Grazing Incidence Synchrotron X-ray Diffraction Research Articles

Superstructures and Correlated Metal Ion Layers in Langmuir−Blodgett Films of Cadmium Soaps Observed with Grazing Incidence X-ray Diffraction

The structures of Langmuir−Blodgett films of cadmium soaps have been investigated using grazing incidence synchrotron X-ray diffraction and imaging plate detection. It was found that the cadmium atoms in the films are arranged with a monoclinic (C) symmetry, while the chains have the same monoclinic structure as that of the metal atoms but without the centered symmetry. The chains are perpendicular to the film surface and arranged in a herringbone pattern. For the first time with X-ray diffraction, a 5 × 1 superstructure present in the film plane and commensurate with the local structure was observed. The meridional diffraction shows a pattern from a set of wide slits, from which thicknesses corresponding to both a monolayer and a bilayer of the cadmium soap films were determined.

Packing Stress Relaxation in Polymer−Lipid Monolayers at the Air−Water Interface: An X-ray Grazing-Incidence Diffraction and Reflectivity Study

Using synchrotron grazing-incidence X-ray diffraction (GIXD) and reflectivity (XR), we have determined the in-plane and out-of-plane structure of phospholipid monolayers at the air−water interface as a function of hydrophilic lipid headgroup size. Di-stearoyl-phosphatidyl-ethanolamine (DSPE) lipid monolayers were systematically modified by chemically grafting hydrophilic poly(ethylene glycol) (PEG) chains of MW = 90 g/mol (2 ethylene oxide, EO, units), MW = 350 g/mol (8 EO units), and MW = 750 g/mol (17 EO units) to the lipid headgroups. The monolayers were studied in the solid phase at a surface pressure of 42 mN/m. At these high lipid packing densities, the PEG chains are submerged in the water subphase. The increased packing stresses from these bulky polymer headgroups distort the unit cell and the in-plane packing modes of the monolayers, leading to large out-of-plane alterations and staggering of the lipid molecules. Surprisingly, a change in the molecular packing of the monolayer toward higher packing densities (lower area per molecule) was observed on increasing the PEG MW to 750 g/mol (17 EO units). This rearrangement of the monolayer structure may be due to a conformational change in the PEG chains.

The Evaporation Resistance of Mixed Monolayers of Octadecanol and Cholesterol

The high resistance to water evaporation exhibited by monolayers of octadecanol is markedly reduced if small quantities of cholesterol are present, an effect that mirrors the behavior of contaminants in octadecanol monolayers. Surface pressure–area isotherms show that octadecanol and cholesterol are almost (but not entirely) immiscible. Grazing-incidence synchrotron X-ray diffraction (GIXD) of the floating monolayers shows that cholesterol produces no diffraction pattern and thus is amorphous, octadecanol gives a clear diffraction peak which can be deconvoluted into a major and a minor peak corresponding to a slightly distorted hexagonal packing of the alkyl chains, and the alcohol-rich mixtures give diffraction patterns like that of pure octadecanol. The latter result shows that cholesterol does not enter the octadecanol domains in the mixed monolayers. The evaporation data are explained by permeation through the irregularly packed domain boundary regions in a pure octadecanol monolayer and by the rapid increase in the size of these regions when cholesterol is added.

Disorder in Langmuir Monolayers. 1. Disordered Packing of Alkyl Chains

Grazing incidence synchrotron X-ray diffraction (GID) studies of Langmuir monolayers of several substances such as 2-monopalmitoyl-glycerol, 2-hexadecanol, and 2-hydroxypalmitic acid reveal diffraction patterns that indicate a disordered packing of alkyl chains. The diffracted intensity is distributed along a characteristic arc in reciprocal space. A relation between in-plane lattice and two parameters, polar tilt angle and tilt azimuth, is derived in order to describe the observed distribution that results from the superposition of the diffracted intensities of different lattices. It is possible to ascribe the diffraction patterns unambiguously to a variation of the tilt azimuth. Variation of the polar tilt angle was ruled out. A detailed analysis of the distributed intensity allows one to obtain at least the polar tilt angle and the lattice spacing perpendicular to the tilt azimuth in the limit of a lattice with nearest-neighbor tilt.

