The evolution of bicontinuous polymeric nanospheres in aqueous solution

Complex polymeric nanospheres in aqueous solution are desirable for their promising potential in encapsulation and templating applications. Understanding how they evolve in solution enables better control of the final structures. By unifying insights from cryoTEM and small angle X-ray scattering (SAXS), we present a mechanism for the development of bicontinuous polymeric nanospheres (BPNs) in aqueous solution from a semi-crystalline comb-like block copolymer that possesses temperature-responsive functionality. During the initial stages of water addition to THF solutions of the copolymer the aggregates are predominantly vesicles; but above a water content of 53% irregular aggregates of phase separated material appear, often microns in diameter and of indeterminate shape. We also observe a cononsolvency regime for the copolymer in THF-water mixtures from 22 to 36%. The structured large aggregates gradually decrease in size throughout dialysis, and the BPNs only appear upon cooling the fully aqueous dispersions from 35 °C to 5 °C. Thus, the final BPNs are ultimately the result of a reversible temperature-induced morphological transition.

How does dense phase CO2 influence the phase behaviour of block copolymers synthesised by dispersion polymerisation?

Using a CO₂ continuous phase for dispersion synthesis of block copolymers can provide a useful handle to control phase behaviour.

Understanding the unusual reorganization of the nanostructure of a dark conglomerate phase

The dark conglomerate (DC) phase exhibited by a bent-core liquid crystal shows remarkable properties including an electric-field tunable chiral domain structure and a large (0.045) reduction of refractive index, while maintaining an optically dark texture when observed under crossed polarizers. A detailed investigation of the system is presented, leading to a model that is fully consistent with the experimental observations. It reports the observation of two distinct regimes in the DC phase: a higher temperature regime in which the periodicity measured by small angle x-ray scattering decreases slightly (0.5%) and a lower temperature regime where it increases considerably (16%). Also, the paper discusses the unusual electric-field-induced transformations observed in both the regimes. These changes have threshold fields that are both temperature and frequency dependent, though the phenomena are observed irrespective of device thickness, geometry, and the alignment layer. The electro-optic behavior in the DC phase corresponds to a number of structural changes leading to unusual changes in physical properties including a small (1%) increase in periodicity and a doubling of the average dielectric permittivity. We propose a model of the DC phase where in the ground state the nanostructure of the phase exhibits an anticlinic antiferroelectric organization. Under an electric field, it undergoes a molecular rearrangement without any gross structural changes leading to an anticlinic ferroelectric order while keeping the overall sponge-like structure of the DC phase intact.

Bio-inspired CO2conversion by iron sulfide catalysts under sustainable conditions

CO₂conversion to small bio-molecules on greigite minerals under room temperature and pressure.

Doubly periodic instability pattern in a smectic-A liquid crystal

We report the observation of a doubly periodic surface defect pattern in the liquid crystal 8CB, formed during the nematic-smectic-A phase transition. The pattern results from the antagonistic alignment of the 8CB molecules, which is homeotropic at the surface and planar in the bulk of the sample cell. Within the continuum Landau-de Gennes theory of smectic liquid crystals, we find that the long period (≈10 μm) of the pattern is given by the balance between the surface anchoring and the elastic energy of curvature wall defects. The short period (≈1 μm) we attribute to a saddle-splay distortion, leading to a nonzero Gaussian curvature and causing the curvature walls to break up.

Simultaneous, Time-Resolved, Saxs/Waxs Studies on Block Copoly(Ether-Urethane) Phase Behaviour

A new experimental technique for the characterisation of the thermal-morphological properties of materials has been developed at Daresbury. Many thermal events, for example melting endotherms, are signals of changes in morphology covering size scales from the atomic to the microscopic, that is Å to μm. There are obvious advantages in collecting both the wide angle (1-20Å) and small angle (20-1000Å) patterns simultaneously to unambiguously characterise such thermal events. The new apparatus comprises a Linkam hot-stage capable of controlled heating and cooling mounted in a combined SAXS/WAXS camera. The camera is equipped with a multiwire quadrant detector (SAXS) located 3.5 m from the sample position and a curved knifeedge detector (WAXS) that covers 120° of arc at a radius of 0.4 m. SAXSIWAXS is possible with a time resolution of 0.1 seconds and heating/cooling rates up to 120 °C min-1.

