adsorption of the multilamellar structures present in the bulk solutions lead to a multilayered film at the air–water interface. The ordering within this film was increased as a result of a 20% area compression mimicking a coalescence event. Finally, PGE foams were shown to be stabilized not only by strong interfacial films but also by agglomerated self-assemblies within the interstitial areas of the foams.
[Show abstract][Hide abstract] ABSTRACT: Postpolymerization modification reactions are widely employed to prepare functional polymer brushes. Relatively little is known, however, about the distribution of functional groups in such postmodified brushes. Using neutron reflectometry and UV–vis spectroscopy as principal tools, this article investigates the p-nitrophenyl chloroformate (NPC)-mediated postpolymerization modification of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes, prepared via surface-initiated atom transfer radical polymerization, with D-10 leucine and D-3 serine. The neutron reflectometry experiments indicate that the postpolymerization modification depends both on brush thickness and grafting density. Whereas for dense brushes, postpolymerization modification with D-10 leucine is limited to the top 200 Å of the brush, independently of the brush thickness, the extent of postmodification can be significantly enhanced by decreasing the grafting density of the brush or by using the more hydrophilic and sterically less demanding D-3 serine, which reflects the ability of this amino acid to more readily penetrate the brush. UV–vis experiments revealed that the NPC activation is also nonuniform, but brush thickness and grafting density dependent, which adds to brush thickness and density and the nature of the amino acid as another of a complex set of variables that determine the final distribution of functional groups in postmodified brushes.
[Show abstract][Hide abstract] ABSTRACT: Semifluorinated alkanes (C(n)F(2n+1)C(m)H(2m+1)), short FnHm display local phase separation of mutually incompatible hydrocarbon and fluorocarbon chain moieties, which has been utilized as a structure-forming motif in supramolecular architectures. The packing of semifluorinated alkanes, nominally based on dodecyl subunits, such as perfluoro(dodecyl)dodecane (F12H12) and perfluoro(dodecyl)eicosane (F12H20), as well as a core extended analogue, 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene) (F11H1-core-H12), was studied at the air/water interface. Langmuir monolayers were investigated by means of neutron reflectivity directly at the air/water interface and scanning force microscopy after transfer to silicon wafers. Narrowly disperse surface micelles formed in all three cases; however, they were found to bear different morphologies with respect to molecular orientation and assembly dimensionality, which gives rise to different hierarchical aggregate topologies. For F12H12, micelles of ca. 30 nm in diameter, composed of several circular or "spherical cap" substructures, were observed and a monolayer model with the fluorocarbon block oriented toward air is proposed. F12H20 molecules formed larger (ca. 50 nm diameter) hexagonally shaped surface micelles that were hexagonally, densely packed, besides more elongated but tightly interlocked wormlike structures. Conversely, F11H1-core-H12 films organized into linear rows of elongated surface micelles with comparable width, but an average length of ca. 400 nm, apparently formed by antiparallel molecular packing.
[Show abstract][Hide abstract] ABSTRACT: Self-assembly of Fe(2+) ions and the rigid ditopic ligand 1,4-bis(2,2':6',2''-terpyridin-4'-yl)benzene results in metallo-supramolecular coordination polyelectrolytes (MEPE). Sequential self-assembly of MEPE and dialkyl phosphoric acid esters of varying chain length via electrostatic interactions leads to the corresponding polyelectrolyte-amphiphile complexes (PAC), which have liquid-crystalline properties. The PACs have a stratified architecture where the MEPE is embedded in between the amphiphile layers. Upon heating above room temperature, the PACs show either a reversible or an irreversible spin-crossover (SCO) in a temperature range from 360 to 460 K depending on the architecture of the amphiphilic matrix. As the number of amphiphiles per metal ion is increased in the sequence 1:2, 1:4, and 1:6, the temperature of the SCO is shifted to higher values whereas the amphiphile chain length does not have a significant impact on the SCO temperature. In summary, we describe in this article how the structure and the magnetic response function of PACs can be tailored through the design of the ligand and the composition. To investigate the structure and the magnetic behavior, we use X-ray scattering, X-ray absorption spectroscopy, differential scanning calorimetry, faraday-balance, and superconducting quantum interference measurements in combination with molecular modeling.