Competition of interactions in monolayers of amphiphilic acid amides at the air–water interface

Phase transitions, morphology and crystal structure of monolayers at the air–water interface of N-alkyl-γ-hydroxy-butyric acid amides with different alkyl chain lengths have been studied by π– A isotherms, Brewster angle microscopy (BAM) and synchrotron grazing incidence X-ray diffraction (GIXD). While the morphological structures were similar, the π– A isotherms and the oblique crystal structure indicate a marked difference between the molecular area in the condensed phase of acid amides with shorter (DHBAA, THBAA and PHBAA) and longer (HHBAA and OHBAA) alkyl chains, resulting from a change in the conformation of the alkyl chains. The increase in molecular area with increasing van der Waals interaction (increase of chain length) can be explained as a result of the competition between the van der Waals interactions of alkyl chains and the rigid hydrogen bondings of acid amide groups.

Molecules Adopting an Inverted U-Shape on the Water Surface by Self-Assembling into Crystalline Monolayers

Model molecules, designed to simulate a chain fold, self-assemble into crystalline monolayers at the air−water interface, the structure of which has been determined, in situ, by synchrotron grazing-incidence X-ray diffraction. The two-dimensional packing arrangement in such monolayers is consistent with a structure composed of ribbons containing translationally related molecules adopting an inverted U-shape. The distance between these molecules along the ribbon is optimum for creating an additional chain fold, leading to the possibility that a polymeric molecule will undulate on the water surface.

Self-Assembly at the Air−Water Interface. In-Situ Preparation of Thin Films of Metal Ion Grid Architectures

Oriented crystalline films, ∼11−20 Å thick, of metal ion complexes of the grid type [Co4L4]8+·8PF6- and [Ag9L6]9+·9CF3SO3-, based on various ligands L, were prepared in-situ at the air-aqueous solution interface by the interaction of the free ligand molecules spread onto aqueous solutions containing Co2+ or Ag+ ions. The structure of the complex architectures composed of a 2 × 2 Co2+ grid coordinated to four ligand molecules and a 3 × 3 Ag+ grid coordinated to six ligand molecules as well as their molecular organization in thin films were characterized by grazing incidence synchrotron X-ray diffraction (GIXD) and specular X-ray reflectivity (XR) measurements performed at the air-aqueous solution interface and by UV, X-ray photoelectron spectroscopy (XPS), and scanning force microscopy (SFM) after film transfer onto various solid supports. The results open perspectives toward an implementation of the air−water interface for the self-assembly and subsequent deposition of organized arrays of complex inorganic architectures onto solid support.

Triple-chain phospholipid monolayers: a scanning force microscopy and grazing incidence X-ray diffraction study

Two triple-chain and one double-chain phospholipid monolayers have been characterised both on water and on different solid substrates by grazing incidence X-ray diffraction and scanning force microscopy. The formation of holes could be observed on water and on a solid support. SFM in contact and tapping mode has revealed surface defects and pinholes in the submicrometer range of all phospholipid films transferred at room temperature on hydrophobic (glass) and hydrophilic (mica) substrates both horizontally and vertically. The double-chain lipids exhibit only a few defects after the transfer, whereas the triple-chain lipids show more and larger holes in the monolayer. SFM contact mode enables the formation of artificial holes in the monolayer on mica in order to estimate the real thickness of the transferred layer. Surprisingly, for all substances investigated the height of the monolayer is too small compared to the length of the molecules. Phospholipids provide suitable model systems for biological membranes. Therefore they have traditionally been used for studying the competitive interaction between polar hydrophilic head groups and hydrophobic chains connected to the glycerol backbone [1–4]. With synchrotron grazing incidence X-ray diffraction (GIXD) measurements one can study the structure of condensed monolayers on the A-scale in considerable detail [5]. In the case of glycerophosphocholines substituted with 2-alkyl branched fatty acids containing long branches, these measurements have shown that the packing of the molecules in monolayers is determined by the space requirements of the chains [6]. However, for glycerophosphoethanolamines (PE) which are able to form a hydrogen bonding network between adjacent head groups there remain many open questions about the influence of lateral disturbances on the phase behaviour in monolayers and multilayers. Therefore, we have investigated double-chain and triple-chain PEs. Over the last few years, it has been shown that scanning force microscopy is a powerful tool for investigating a large variety of materials with high spatial resolution [7–10]. Morphological information, such as topographic features of monolayers and multilayers, height of structures, step sites and also friction and elastic compliance in the local regions have been shown. There have been several SFM and AFM/LFM studies of self-assembled phospholipid layers [11], but until now only very few AFM images on phospholipid monolayers or bilayers have been published [12, 13]. Moreover, it is known that there is a reorganisation of lipid films after the transfer in air [14]. This reorganisation is faster for molecules having short hydrocarbon chains, such as cadmium palmitate, than for longer-chain molecules [15]. The observation and characterisation of microstructures of organic thin films, such as monolayers and multilayers of phospholipids on solid substrates, are of particular importance for further practical application of the organic films as a matrix for functional organics or biomolecules in biosensors and biochemical probes. The aim of the current research is to characterise doubleand triple-chain phospholipids on water and on a solid support using the combination of SFM/LFM and GIXD. 1 Experimental details 1.1 Monolayer preparation The triple-chain phospholipids 1(2-hexadecylstearoyl)-2hexadecyl-glycerophosphoethanolamine 1(2C16-18:0)-2HPE (I) and 1(2-hexadecylstearoyl)-2-hexadecyl-glycerophospho-N-methylethanolamine 1(2C16-18:0)-2H-NMe (II) were synthesised according to [16]. The corresponding chemical structures are presented in Figures 1a and 1b. The lipids were purified by column chromatography. High-performance liquid chromatography (HPLC) and electron mass spectroscopy have been used for analytical characterisation. All lipids were spread from an approximately 1 mM p.a.-grade chloroform (Merck, FRG) solution. The preparation of the Langmuir–Blodgett films was performed using a circular trough with a subphase temperature of 20 ◦C. The subphase was ultrapure water (pH 5.5) with a specific resistance above