Scatter: software for the analysis of nano- and mesoscale small-angle scattering

Scatteris a new software for analysis, modeling and fitting of one- and two-dimensional small-angle scattering data of non-ordered, partially ordered or fully ordered nano- and mesoscale structures. The calculations are based on closed analytical expressions for the scattering intensity, enabling efficient evaluation of form factors and structure factors. The software allows one to sequentially fit large series of scattering curves and scattering patterns automatically. It provides further tools for data loading, beam centering, calibration, zooming, binning, lattice identification, calculation of density profiles and size distributions, and visualization of real-space structures. Presentations of experimental and calculated data can be saved as is for presentations or exported for further graphical or mathematical treatment.

Time-resolved x-ray studies of the dynamics of smectic- A layer realignment by magnetic fields

While the rotation of smectic layers under an applied field may at first appear to be a relatively simple problem, the dynamic processes involved are rather complex. An applied field produces a torque on the liquid crystal director, but has no direct influence on the smectic layers. If the director is reoriented significantly, however, the layers must also reorient in order to accommodate this (the layered structure is produced by short-range molecular interactions). Indeed, if the liquid crystalline order is not maintained during the realignment then matters become even more complex. In this paper we use time-resolved x-ray scattering to investigate the realignment of smectic- A layers in thin-film devices using a magnetic field. No evidence is found for continuous rotation of the smectic layers under any circumstances in such devices, a result that is not found when using bulk samples. No evidence indicating the formation of the nematic phase is observed during realignment. A molecular-dynamics technique is used to model the system which indicates that the sample becomes significantly disorganized during the realignment process when large angular rotations are induced.

Direct Observation of the High Magnetic Field Effect on the Jahn-Teller State in TbVO4

We report the first direct observation of the influence of high magnetic fields on the Jahn-Teller (JT) transition in TbVO(4). Contrary to spectroscopic and magnetic methods, x-ray diffraction directly measures the JT distortion; the splitting between the (311)/(131) and (202)/(022) pairs of Bragg reflections is proportional to the order parameter. Our experimental results are compared to mean-field calculations, taking into account all possible orientations of the grains relative to the applied field, and qualitative agreement is obtained.

Field-induced realignment of a smectic nanodroplet in an external magnetic field: a numerical investigation

The field-induced realignment of a smectic-A phase is in principle a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved X-ray scattering experiments have revealed major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process. In order to obtain a deeper insight into this process, we have carried out a dissipative particle dynamics study of the realignment kinetics of a nanodroplet of a smectic-A liquid crystal suspended in an isotropic fluid following a switch in the direction of an applied magnetic field. The strength of the mesogen-field interaction is small compared to the inter-molecular interactions. The reaction of the smectic configuration to the field switch was found to depend on the balance between the inter-molecular interactions stabilising the formation of the smectic layering and the interaction of the mesogens with the external field. It is found that the rotational behaviour of the smectic layers under the influence of an external magnetic field arises from a combination of stochastic translational displacements and rotational motions of the centres of mass of the mesogens in the nanodroplets. The simulations indicate that X-ray scattering and NMR experiments monitoring the orientational order are sensitive to different aspects of the realignment process.