Journal of the American Chemical Society 12/2010; 133(3). DOI:10.1021/ja108416a · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Grazing incidence small-angle X-ray scattering (GISAXS) is used for nondestructive characterization of colloidal crystals of different numbers of hexagonally dense packed layers fabricated by convective self-assembly. Whereas small crystallites with random orientation were obtained in case of monolayers, the scattering data obtained from multilayer samples revealed colloidal domains over areas of a few centimeters where the single crystalline domains are mainly aligned along the growth direction. The data indicates an increasing degree of self-organization going from monolayers to multilayers. Within the multilayer samples, the stacking sequence of the hexagonally packed layers is evaluated using a numerical model for fitting the X-ray scattering data containing the stacking parameter, a. Compared with an expected complete random stacking with a = 0.5, the fitted stacking parameter, a = 0.63 ± 0.01, averaged over a sample area of about 1 mm(2) indicates a preference for a cubic stacking sequence. This value is smaller than reported by various local probe techniques.
The Journal of Physical Chemistry B 10/2010; 114(39):12473-9. DOI:10.1021/jp103943y · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Metal ion induced self-assembly of the rigid ligand 1,4-bis(2,2':6',2 ''-terpyridine-4'-yl) benzene (1) with Fe(II), Co(II), Ni(II) and Zn(II) acetate in aqueous solution results in extended, rigid-rod like metallosupramolecular coordination polyelectrolytes (MEPE-1). Under the current experimental conditions the molar masses range from 1000 g mol(-1) up to 500 000 g mol(-1). The molar mass depends on concentration, stoichiometry, metal-ion and time. In addition, we present viscosity measurements, small angle neutron scattering and AFM data. We introduce a protocol to precisely control the stoichiometry during self-assembly using conductometry. The protocol can be used with different terpyridine ligands and the above-mentioned metal ions and is of paramount importance to obtain meaningful and reproducible results. As a control experiment we studied the mononuclear 4'-(phenyl)2,2':6',2 ''-terpyridine (3) complex with Ni(II) and Zn(II) and the flexible ligand 1,3-bis[4'-oxa(2,2': 6',2 ''-terpyridinyl)] propane (2) with Ni(II) acetate (Ni-MEPE-2). This ligand does not form extended macroassemblies but likely ring-like structures with 3 to 4 repeat units. Through spin-coating of Ni-MEPE-1 on a solid surface we can image the MEPEs in real space by AFM. SANS measurements of Fe-MEPE-1 verify the extended rigid-rod type structure of the MEPEs in aqueous solution.
[Show abstract][Hide abstract] ABSTRACT: Metal-ion-induced self-assembly in aqueous solution of the rigid ligand 1,4-bis(2,2′:6′,2′′-terpyridine-4′-yl)benzene (1) with Fe(OAc)2 and Ni(OAc)2 is investigated with viscosimetry, SANS, and AFM. Ligand 1 forms extended, rigid-rod like metallo-supramolecular coordination polyelectrolytes (MEPEs) with a molar mass of up to 200 000 g mol−1 under the current experimental conditions. The molar mass depends on concentration, stoichiometry, and time. By spin-coating MEPEs on a solid surface, we can image the MEPEs in real space by AFM. Both AFM and SANS confirm the extended rigid-rod-type structure of the MEPEs. As a control experiment, we also studied the flexible ligand 1,3-bis[4′-oxa(2,2′:6′,2′′-terpyridinyl)]propane (2). Ligand 2 does not form extended macro-assemblies but likely ringlike structures with three to four repeat units. Finally, we present a protocol to control the stoichiometry during self-assembly using conductometry, which is of paramount importance to obtain meaningful and reproducible results.