Comparing Molecular Packing and Textures of Langmuir Monolayers of Fatty Acids and Their Methyl and Ethyl Esters

Langmuir monolayers of fatty acids and their methyl and ethyl esters show different textures. Stripe textures are observed in myristic acid monolayers. Continuous changes of molecular orientation occur in palmitic acid layers as well, although there are no stripe textures. In monolayers of fatty acid methyl and ethyl esters such continuous changes of orientation are not observed. The domains of methyl palmitate and ethyl palmitate are subdivided into segments of equal brightness. Grazing incidence synchrotron X-ray diffraction (GID) studies were performed to analyze the reason for this deviating behavior at the molecular level. The diffraction patterns of myristic acid suggest that the tilt azimuth is dispersed by about 5° around a nearest-neighbor direction. Such dispersions of the tilt azimuth occur in palmitic acid and methyl palmitate layers as well. The monolayers of these substances differ from those of myristic acid only in the degree of the dispersion.

Atomic Force Microscopy and X-ray Studies of Three-Dimensional Islands on Monolayers

The structure and morphology of the three-dimensional aggregates of arachidic acid were studied by atomic force microscopy (AFM) after transfer onto a silicon wafer and by synchrotron grazing incidence X-ray diffraction (GID) at the helium/water interface. The 3D aggregates on the wafer observed by AFM are islands of granulelike and platelike shapes. The plate islands consist of terraces of different heights. The crystal structure of the plate islands was determined through molecular resolution by AFM. They have the C-crystal structure of arachidic acid at the beginning of the relaxation. During further relaxation the B-crystal structure becomes apparent. The islands with B-crystal structure dominate at the end of the relaxation process. Regions with a disordered molecular structure are found beside the ordered crystal structures. In all ordered regions of a single island the same crystal structure is observed. Using GID, the B- and C-crystal structures of the 3D aggregates of arachidic acid were also found at the helium/water interface. No peaks of any crystal structure are observed until 20% of the monolayer material is transformed into 3D aggregates. Then first the peaks of the B-crystal structure appear, which increase with the relaxation time. Additionally small peaks corresponding to the C-crystal structure are found.

Crystalline Mono‐ and Multilayer Self‐Assemblies of Oligothiophenes at the Air–Water Interface

AbstractThe formation of Langmuir monolayers at the air–water interface has long been believed to be limited to amphiphilic molecules containing a hydrophobic chain and a hydrophilic headgroup. Here we report the formation of crystalline mono‐ and multilayer self‐assemblies of oligothiophenes, a class of aromatic nonamphiphilic molecules, self‐aggregated at the air–water interface. As model systems we have examined the deposition of quaterthiophene (S4), quin‐quethiophene (S5), and sexithiophene (S6) from chloroform solutions on the water surface. The structures of the films were determined by surface pressure–area isotherms, by scanning force microscopy (SFM) after transfer of the films onto atomically smooth mica, by cryo‐transmission electron microscopy (Cryo‐TEM) on vitreous ice, and by grazing incidence synchrotron X‐ray diffraction (GID) directly on the water surface. S4 forms two polymorphic crystalline multilayers. In polymorph α, of structure very similar to that of the three‐dimensional solid, the molecules are aligned with their long molecular axis tilted by about 23° from the normal to the water surface. In polymorph β the long molecular axis is perpendicular to the water surface. S5 self‐aggregates at the water surface to form mixtures of monolayers and bilayers of the β polymorph; S6 forms primarily crystalline monolayers of both α and β forms. The crystalline assemblies preserve their integrity during transfer from the water surface onto solid supports. The relevance of the present results for the understanding of the early stages of crystal nucleation is presented.