The structural principles of multidomain organization of the giant polypeptide chain of the muscle titin protein: SAXS/WAXS studies during the stretching of oriented titin fibres

Elasticity of titin is a key parameter that determines the mechanical properties of muscle. These include reversibility, i.e., the muscle's capacity to change its length many-fold and return to its original state, and the transduction of passive tension generated by the stretched muscle. The morphology and elastic properties of oriented fibres of titin molecules were studied using SAXS and WAXS (small- and wide-angle X-ray scattering, respectively) and mechanical techniques. We succeeded in obtaining oriented filaments of purified titin suitable for diffraction measurements. Our X-ray data suggest a model of titin as a nanoscale, morphological, and aperiodical array of rigid Ig- and Fn3-type domains covalently connected by conformationally variable short loops. The line group symmetry of the model can be defined as SM with axial translation tau(infinity). Both tension transduction and high elasticity of titin can be explained in terms of crystalline polymer physics. Titin stretching experiments show that each individual titin macromolecule can adopt a novel two-phase state within the fibre. Conversion between high elasticity and strength can be explained as a phase transition under external tension. In the terms of the concept of orientational melting the origin of the functional heterogeneity along the titin strand becomes interpretable.

Magnetic-field-induced orientational order in the isotropic phase of hard colloidal platelets

The magnetic-field-induced orientational order in the isotropic phase of colloidal gibbsite [Al(OH)3] platelets is studied by means of optical birefringence and small-angle x-ray scattering (SAXS) techniques. The suspensions display field-induced ordering at moderate field strengths (a few Tesla), which increases with increasing particle concentration. The gibbsite particles align their normals perpendicular to the magnetic field and hence possess a negative anisotropy of their diamagnetic susceptibility Delta(chi). The results can be described following a simple, Onsager-like approach. A simplified model is derived that allows one to obtain the orientational distribution function directly from the scattering data. However, it leads to an underestimate of the diamagnetic susceptibility anisotropy Delta(chi). This accounts for the difference between the Delta(chi) values provided by the two experimental techniques (SAXS and magneto-optics). The order of magnitude Delta(chi) approximately 10(-22) J/T(2) lies in between that of goethite suspensions and that of suspensions of organic particles.

A new experimental cell for in situ and operandoX-ray absorption measurements in heterogeneous catalysis

A new X-ray absorption cell dedicated to in situ and operando experiments in heterogeneous catalysis has been built and tested. The cell consists of several boron nitride and stainless steel plates linked together using graphite seals. It allows the measurement of XANES and EXAFS spectra of heterogeneous catalysts within a wide range of photon energies in transmission mode under the flow of various oxidative and reductive gas mixtures at elevated temperatures. The cell is compact and easy to build. Catalysts are loaded into the cell as powders. The use of boron nitride and a small beam pathlength in the cell result in a low absorption of the X-ray beam at lower energies. The cell was tested by in situ characterizing cobalt species during oxidative and reductive pre-treatments of a silica-supported Fischer-Tropsch catalyst. An operando study of methanol conversion over alumina-supported molybdenum catalysts was also carried out.

Responsive brushes and gels as components of soft nanotechnology

Progress in the development of generic molecular devices based on responsive polymers is discussed. Characterisation of specially synthesised polyelectrolyte gels, "grafted from" brushes and triblock copolymers is reported. A Landolt pH-oscillator, based on bromate/ sulfite/ferrocyanide, with a room temperature period of 20 min and a range of 3.1 < pH < 7.0, has been used to drive periodic oscillations in volume in a pH responsive hydrogel. The gel is coupled to the reaction and changes volume by a factor of at least 6. A continuously stirred, constant volume, tank reactor was set-up on an optical microscope and the reaction pH and gel size monitored. The cyclic force generation of this system has been measured directly in a modified JKR experiment. The responsive nature of polyelectrolyte brushes, grown by surface initiated ATRP, have been characterised by scanning force microscopy, neutron reflectometry and single molecule force measurements. Triblock copolymers, based on hydrophobic end-blocks and either polyacid or polybase mid-block, have been used to produce polymer gels where the deformation of the molecules can be followed directly by SAXS and a correlation between molecular shape change and macroscopic deformation has been established. The three systems studied allow both the macroscopic and a molecular response to be investigated independently for the crosslinked gels and the brushes. The triblock copolymers demonstrate that the individual response of the polyelectrolyte molecules scale-up to give the macroscopic response of the system in an oscillating chemical reaction.

