Contrast Variation Method Research Articles

Agarose Sols and Gels Revisited

AbstractAgarose sols have been seen for long as solutions of flexible chains that, on cooling, produce thermoreversible gels through double‐helix formation. Investigations of the chain conformation in the sol state by small‐angle neutron scattering reveals instead a rigid chain with a very large persistence length (lp > 9 nm). The chain cross‐section radius and mass per unit length correspond to characteristics of helices as those described by Foord and Atkins. These results lead one to a reappraisal of the occurrence of double helices in the gelation process, as they rather suggest a transition of the type loose‐single helix⇒tight single helix. Studies of gels from agarose/water/cosolvent where the cosolvent is Dimethyl Sulfoxide (DMSO), Dimethyl Formamide (DMF), and Methyl Formamide (MF) have led one to conclude on the formation of agarose/water/ cosolvent ternary complexes. The contrast variation method by neutron scattering gives further support to this assumption. Finally, determination of the gel nanostructure allows one to account for the two regimes observed for the variation of the elastic modulus vs concentration.

Crystallographic study and comparisons of compoundsA2Sr2B2C2Ox

It is sometimes difficult and/or time-consuming to obtain crystals of suitable size for single crystal X-ray diffraction measurements. However if a microcrystalline material can be obtained, it is still possible to perform powder diffraction experiments. Although the collapse of the three-dimensional reciprocal space into a one-dimensional powder diffraction pattern gives rise to a severe loss of information, the availability of high-resolution powder diffractometers and synchrotron sources enables the recording of high-quality powder patterns from proteins. From these spectra, a set of valid intensities can be extracted and/or Rietveld refinement can be carried out [1-2]. It is thus possible to investigate the binding of ligands or heavy atoms to biological macromolecules [3]. We present here results obtained recently in this field. Emphasis will be put on low-resolution phase determination by the methods of isomorphous replacement and contrast variation. By using extracted intensities from a series of powder diffraction patterns, the location of heavy-atom is possible and a low resolution structure envelope could even been obtained for some small proteins. We present here data for PPE (porcine pancreatic elastase) in which the radiation-induced anisotropic lattice expansion was used as a way to uncorrelate the intensities of overlapping peaks. On the other hand, contrast variation by solvent exchange is commonly used in small-angle scattering studies, both with X-rays and neutrons, and allows ab initio phasing andmolecular envelope determination [2]. We present here our first results on using the solvent contrast variation method in powder diffraction studies.

Structure of raft-model membrane by using the inverse contrast variation neutron scattering method

By means of the inverse contrast variation method in small-angle neutron scattering, we have studied the structure of a small unilamellar vesicle (SUV) composed of ganglioside, cholesterol and dipalmitoyl-phosphocholine. The SUV treated has a similar lipid composition as in a plasma membrane with microdomains, so-called rafts. The present results indicate an asymmetric distribution of lipid components within the bilayer of the vesicle, that is, a predominant distribution of ganglioside and cholesterol at the outer leaflet of the vesicle bilayer. The deviation from the linearity in a pseudo-Stuhrmannplot strongly suggests the presence of a large heterogeneity of lipid composition in a bilayer, namely a clustering of ganglioside and cholesterol molecules. This deviation is enhanced by temperature elevation, meaning that ganglioside–cholesterol clusters become larger with holding liquid-ordered ( L o) phase.

Diffusive solvent dynamics in a polymer gel electrolyte studied by quasielastic neutron scattering

A quasielastic neutron scattering study has been performed on a polymer gel electrolyte consisting of lithium perchlorate dissolved in ethylene carbonate/propylene carbonate and stabilized with poly(methyl methacrylate). The dynamics of the solvent, which is crucial for the ion conduction in this system, was probed using the hydrogen/deuterium contrast variation method with nondeuterated solvent and a deuterated polymer matrix. Two relaxation processes of the solvent were studied in the 10-400 microeV range at different temperatures. From analysis of the momentum transfer dependence of the processes we conclude that the faster process ( approximately 100 microeV) is related to rotational diffusion of the solvent and the slower process ( approximately 10 microeV) to translational diffusion of the solvent. The translational diffusion is found to be similar to the diffusion in the corresponding liquid electrolyte at short distances, but geometrically constrained by the polymer matrix at distances beyond approximately 5 A. The study indicates that the hindered diffusion of the solvent on a length scale of the polymer network interchain distance ( approximately 5-20 A) is sufficient to explain the reduced macroscopic diffusivity and ion conductivity of the gel electrolyte compared to the liquid electrolyte.

