Grazing Incidence X-ray Diffraction Experiments Research Articles

Grazing Incidence Synchrotron X-ray Diffraction of Polymerizing Langmuir Monolayers

Octadecyltrimethoxysilane (OTMS) was spread on the water surface of a Langmuir trough. Utilizing the high-intensity synchrotron X-ray radiation at the Advanced Photon Source at Argonne National Laboratory, the kinetics of film growth of OTMS as a function of the subphase pH was studied. In situ grazing incidence X-ray diffraction experiments at the air/water interface revealed that OTMS self-assembles into small domains and that the number of these ordered domains increases as OTMS undergoes hydrolysis/condensation reactions. The hexagonal packing of alkyl chains found in the OTMS monolayer is consistent with the ordering found in bulk OTMS by powder X-ray diffraction.

Interaction between phospholipids and new Gemini catanionic surfactants having anti-HIV activity

Abstract Monolayers at the air/water interface are used as 2D models of biological membrane to study the interaction of a new Gemini catanionic surfactant, Gem16,12, with the phospholipid DPPC. Gem16,12 is a synthetic analog of galactosylceramide (a cell receptor for HIV-1) and shows both good anti-HIV activity and low toxicity. Adsorption kinetics measurements are performed at Gemini concentrations below the critical aggregation concentration (CAC) and show a decrease of surface tension with increasing concentration. If the surface is covered with a DPPC monolayer at molecular areas with still zero surface pressure the adsorption of Gem16,12 is faster and leads to a higher surface pressure compared with the pure air/water interface. Film balance, fluorescence microscopy and grazing incidence X-ray diffraction (GIXD) experiments were performed after reaching the equilibrium surface tension. The experiments show an incorporation of Gem16,12 molecules into the fluid phase as well as into the condensed domains of a DPPC monolayer. GIXD experiments confirmed these results showing a smaller tilt angle in a DPPC monolayer on a Gemini subphase compared to DPPC on pure water. Compressing such a mixed monolayer leads first to the squeezing out of Gem16,12 molecules from the fluid part of the monolayer and above 30 mN/m to the squeezing out from the condensed part.

Equation of State for Monolayers under Consideration of the Two-dimensional Compressibility in the Condensed State

The equation of state for Langmuir monolayers proposed earlier (Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. B 1999, 103, 145.) is generalized for the case that one, two or more phase transitions in the monolayer take place, provided that the two-dimensional compressibility undergoes a jump at the phase transition, whereas within any condensed phase of the monolayer this compressibility is proportional to the surface pressure. N-Tetradecyl-β-hydroxy-propionic acid amide (THPAA) monolayers undergo two phase transitions during the compression, demonstrated experimentally both by the cusps of the surface pressure−area (Π−A) isotherm and the results of grazing incidence X-ray diffraction (GIXD). The theoretical Π−A isotherm agrees satisfactorily with the experimental data if the THPAA molecular characteristics obtained from independent GIXD experiments are taken as model parameters.

Structure and Elastic Properties of 10−12 Pentacosadiyonic Acid Langmuir Films

Monolayers of the polymerizable 10−12 pentacosadiyonic acid amphiphile spread on a basic buffer are studied both before and after polymerization. We have established the phase diagram of the unpolymerized Langmuir film by isotherm measurements, Brewster angle microscopy, and grazing incidence X-ray diffraction experiments. Diffuse scattering experiments performed at the European Synchrotron Radiation Facility allowed us to investigate the structure and elastic properties of the different monomer phases and the modifications induced by polymerization.

Herringbone ordering and lattice distortions in a planar-molecule model for langmuir monolayers

A model of planar molecules, made up of "atoms" interacting by Lennard-Jones potentials and arranged to mimic the cross section of alkyl chains, is used to study the problem of backbone plane ordering in Langmuir monolayers. It is shown that two minima of the interaction energy are reached if molecules lie on the sites of a centered rectangular lattice in a herringbone configuration with two different dihedral angles. These orientationally ordered phases can be related to the so-called herringbone and pseudoherringbone structures, whose lattice distortions qualitatively agree with those determined by means of grazing incidence x-ray diffraction experiments on Langmuir monolayers. A third energy minimum is obtained for a configuration of parallel molecules on an oblique lattice, which has also been observed in some experiments. The competition between the three phases is investigated, upon varying geometric parameters of the model molecules and surface pressure. The effect of temperature is analyzed in a mean field approximation, by taking into account the orientational entropy contribution on a lattice system with variable unit cell parameters. In this framework the transition to an orientationally disordered phase is also pointed out.

Strain status of self-assembled InAs quantum dots

Grazing incidence x-ray diffraction experiments employing the asymmetric (202) Bragg diffraction have been performed to characterize self-assembled InAs quantum dots grown by molecular-beam epitaxy. We find that the strain is elastically relaxed with different components. The volume distribution of partially strained InAs inside islands is peaked at intermediate strain values. The fraction of both almost fully strained and totally relaxed InAs is found to be small. In addition, a small volume fraction of relaxed InxGa1−xAs is found.