Field-induced alignment of a smectic-Aphase: A time-resolved x-ray diffraction investigation

The field-induced alignment of a smectic-A phase is, in principle, a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved nuclear magnetic resonance spectroscopy has revealed this complexity in the case of the director alignment, but provides no direct information on the motion of the layers. Here we describe a time-resolved x-ray diffraction experiment using synchrotron radiation to solve the challenging problem of capturing the diffraction pattern on a time scale which is fast in comparison with that for the alignment of the smectic layers. We have investigated the alignment of the smectic-A phase of 4-octyl-4(')-cyanobiphenyl by a magnetic field. The experiment consists of creating a monodomain sample of the smectic-A phase by slow cooling from the nematic phase in a magnetic field with a flux density of 7 T. The sample is then turned quickly through an angle phi(0) about an axis parallel to the x-ray beam direction but orthogonal to the field. A sequence of two-dimensional small angle x-ray diffraction patterns are then collected at short time intervals. Experiments were carried out for different values of phi(0), and at different temperatures. The results show that the alignment behavior changes fundamentally when phi(0) exceeds 45 degrees, and that there is a sharp change in the alignment process when the temperature is less than 3 degrees C below the smectic-A-nematic transition. The results of the x-ray experiments are in broad agreement with the NMR results, but reveal major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process.

The phase behaviour of skin lipid mixtures based on synthetic ceramides

The lipid lamellae present in the outermost layer of the skin, the stratum corneum (SC), form the main barrier for diffusion of molecules across the skin. The main lipid classes in SC are cholesterol (CHOL), free fatty acids (FFA) and at least nine classes of ceramides (CER), referred to as CER1 to CER9. In the present study the phase behaviour of four synthetic CER, either single or mixed with CHOL or CHOL and FFA, has been studied using small and wide angle X-ray diffraction. The lipid mixtures showed complex phase behaviour with coexistence of several phases. The results further revealed that the presence of synthetic CER1 as well as a proper composition of the other CER in the mixture were crucial for the formation of a phase with a long periodicity, characteristic for SC lipid phase behaviour. Only a mixture containing synthetic CER1 and CER3, CHOL and FFA showed similar phase behaviour to that of SC.

A SAXS/WAXS XAFS study of crystallisation in cordierite glass

New Cr X-ray absorption fine structure (XAFS) data have been combined with the results of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) experiments to probe in detail the crystallisation mechanism in cordierite (Mg2Al4Si5O18) glass doped with 0.34 mol% Cr2O3. By direct comparison with chromo-aluminate spinels (MgCr2xAl2(1 - x)O4) Cr XAFS is used to determine the composition of the devitrified Cr species. This is identified as MgCr(0.18)Al(1.82)O4, which can be directly related to the Cr content in the starting glass and as a result the total crystalline volume in the fully developed ceramic is predicted to be 4%. In situ WAXS not only reveals the presence of the spinel phase but also a silica-rich stuffed quartz phase. This grows independently of the spinel and is probably nucleated from the glass surface. From our knowledge of the compositions of both crystalline phases we are able to deduce that the SAXS contrast between the surrounding glass and the spinel crystallites is 30 times greater than that between the quartz crystallites and the glass matrix, and therefore dominates the measured scattered intensity and the SAXS invariant that is derived from it. As a consequence we are able to show that the spinel crystalline volume fraction inherent in the SAXS is in close agreement with the 4% value obtained from the Cr XAFS. Furthermore in situ SAXS reveals the gradual development of the spinel particle size and shape during heat treatment. This is conducted in the super-cooled region just above the glass transition temperature, Tg. By employing a two-step annealing process nucleation can be separated from growth and from time-resolved SAXS measurements the alumino-chromate nanocrystals are found to be closely monodispersed. Over a total time course of 600 min they grow from rough crystallites to smooth spherical particles of radius 21 +/- 2 nm, with a final density of (1.2 +/- 0.4) x 10(21) m(-3). As the process of ceramic formation takes place in the viscous melt, growth is indeed found to be limited by diffusion and is complete when all the Cr is exhausted. We use this comprehensive in situ study of crystallisation in cordierite glass to demonstrate the advantages of combining SAXS, WAXS and XAFS for probing the time-resolved chemistry, the microstructure and its development from nucleation sites, that underpins the processing of nanoparticle ceramics.