Structure of Hydrogen and Hydrophobically Bonded Amphiphilic Copolymer with Poly(methacrylic acid) Complexes as Revealed by Small Angle Neutron Scattering

AbstractWe study by SANS the structure of intermolecular complexes formed through hydrogen bonding and hydrophobic interactions between poly(methacrylic acid) (PMA) and a neutral copolymer surfactant (PEO‐PPO‐PEO). The contrast variation method enables us to probe the structure factor of each polymer in the complex and their cross structure factor. The number of copolymer chains, which results from the cooperative action of hydrogen bonding and hydrophobic interactions increases as the charge of the polyacid decreases. The aggregation preserves the micellar core‐corona organization of the copolymer and shrinks the polyacid chains which adopt a similar compact structure. Finally, the structure of the aggregates is compared to that of PEO‐PMA homopolymer complex observed by SANS.

Analysis of the spatial structure of rigid polyphenylene dendrimers by small-angle neutron scattering

The analysis of the spatial structure of a rigid polyphenylene dendrimer G4-M of fourth generation by small-angle neutron scattering (SANS) is presented. This dendrimer is composed of phenyl units and is therefore devoid of any flexible unit. The scattering intensity of dilute solutions of the dendrimer was measured by SANS at different contrast which was adjusted by mixtures of protonated and deuterated toluene. Hence, the method of contrast variation could be applied and the data yield the scattering function extrapolated to infinite contrast. The comparison of this data with simulations demonstrates that the scaffold of the dendrimer is rigid as expected from its chemical structure. The positions of the various units setting up consecutive shells of the dendrimer are relatively well localized and the entire structure cannot be modeled in terms of spherically symmetric models. No backfolding of the terminal groups can occur and the model calculations demonstrate that higher generations of this dendritic scaffold must exhibit a dense shell and a congestion of the terminal groups. This finding is directly corroborated by recent solid-state NMR data. All results show that the rigid dendrimer investigated here presents the first example for a dendritic structure whose segment density does not have its maximum at the center. Rigid scaffolds are therefore the only way to achieve the goal of a “dense-shell” dendrimer whereas flexible scaffolds leads invariably to the “dense-core” case.

SANS analysis of perfluorinated lyotropic lamellar phase—ion condensation effect

In charged systems, the counterion distribution is an important information to understand their equilibrium structure. Small angle neutron scattering used with a contrast variation method is a suitable technique to analyze this distribution in aqueous and confined media. We had applied this method to Nafion membranes, a complex ionomer and disordered charged system. In this paper, the same approach is used for a lyotropic liquid crystal (LC) system: a lamellar phase of perfluoropolyether surfactants. As for Nafion system, we show strong counterion condensation at the surfactant–water interface that depends on the long-range period characterizing such an LC system.

An analysis of calcium carbonate/polymer hybrid crystals applying contrast variation SANS

The geometry of calcium carbonate (CaCO 3)/polymer hybrid crystals was investigated by means of the contrast variation small angle neutron scattering. Our sophisticated contrast variation method led to decomposition of the measured scattering intensities into partial scattering functions of each component. These decomposed partial scattering functions gave detailed information on each component in the hybrid particle. Especially, on the basis of the Babinet principle (or incompressibility hypothesis), the comparison of the cross terms led to the relationships of each scattering amplitude. In this way, we could determine the geometry of the hybrid crystals in detail.

PNIPAM-co-polystyrene Core−Shell Microgels: Structure, Swelling Behavior, and Crystallization

The present contribution presents the single-step preparation and characterization of poly(N-isopropyl acrylamide)-co-polystyrene core-shell microgels with varying polystyrene content. The swelling behavior of the particles is investigated using dynamic light scattering and differs significantly from the swelling behavior of poly(N-isopropyl acrylamide) homopolymer particles. The lower critical solution temperature is found to be shifted to lower temperatures upon increasing the polystyrene content of the particles. The core-shell structure of the particles is revealed by means of small angle neutron scattering (SANS) using the method of contrast variation. Additionally, the formation of mesoscopic crystals of these particles is investigated by means of scanning electron microscopy and also by SANS. The particles seem to have preferable properties with respect to crystallization compared to homopolymer microgels.