Oriented Crystalline Thin Films of Tetracosanedioic Acid and Its Metal Salts at the Air-Aqueous Solution Interface

Langmuir monolayers are conventionally prepared by spreading, on a water surface, amphiphilic molecules containing hydrophilic head groups, which interact with the water molecules, and hydrophobic chains, which favor lateral aggregation in air. The crystalline packing of the molecules in the monolayer has been determined in situ, with almost atomic resolution by means of synchrotron X-ray diffraction using a monochromatic beam at grazing incidence. With the use of this technique, the two-dimensional crystalline packing arrangement of monolayers of long-chain alcohols, carboxylic acids, amides, aamino acids, and phospholipids could be determined. Recently, the formation at the air±water interface of crystalline monolayer and multilayer films obtained from symmetric molecules with end groups equally hydrophobic or hydrophilic, such as long-chain linear fluorocarbons and hydrocarbons, oligothiophenes (containing four, five, and six rings), and a,w-alkanediols, has been reported. In these films at low surface pressure the long molecular axis is aligned either normal to the water surface or tilted by up to 40 from the normal. Moreover, the formation of crystalline interdigitated trilayers from water-insoluble long-chain mandelic acid and phenylethylamine molecules contained in the subphase was proven by grazing incidence X-ray diffraction (GID). The changes in packing arrangement of monolayers composed of long-chain carboxylic acids, as induced by the presence in the subphase of Cd, Ca, or Pb ions, have also been determined by GID. Over pure water and in the uncompressed state, the chains are tilted by approximately 30 from the normal to the water surface, whereas over solutions containing the metal ions, the chains are vertically aligned. This report describes the self-assembly process of a,wtetracosanedioic acid, HOOC±(CH2)22±COOH, on aqueous subphases containing divalent Cd and Pb ions. These positive ions have a dramatic influence on the packing of the diacid molecules: they bind to both carboxylate ends of the molecule and force the long hydrocarbon chain to lie parallel to the water surface and to self-assemble as multilayer domains about 50 Š thick, but with the molecules and the bound ions perfectly ordered laterally over distances up to 1000 Š. GID and specular X-ray reflectivity (XR) were used to characterize the oriented crystalline multilayers of these cadmium and lead salts. Such multilayer films preserve their orientation during the transfer from the water surface onto a mica support, as imaged by scanning force microscopy (SFM), and can be used in the preparation of quantum dots (see following communication). The GID experiments were carried out at the synchrotron beamline BW1, HASYLAB, on the liquid surface diffractometer, as described elsewhere. The samples were prepared by spreading solutions of the diacid onto pure water and onto aqueous subphases containing the metal ions, at 15 C, to give a nominal molecular area of 36 Š (trough area divided by the number of spread molecules). Spreading was followed by reducing the surface area of the film to a nominal molecular area of 18 Š, with a maximum surface pressure increase of 1 mN/m and cooling down to 5 C. Preliminary evidence for the interaction between the diacid molecules and the metal ions present in the subphase leading to salt formation was obtained from the surface pressure±area isotherms (Fig. 1a). While the diacid spread on water leads to a small increase in surface pressure only at relatively small nominal molecular areas, the isotherm performed on the subphases containing ions resulted in a pressure increase of about 20 Š/molecule. On the basis of the isotherm alone, one might assume conventional monolayer formation for the latter system. However, as we shall see, the GID results provide unambiguous evidence for the formation of multilayers with the diacid hydrocarbon chain aligned parallel to the liquid surface.

Structure of the liquid-vapor interfaces of metals and binary alloys

The current status of our understanding of the structure of the liquid-vapor interfaces of metals and binary alloys is briefly described, with emphasis on the comparison between theoretical predictions and the results of recent X-ray reflection and grazing incidence X-ray diffraction experiments.

Synchrotron X-ray diffraction studies of inorganic materials and heterogeneous catalysts

Abstract The special features of synchrotron X-radiation and the types of instrumentation required for a range of synchrotron X-ray diffraction experiments are outlined. A diverse range of applications to inorganic and heterogeneous catalyst systems are discussed. Grazing incidence X-ray diffraction experiments have revealed a number of surface structures and followed surface phase transformations; similar studies of boundary and interfacial structures are cited. Experiments on catalytic metals dispersed on supports have exploited anomalous scattering effects. High-resolution powder diffraction has assisted phase identifications, provided quantitative data on peak broadening (such as arise from particle size effects or stacking disorder), enabled studies of subtle superstructures and a growing number of ab initio structure solutions, and permitted Rietveld refinements of a range of inorganic materials. The potential for performing measurements on individual crystallites in the micrometre size regime has been demonstrated; a small number of successful single-crystal structure refinements have been described. Single-crystal Laue methods have been also applied successfully to structure refinements and solutions; this configuration potentially permits data sufficient for structure definition to be recorded on a nano-microsecond time scale. A range of studies that are time-resolved (currently on somewhat longer scales), and various studies under non-ambient conditions (such as at high or low temperatures, at extreme pressures, or under applied magnetic and electric fields) have also been described. These various applications of synchrotron X-ray diffraction techniques to inorganic and heterogeneous catalyst systems both illustrate already substantial contributions to our understanding, and promise still greater advances from future applications.