High-resolution small-angle x-ray diffraction study of long-range order in hard-sphere colloidal crystals

The long-range order parameters in single crystals of hard colloidal spheres grown in sediments of colloid-polymer mixtures are determined using synchrotron small-angle x-ray diffraction with a resolution of 10(-6) of the wave vector. The interplanar positional order derived from the width of lattice reflections extends over at least 500 lattice planes. The lattice planes are orientationally correlated within approximately 0.1 degrees throughout the crystals, whereas the stacking of hexagonal planes remains random.

The SAXS/WAXS software system of the DUBBLE CRG beamline at the ESRF

The small/wide-angle X-ray scattering (SAXS/WAXS) system on the DUBBLE CRG beamline at the ESRF is used for both static and time-resolved measurements. The integrated system developed for control and data reduction deals effectively with the high rates of incoming data from the different detector systems, as well as the presentation of results for the user. To ensure that the data may be used directly by a wide range of packages, they may be recorded in a number of output formats, thus serving as a practical test bed where developing standards may be compared and contrasted. The software system implements proposals raised at the canSAS meetings to promote a limited set of standard data formats for small-angle scattering studies. The system presented can cope with a volume of results in excess of 10 Gbytes of data per experiment and shows the advantages achieved by minimizing the dependence on raw-data formats.

Scattering from biopolymers with helical symmetry in solution

Small angle X-ray fibre diffraction on aligned elongated macromolecules has been a routine tool for the structure of materials that either can not be crystallized or in cases in which one wants to study the structure of the molecule as close as possible to those occurring in solution. One of the problems with this approach is that quite often it is not possible to obtain completely aligned samples but only samples in which there is an angular distribution around the main orientation axis. This has as a consequence that atlarger scattering angles the contributions from different layer lines start to overlap. A possible method to overcome this problem can be found in combining results from small angle solution scattering with small angle fibre diffraction from molecules aligned with their long axis parallel with respect to the X-ray beam. In the latter case one only observes the projection of the molecule on the basal plane. We show how this method can be applied to hydrated microtubules.

Force generation and shift of mass between myosin and actin in skinned striated muscle fibres at low calcium concentrations

Skinned muscle fibres from the gracilis muscle of the rabbit were used to record small angle X-ray diffraction spectra under various contractile conditions. The intracellular calcium concentration, expressed as pCa, was varied between 8.0 and 5.74. Equatorial diffraction spectra were fitted by a function consisting of five Gaussian curves and a hyperbola to separate the (1.0), (1.1), (2.0), (2.1) and Z-line diffraction peaks. The hyperbola was used to correct for residual scattering in the preparation. The ratio between the intensities of the (1.1) and (1.0) peaks was defined as the relative transfer of mass between myosin and actin, due to crossbridge formation after activation by calcium. The relation between the ratio and the relative force of the fibre (normalized to the force at pCa 5.74 and sarcomere length 2.0 microns) was linear. At high pCa (from pCa 6.34 to 8.0) no active force was observed, while the ratio still decreased. Sarcomere length was recorded by laser diffraction. The laser diffraction patterns did not show changes in sarcomere length due to activation in the high pCa range (between 8.0 and 6.34). From these results the conclusion is drawn that crossbridge movement occurs even at subthreshold calcium concentrations in the cell, when no active force is exerted. Since no force is generated this movement may be related to crossbridges in the weakly bound state.

The Dutch-Belgian beamline at the ESRF

A brief description is given of the design principles and layout of the Dutch-Belgian beamline at the ESRF. This beamline optimizes the use of the available bending-magnet radiation fan by splitting the beam into two branches, each accommodating two experimental techniques.

