[Show abstract][Hide abstract] ABSTRACT: At a non-polar air/water interface Quillaja bark saponin (QBS) adsorbs with the hydrophobic aglycone part directed towards the air. In contrast, at Si/water interface QBS adsorbs in an opposite orientation, through weak hydrogen bonds with the Si surface.
[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.
Biochimica et Biophysica Acta 04/2014; · 4.66 Impact Factor
[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: 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; · 10.68 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. · 3.61 Impact Factor
[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. · 10.68 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. · 0.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Surface relief gratings were inscribed on azobenzene polymer films using a pulselike exposure of an Ar+ laser. The inscription process was initiated by a sequence of short pulses followed by much longer relaxation pauses. The development of the surface relief grating was probed by a He−Ne laser measuring the scattering intensity of the first-order grating peak. The growth time of the surface relief grating was found to be larger than the length of the pulses used. This unusual behavior can be considered as a nonlinear material response associated with the trans−cis isomerization of azobenzene moieties. In this study the polymer stress was assumed to be proportional to the number of cis-isomers. One-dimensional viscoelastic analysis was used to derive the polymer deformation. The rate of trans−cis isomerization increases with the intensity of the inscribing light; in the dark it is equal to the rate of thermal cis−trans isomerization. The respective relaxation times were estimated by fitting theoretical deformation curves to experimental data.
Journal of Physical Chemistry B - J PHYS CHEM B. 08/2004; 108(39).
[Show abstract][Hide abstract] ABSTRACT: Surface relief gratings on azobenzene containing polymer films were prepared under irradiation by actinic light. Finite element modeling of the inscription process was carried out using linear viscoelastic analysis. It was assumed that under illumination the polymer film undergoes considerable plastification, which reduces its original Young's modulus by at least three orders of magnitude. Force densities of about 10(11) N/m3 were necessary to reproduce the growth of the surface relief grating. It was shown that at large deformations the force of surface tension becomes comparable to the inscription force and therefore plays an essential role in the retardation of the inscription process. In addition to surface profiling the gradual development of an accompanying density grating was predicted for the regime of continuous exposure. Surface grating development under pulselike exposure cannot be explained in the frame of an incompressible fluid model. However, it was easily reproduced using the viscoelastic model with finite compressibility.
The Journal of Chemical Physics 03/2004; 120(8):4039-45. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the results of time-dependent x-ray scattering measurements during the formation of a buried, modulated density structure below polymer film surface created at temperatures above the glass transition temperature TG during the thermal erasure of a holographically written surface relief grating. The surface relief gratings are initially formed on polymer films containing azobenzene side groups by exposure to a polarization holographic pattern made using light at 488 nm. The material responds to the actinic light by forming a sinusoidal surface relief pattern in a one-step process. Theoretical predictions show that the surface relief grating formation is accompanied by a density grating just below the film surface. Upon annealing above polymers glass transition temperature the surface relief grating melts while some of the density modulation in the bulk remains and a nonsinusoidal density grating is rebuilt after long term annealing at a temperature approximately 30 K above the glass transition temperature. The growth of density volume structure is assisted by exposure to red laser light during annealing. The observed homopolymer pDR1M was found to be the most effective material to build the modulated buried density grating. This might be due to the formation of clusters below the former peak positions of the surface relief grating. A quantitative description and preliminary model is given for this behavior.
Journal of Applied Physics 03/2003; 93(6). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the results of time-dependent x-ray and visible light (VIS) scattering measurements during formation of surface relief grating (SRG). These gratings are formed on polymer films containing azobenzene side groups during pulselike exposure with a holographic pattern of circularly polarized light at 488 nm. The SRG formation is accompanied by a density grating just below the film surface. Assuming viscoelastic flow, a change in polymer's elastic properties upon light exposure can explain the massive material transport. Finite element calculations reveal a dynamic model of grating formation characterized by different relaxation times. The simultaneous formation of a surface relief grating and of a density grating explains quantitatively the findings of the VIS experiment, but only qualitatively the findings of the x-ray measurements.