Phase Transitions in Langmuir−Blodgett Films of Cadmium Stearate: Grazing Incidence X-ray Diffraction Studies

Variations in the structure of multilayer Langmuir-Blodgett (LB) films of cadmium stearate with temperature have been monitored by grazing incidence synchrotron X-ray diffraction. At room temperature the film structure comprises a centered rectangular unit cell in the plane of the substrate with the hydrocarbon chains normal to the substrate surface. As the temperature increases up to 90 degrees C, the diffraction pattern due to this in-plane structure becomes diffuse and the intensity decreases. Changes in the Q-values of the in-plane spots indicate an anisotropic expansion of the unit cell. At around 100 degrees C the film undergoes a phase transition from a layered three-dimensional crystalline structure to an array of rod structures packed hexagonally. The rods are oriented parallel to the surface of the film. The core of each rod is 6.7 Angstrom in diameter and appears to be composed of cadmium cations and the carboxylate groups of the fatty acid chains. The aliphatic chains are randomly arranged around these rods. The rod structure is retained on cooling, but the observed correlation length is reduced.

Separation of Enantiomers and Racemate Formation in Two-Dimensional Crystals at the Water Surface from Racemic α-Amino Acid Amphiphiles: Design and Structure

Studies are presented on the two-dimensional (2-D) crystalline packing arrangements of enantiomerically pure and racemic α-amino acid RHC(NH3+)CO2- monolayers on water and on glycine aqueous solutions, as determined by synchrotron grazing incidence X-ray diffraction. The amphiphiles have been designed such that their racemic mixtures form 2-D crystals which are either heterochiral (for R = CnH2n+1−, n = 10, 12, 16) due to the tendency for herringbone chain arrangements via glide symmetry or homochiral (for R = CnH2n+1CONH(CH2)4−, n = 11, 17, 21) by virtue of hydrogen bonding by translation of the amide group in the chains leading to a spontaneous separation into islands of opposite chirality. The two different crystalline motifs led to a correlation between their packing arrangements and induced oriented nucleation of 3-D crystals of α-glycine by these monolayers. The relevance of the present results to the possibility of ordering and spontaneous segregation of racemates of the natural hydrophobic α-amino acids at the air-solution interface is discussed.

Elucidation of Multilayer Growth of Amphiphiles on Liquid Surfaces

The crystalline properties of the amphiphilic systems N-methylarachidamide (NM) and of N,N‘-dimethyldocosanediamide (NND) in the uncompressed state at 5 °C on surfaces of water and liquid formamide were studied by grazing incidence synchrotron X-ray diffraction (GID). On water, the amphiphile NM forms a crystalline monolayer. On formamide, NM behaves differently. One h after deposition on the liquid surface, GID patterns indicated coexistence of two different crystalline monolayer structures. Three h later, a crystalline bilayer had formed. The fundamental question as to whether the interlayer growth took place in the liquid subphase or in the air was elucidated with the help of the tailor-made additive N-benzylarachidamide (NB). Addition of 10% of NB in the spreading solution yielded only a monolayer. From this result we conclude that bilayer growth of pure NM must occur from the liquid side. It also helps to establish a similar interlayer growth mechanism for the trilayer formation of pure arachidamide ...

Controlling the structure of transparent Langmuir–Blodgett films for nonlinear optical applications

Langmuir–Blodgett (LB) multilayers of (E)-N-alkyl-4-[2-(4-docosyloxyphenyl)ethenyl]pyridinium bromide, CnH2n+ 1—Py+—CHCH—C6H4—O—C22H45Br–(dye I), are centrosymmetric (Y-type) for n⩽ 6 and non-centrosymmetric (Z-type) for 8 ⩽n⩽ 20. The areas obtained from grazing incidence synchrotron X-ray diffraction indicate that the molecules adopt a ‘stretched’ rather than a ‘U-shaped’ configuration and, unlike conventional amphiphilic materials, the long chain homologues do not invert during deposition. The second-harmonic intensity from films of the higher homologues (n 10) increases quadratically with the number of LB layers (I2ω(N)=I2ω(1)N2) whereas for the lower homologues (n⩽ 6) the intensity is negligible for even numbers of layers. The nonlinear optical properties of the sulfur (dye II), selenium (dye III) and quinolinium (dye IV) analogues are also reported and, in each case, quadratic enhancement of the second-harmonic intensity has been observed for suitable combinations of alkyl chain lengths. The second-order susceptibility, χ(2)zzz, of a 100 layer LB film of (E)-N-dodecyl-4-[2-(4-docosyloxyphenyl)ethenyl]quinolinium bromide (dye IV) is 20 pm V–1.