[Show abstract][Hide abstract] ABSTRACT: Crown ether functionalised conducting polymer films were used to complex barium ions from acetonitrile solution. It was found that fully-functionalised N-derivatized polypyrrole films do not possess adequate mechanical stability, but dilution with unfunctionalised bithiophene co-monomer leads to a series of copolymer films with excellent stability. Film reactivity, composition and structure were investigated using electrochemical, nanogravimetric, FTIR, XPS and neutron reflectivity techniques. The first three of these provided spatially integrated barium populations and neutron reflectivity provided spatially resolved compositional profiles. Measurements at various stages of film fabrication yielded spatial distributions of co-monomer, crown ether, solvent and barium (as perchlorate) components. Critically, the amount of free volume to accommodate crown motifs and barium within the film was limited by the film's internal microstructure and solvent content; the low solvent volume fraction creates a different local environment to solution.
[Show abstract][Hide abstract] ABSTRACT: Self-assembly of Fe(2+) or Ni(2+) ions and the ditopic ligand 6,6',6''-bis(2-pyridyl)-2,2':4',4'':2'',2'''-quaterpyridine (btpy) through coordinative binding results in rodlike metallosupramolecular coordination polyelectrolytes (Fe-MEPE or Ni-MEPE). Sequential self-assembly with dihexadecyl phosphate (DHP) via electrostatic interactions between MEPE and DHP leads to the corresponding polyelectrolyte amphiphile complex (PAC) with liquid crystalline properties. The MEPE rods are embedded in between the interdigitated DHP layers. Upon heating above room temperature, the Fe-PAC shows an irreversible spin-crossover (SCO) from a diamagnetic low-spin (LS) to a paramagnetic high-spin (HS) state accompanied by a color change from dark blue to pale blue. The SCO is nearly complete (95%) and directly associated with the structure changes induced by the melting of the amphiphilic matrix. The original Fe-PAC architecture does not reassemble upon cooling and remains in a disordered frozen HS state. However, dissolving the heated PAC induces reassembly, and the original dark blue, diamagnetic, ordered material is completely recovered. In comparison to Fe-PAC, Ni-PAC shows the same lamellar structure and the same temperature depended structure changes but has a constant magnetic moment. In contrast to Fe-PAC, in neat Fe-MEPE the SCO depends on the history of the sample and in particular on the amount of included solvent as thermogravimetric analysis, differential scanning calorimetry (DSC), and magnetic measurements indicate. Solid MEPE does not have liquid crystalline properties, and, therefore, the induced structure changes upon heating are constrained by the solid-state architecture, and thus, the SCO in Fe-MEPE is incomplete.
Journal of the American Chemical Society 03/2009; 131(8):2934-41. DOI:10.1021/ja808278s · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on in situ investigations of colloidal ordering during gravity sedimentation from a colloidal suspension onto a prepatterned support using a polymeric surface relief grating (SRG) as the support. The ordering of colloids with a diameter of 420 nm was investigated by means of grazing-incidence small-angle X-ray scattering (GISAXS) and transmission SAXS using a preparation cell guaranteeing stable temperature and humidity. GISAXS was used for in situ monitoring of the time evolution of colloidal ordering within the whole illuminated sample area. The onset of ordering was indicated by the increase of integrated intensity within a small time frame shortly before complete evaporation of the dispersant. Single domains of coated samples were investigated ex situ by SAXS in transmission geometry where the irradiated sample area was 200 x 200 microm(2) only. Domains with the typical size of a few millimeters were observed varying in orientation and crystallographic structure for various positions at the sample. They were mainly oriented along the grooves of the grating, confirming the influence of the underlying grating on colloidal ordering.
[Show abstract][Hide abstract] ABSTRACT: The development of surface relief and density patterns in azobenzene polymer films was studied by diffraction at two different wavelengths. We used x-ray diffraction of synchrotron radiation at 0.124 nm in combination with visible light diffraction at a wavelength of 633 nm. In contrast to visible light scattering x-ray diffraction allows the separation of a surface relief and a density grating contribution due to the different functional dependence of the scattering power. Additionally, the x-ray probe is most sensitive for the onset of the surface grating formation.
Molecular Crystals and Liquid Crystals 08/2006; 446(2006: pp. 111–121):111-121. DOI:10.1080/15421400500383345 · 0.49 Impact Factor