[Show abstract][Hide abstract] ABSTRACT: Octa-substituted tetraazaporphyrins with amphiphilic 3,6-dioxaheptylthio and 3,6,9-trioxadecylthio chains and their metal complexes (Co(II), Ni(II), Cu(II), Zn(II)) were synthesised in order to compare their thermotropic and lyotropic mesomorphism as well as their thin film properties with related tetraazaporphyrins, phthalocyanines and triphenylenes, which have been reported previously. The Co, Ni, and Cu complexes melted into hexagonal columnar mesophases, whereas the Zn(II) complexes and the free-base porphyrins did not display mesomorphism. In fact, the zinc complexes did not crystallise on cooling, rather giving highly viscous isotropic oils at room temperature that solidified below −50°C to glasses. This unusual behaviour may be caused by axial interactions between the zinc ion and the oxygen of the polyether groups. All 3,6,9-trioxadecylthio substituted derivatives solidified far below 0°C giving rise to columnar hexagonal mesophases at room temperature for the Co(II), Ni(II), and Cu(II) complexes. The columnar hexagonal mesophases of the long chain derivatives spontaneously align homeotropically when sandwiched between two substrates, in contrast to the short chain analogues. Both the long chain and the short chain Co(II) complexes displayed a rather disordered hexagonal columnar packing as revealed by X-ray diffraction. Binary mixtures of the short chain derivatives with non-polar or polar organic solvents did not display additional lyotropic mesophases. However, the transition temperatures and enthalpies, as well as the texture of the hexagonal columnar mesophase were affected, assuming a solubility of the solvent in the thermotropic mesophase as shown for the Cu(II) derivative. All long chain derivatives were also soluble in water but, again, lyotropic mesomorphism was observed neither with water nor with organic solvents. The film forming properties of some derivatives were investigated by spin-coating and the Langmuir–Blodgett (LB) technique. Homogeneous films of the mesomorphic short chain copper complex were obtained by the spin-coating method. The films displayed a layer structure with edge-on orientation at ambient temperature, although they were of crystalline morphology, as shown by X-ray reflectivity measurements and polarised UV/VIS spectroscopy. Only monolayer films of short and long chain derivatives could be transferred by the LB method. Again, determination of the thickness of the monolayer by X-ray agreed with an edge-on orientation of the tetraazaporphyrins. In contrast, the extrapolated molecular areas on the water surface suggested a flat-on orientation of the macrocycle with the oligo(oxyethylene) chains being dissolved in the sub-phase.
Journal of Materials Chemistry 01/2001; 11(6). · 5.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Anisotropic films of functionalised polymers such as the liquid crystalline poly (olefin sulfone)s (POS) and maleic anhydride co- and terpolymers (MAC, containing azobenzene side-groups too) are studied. Starting from isotropic spin-coated films annealing results in homeotropically aligned films by spontaneous self-assembling. X-ray specular scattering (XRS) and X-ray grazing incidence diffraction (GIXD) indicate a layered structure across the film parallel to the interface and significant in-plane order in thin films. Polarised UV-VIS spectroscopy investigations show that the homeotropic orientation of the side groups causes the formation of “card packed” or H-aggregates. For azobenzene containing MAC terpolymer an initial UV-irradiation followed by a polarised VIS-irradiation causes a light-induced co-operative reorientation of both types of side-groups and results in a lamellar stacking which is tilted. Orientation and aggregation are therefore driven by the liquid crystallinity and the segregation tendency of POS and MAC in constrained films. This seems to be an important factor for such spontaneous ordering processes.
[Show abstract][Hide abstract] ABSTRACT: The solid state diazotation of sulfanilic acid monohydrate (1) on (010) gives island features and craters on the surface. The remaining surface roughness stays low enough to permit depth dependent grazing incidence X-ray diffraction (GID) measurements, but only when the gas applications are slow. The theoretical background of the unprecedented GID measurements is given. GID measurements were undertaken below, at and above the critical angle of incidence to differentiate the processes at the surface and in the crystal bulk. The phase rebuilding and phase transformation processes could be directly detected in the bulk of the crystal supporting the common AFM interpretations which are derived from the formation of surface features in correlation with crystal packing data. The combined consideration of depth resolved in-plane GID and AFM experiments leads to a comprehensive description of the gas-solid process.
Molecular Crystals and Liquid Crystals. 02/1997; 293(1):261-275.
[Show abstract][Hide abstract] ABSTRACT: Amphotropic copolymers containing rod-like side groups combine the ability to form Langmuir-Blodgett multilayer assemblies with thermotropic liquid crystalline selforganization on temperature changes. Photochromic azobenzene side groups affect the supramolecular structure in such copolymers in dependence on irradiation wavelength and polarization of the incident light beam as well as temperature. The aggregation phenomena at the polymer air interface caused by the amphotropic properties of the material were reversibly modified by the light induced variation of the molecular shape of the photochromic moieties. The vertically layered structure had been destroyed and rebuilt upon irradiation and annealing the LB films. Moreover, the optical in-plane anisotropy was erased on irradiation and reinduced using linearly polarized light.Combined investigations (angular dependent UV-VIS measurements, energy dispersive SAXS and GID, high resolution AFM) provide new insights into the micromorphology as well as the surface structure of the condensed initial and transformed LB films.
Molecular Crystals and Liquid Crystals. 03/1995; 262(1):157-166.
[Show abstract][Hide abstract] ABSTRACT: Photochromic LB-multilayers of amphotropic copolymers having rod-like azobenzene moieties as side groups which exhibit thermotropic liquid crystalline behaviour in the bulk have been characterized by UV-VIS spectroscopy as well as by angular dependent and energy dispersive SAXS. Upon irradiation with UV-light the initial optical in-plane anisotropy and the vertical layered structure of LB-films can be destroyed. A new layered structure can be rebuilt on irradiation with visible light or by annealing. Using linearly polarized light for the irradiation an optical anisotropy (POA) has been induced or reversibly modified by a photoselection process.