Self-Aggregated Two-Dimensional Crystal Structure of the Mixed Monolayer of Triacontanoic Acid and Nonacosylamine. Evidence in Favor of an Ordered Arrangement of Ionized Head Groups

Grazing incidence synchrotron X-ray diffraction measurements were made on an uncompressed monolayer of 1:1 triacontanoic acid (C29H59COOH) and nonacosylamine (C29H59NH2), spread on pure water and on aqueous ZnCl2 solution. The measured diffraction data of the mixed monolayer on water provides clear evidence of a complete mixing of the acid and amine molecules into a two-dimensional crystal where the hydrocarbon chains pack in a herringbone motif. A model has been proposed in which the ionized acid and amine moieties are ordered and form a continuous hydrogen-bonded array.

Interaction of Phosphotungstate Ions with Phospholipid Monolayers: A Synchrotron X-ray Study

The surface pressure-area isotherms of monolayers of several phospholipids are markedly altered by the presence of phosphotungstate ions in the aqueous subphase at low values of pH (Gorwyn, D.; Barnes, G. T. Langmuir 1990, 6, 222). These changes have now been studied by grazing incidence synchrotron X-ray diffraction. Where the monolayer was in a condensed monolayer state the 2 theta scans revealed a single peak consistent with hexagonal packing of the acyl chains despite the diacyl structure of the phospholipids. Addition of phosphotungstate to the subphase at pH 2 caused this peak to disappear, suggesting a disruption of the regular chain packing, and examination of the concomitant surface pressure-area isotherms showed that there had been a transition from a condensed to an expanded phase.

Control of Structure and Growth of Polymorphic Crystalline Thin Films of Amphiphilic Molecules on Liquid Surfaces

The spontaneous formation and coexistence of crystalline polymorphic trilayer domains in amphiphilic films at air-liquid interfaces is demonstrated by grazing incidence synchrotron x-ray diffraction. These polymorphic crystallites may serve as models for the early stages of crystal nucleation and growth, helping to elucidate the manner in which additives influence the progress of crystal nucleation, growth, and polymorphism and suggesting ways of selectively generating and controlling multilayers on liquid surfaces. Auxiliary molecules have been designed to selectively inhibit development of the polymorphs, leading primarily to a single phase monolayer.

Two-dimensional structures of crystalline self-aggregates of amphiphilic alcohols at the air-water interface as studied by grazing incidence synchrotron x-ray diffraction and lattice energy calculations

The two-dimensional (2D) crystal structures of self-aggregated clusters of amphiphilic alcohols (C n H 2n+1 OH, n=23, 30, 31, and C 19 H 39 CO 2 (CH 2 ) n OH, n=9, 10) on pure water at a temperature of 5 o C have been determined to near-atomic resolution in order to understand the relative abilities of these monolayers to induce ice nucleation. The structures were solved making use of grazing incidence synchrotron X-ray diffraction (GID) data, complemented by lattice energy calculations

Interaction of (1 x 2)-reconstructed Si(100) and Ag(110):Cs surfaces with C60 overlayers.

The structure of the Si(100)-(1\ifmmode\times\else\texttimes\fi{}2) surface, after room-temperature deposition of a thin film of ${\mathrm{C}}_{60}$, has been examined with grazing-incidence synchrotron x-ray diffraction. The (1\ifmmode\times\else\texttimes\fi{}2) surface reconstruction is well preserved under the ${\mathrm{C}}_{60}$ overlayer. The measured fractional-order diffraction intensities, similar to those reported for the clean Si(100)-(1\ifmmode\times\else\texttimes\fi{}2) surface, are analyzed in terms of a dimer model. A similar investigation has been carried out for another (1\ifmmode\times\else\texttimes\fi{}2) reconstructed surface, a Cs-doped Ag(110), to test the generality of the preservation of surface reconstructions under a ${\mathrm{C}}_{60}$ cap layer. In this case, the (1\ifmmode\times\else\texttimes\fi{}2) reconstruction is suppressed by the deposition of ${\mathrm{C}}_{60}$.