Sample environments and techniques combined with small angle X-ray scattering

The number of synchrotron radiation-based Small Angle X-ray Scattering beamlines has increased considerably over the last decade. With the high X-ray flux and collimation of these beamlines it not only has become possible to perform time-resolved experiments on time scales down to the millisecond/frame range, but also it allows experimenters to utilise new sample environments and use simultaneous several experimental techniques on one sample. An overview of recent developments in this field is given.

The susceptibility of pure tubulin to high magnetic fields: a magnetic birefringence and x-ray fiber diffraction study

The orientational behavior of microtubules assembled in strong magnetic fields has been studied. It is shown that when microtubules are assembled in a magnetic field, they align with their long axis parallel to the magnetic field. The effect of several parameters known to affect the microtubule assembly are investigated with respect to their effect on the final degree of alignment. Aligned samples of hydrated microtubules suitable for low-resolution x-ray fiber diffraction experiments have been produced, and the results obtained from the fiber diffraction experiments have been compared with the magnetic birefringence experiments. Comparisons with earlier fiber diffraction work and small-angle x-ray solution scattering experiments have been made.

Time-Resolved Small-Angle X-ray Scattering Combined with Wide-Angle X-ray Scattering

The high X-ray intensity of synchrotron radiation (SR) beamlines makes it possible to perform time-resolved small-angle X-ray scattering (SAXS) experiments. The information that can be obtained by collecting the wide-angle diffraction pattern simultaneously not only increases the information content of an experiment but also increases the reliability of the time-correlations between SAXS and WAXS (wide-angle X-ray scattering) patterns. This is a great advantage for experiments with a time resolution below the level of 1 s per frame. With appropriate instrumentation, this is a time domain that is routinely accessible for a large group of research fields. This has had a considerable impact upon the understanding of fundamental aspects of phase transformations. Not only fundamental processes but also more applied fields have benefited from these developments. In polymer research this has led to a situation in which it has become possible to simulate materials processing techniques on-line. With the advent of third-generation synchrotron-radiation sources (e.g. ESRF, APS, Spring8), it has become possible to develop SAXS/WAXS beamlines that will open up new research opportunities by utilizing the higher intensity, the tuneability and the higher collimation offered by these SR sources. However, some of the instrumentation limits in detector and sample environments that have become apparent in research on second-generation synchrotron-radiation sources still have not been appropriately addressed, which means that in some fields it will not be possible to take full advantage of the superior X-ray beam quality that third-generation synchrotrons can offer. A way in which these instrumentation limits can be overcome is discussed, and the instrumentation for a new bending-magnet beamline at the ESRF is used as an example.

A combined SAXS/WAXS investigation of the phase behaviour of di-polyenoic membrane lipids

Real-time measurements of the SAXS/WAXS diffraction patterns of aqueous dispersions (1:1 wt/wt) of the di-polyenoic lipids di-18:2 PC, di-18:3 PC, di-18:2 PE and di-18:3 PE were made over the temperature range 10 degrees to about -80 degrees C. The results of these measurements were compared to similar measurements performed on the corresponding di-18:0 and di-18:1 derivatives. SAXS measurements of the temperature dependence of lamellar repeat distances show that the di-polyenoic lipids undergo broad second-order transitions between their gel and liquid-crystal lamellar phases spanning 30-40 degrees C. The di-18:1 and di-18:0 derivatives, in contrast, undergo abrupt first-order transitions. The gel phases of the di-18:0 derivatives are characterised by two-component WAXS patterns with a sharp component close to 0.42 nm and a broader component at narrower spacings. On cooling, these lipids appear to undergo an initial transition to an L beta, phase followed by a conversion to an Lc phase. The gel phases of the di-18:1 derivatives also show two-component patterns but with the sharp component centred closer to 0.44 nm. The di-polyenoic lipids, in contrast, are characterised by a single broad peak centred at a spacing of about 0.42 nm, close to that of conventional L beta phases. The changes in lamellar repeat distance accompanying the transitions in the di-monoenoic and di-polyenoic lipids, all of which occur in the frozen state, are very similar, indicating that the acyl chains of the polyenoic lipids are close to their maximum extension in the gel state. The WAXS patterns of the polyenoic lipids suggest that the saturated upper parts of the acyl chains are packed on a regular hexagonal lattice while their polyunsaturated termini remain relatively disordered.

Phase behavior of isolated skin lipids

Ceramides were isolated from the pig stratum corneum (SC) and mixed in varying molar ratios with either cholesterol or with cholesterol and free fatty acids. The phase behavior of the mixtures was studied by small-(SAXD) and wide-angle (WAXD) X-ray diffraction. Ceramides alone did not exhibit a long range ordering. Upon addition of cholesterol to ceramides, lamellar phases were formed and a hexagonal lateral packing was detected similar to that seen in intact SC. At a cholesterol/ceramide molar ratio of 0.1, only one reflection at 5.9 nm was observed. At a cholesterol/ceramide molar ratio of 0.2, three reflections corresponding to 12.3, 5.56, and 4.26 nm appeared. The reflections were based on two phases. Increasing the cholesterol/ceramide ratio to 0.4, the peak positions were slightly shifted. The diffraction pattern revealed the presence of two lamellar phases with periodicities of 12.2 and 5.2 nm, respectively. The positions of the peaks remained unchanged when the cholesterol/ceramide ratio was increased up to 1.0. At a cholesterol/ceramide molar ratio of 2.0, the intensity of various peaks based on the 12.2 nm phase decreased in intensity. The phase behavior of the cholesterol/ceramide mixtures in a ratio between 0.4 and 1.0 was very similar to that found in intact pig SC in which two lamellar phases with periodicities of 6.0 and 13.2 nm are present. Our data further indicate that the formation of the 5.2 nm lamellar phase requires a higher cholesterol content than the formation of the 12.2 nm lamellar phase. Furthermore, when the relative amount of cholesterol is very high, the 5.2 nm phase is the most pronounced one. Addition of free fatty acids increased the solubility of cholesterol, indicating the role free fatty acids may play for the skin barrier function. The phase behavior of cholesterol/ceramide/fatty acid mixtures was found to be dependent on the chain length of fatty acids used. Namely, addition of short-chain free fatty acids (C14-C18) did not change the periodicity of the 12.2 and 5.2 nm phases, but induced the formation of an additional 4.2 nm phase. In the presence of long-chain free fatty acids (C16-C26), the periodicity of the lamellar phases was slightly increased (to 13.0 and 5.3 nm, respectively) but no additional 4.2 nm phase was formed. These results indicate that the lipid phase behavior of the cholesterol/ceramide/free fatty acid mixtures closely mimics that of the intact stratum corneum only in the presence of long-chain free fatty acids.

Lipid organization in pig stratum corneum

The lipid and keratin structure of pig stratum corneum has been elucidated by small- and wide-angle X-ray diffraction. The measurements were carried out as a function of hydration and temperature. In addition, the stratum corneum was measured after recrystallization of the lipids at various temperatures. The results led us to conclude that the intercellular lipids in the stratum corneum are organized in at least two different lamellar structures with repeat distances of 6 and 13.2 nm. There is an indication for the presence of a third phase with a periodicity of 9 nm. The wide-angle pattern revealed a hexagonal (0.414 nm spacing) and liquid lateral packing (approximately 0.46 nm spacing). The 0.414 nm reflection started to decrease in intensity between 60 and 66 degrees C and disappeared between 72 and 95 degrees C. Furthermore, crystalline cholesterol has been indicated by both, wide- and small-angle X-ray diffraction, while the reflections of alpha-keratin were observed in the wide-angle X-ray diffraction pattern.

Real-time simultaneous wide- and small-angle fibre diffraction

A combination of two independent imaging area-detector systems controlled by a single data-acquisition system, provides a powerful system for X-ray diffraction studies of time-resolved phenomena over a wide q range, in samples with intrinsic or induced structural orientation. With this system we have observed a transient, tensile-stress-induced, orthorhombic-to-monoclinic transition in high-density polyethylene.