Determination of Asymmetric Structure of Ganglioside-DPPC Mixed Vesicle Using SANS, SAXS, and DLS

Functions of mammalian cell membrane microdomains being rich in glycosphingolipids, so-called rafts, are now one of the current hot topics in cell biology from the intimate relation to cell adhesion and signaling. However, little is known about the role of glycosphingolipids in the formation and stability of the domains. By the use of the inverse contrast variation method in small-angle neutron scattering (SANS), combined with small-angle x-ray scattering (SAXS) and dynamic light scattering (DLS), we have determined an asymmetric internal structure of the bilayer of the small unilamellar vesicle (SUV) of monosialoganglioside (GM1)-dipalmitoylphosphatidylcholine (DPPC) mixture ([GM1]:[DPPC]=0.1:1). A direct method using a shell-model fitting with a size distribution function describes consistently all experimental results of SANS, SAXS, and DLS. We have found that GM1 molecules predominantly localize at SUV outer surface to form a highly hydrophilic layer which is dehydrated with the rise of temperature from 25°C to 55°C accompanied by the conformational change of the oligosaccharide chains. The average SUV size determined is ∼200Å, which is comparable to the reported value 260±130Å of glycosphingolipids microdomains. The present results suggest that the preferential asymmetric distribution of gangliosides is essential to define the size and stability of the domains.

Self-Assembly at High Pressures: SANS Study of the Effect of Pressure on Microstructure of C8E5 Micelles in Water

We present the results of a high-pressure small-angle neutron scattering study of the effect of pressure on surfactant microstructure. The study was carried out on a solution of 1 wt % C8E5 in D2O at 29.4 °C and pressures up to 310 MPa. The C8E5 micelles that form under these conditions are noninteracting. We find that applying pressure leads to a pronounced decrease in the micelle radius of gyration and the forward scattering intensity over the pressure range from ambient to 150 MPa. The partial molecular volume of the surfactant and the extent of hydration of the surfactant head groups in the micelle were also determined using the method of solvent contrast variation. Both quantities decrease with the application of pressure up to 150 MPa. Core−shell model fits to the scattering spectra over the entire q-range indicate that the shell radius decreases, while the hydrophobic core radius increases slightly with pressure. The pressure dependence of the shell radius is notably similar to that observed for the radius of gyration. Collectively, these observations lead to the conclusion that the effect of pressure on C8E5 micellization is to induce the dehydration of surfactant head groups and the collapse of the hydrophilic micelle shell at pressures between ambient and 150 MPa.

Benefits of polarized small-angle neutron scattering on magnetic nanometer scale structure modeling

Recent use of polarized neutron technique in small-angle scattering (SANS) have led to impressive results in the case of magnetic nanometer-scale structure analysis. In some particular cases this method offers the possibility to survey structure models with the necessary accuracy for the first time. The different cross sections for spin-up and spin-down neutron scattering on magnetic precipitates can be combined with the method of chemical contrast variation. All data fitting using structure models will benefit of that kind of constraints. The analysis of the interference term of nuclear and magnetic scattering respectively enables the extraction of additional information about the composition and magnetization profiles of the samples. Here we place emphasis on the difference of spin-up and spin-down neutron scattering intensities to obtain this information. This technique profits by the clear distinction between magnetic and nonmagnetic scattering contributions and the strong auxiliary conditions for model fitting procedures. Depending on the relative orientations of the external magnetic field, the local magnetization of the precipitates and the scattering vector, significant scattering patterns can be scrutinized. Beside general formulas for some special cases of present experimental interest we exercise the approach to a nontrivial case of data obtained from polarised SANS experiments at the Berlin Neutron Scattering Center (BENSC).

Neutron scattering of colloidal particle dispersions; contrast variation with homogeneous and granular solvents

Abstract The well known method of contrast variation is based on the assumption that the low-molecular solvent may be considered as a homogeneous background. The colloidal dispersion is described as a quasi- one component system. In this paper this assumption is analyzed in detail. The system is described as a two-component one and use has been made of our recently derived new equation for the scattering of mixtures. It is found that when solvents are condensed liquids, this assumption is accurate when the colloid particles are not too small, say larger than 3 nm. In this limiting case the partial structure factors colloid–solvent, S12, and solvent–solvent, S22, are dependent on the partial structure factor of the colloid, S11. When the solvent compressibility is not negligible, or when extra components—such as depletion agents are present, deviations occur. This last situation is considered in detail for a special type of depletion agent of which the size may be comparable to that of the colloid particles. It is found that the usual contrast variation procedure cannot be applied unless the depletion agent is fully matched with the low-molecular solvent. When this is not the case the procedure must be adapted with the bonus that the (often negative) adsorption of the depletion agent by the colloid can be obtained. Two examples from the literature are given of light scattering from mixtures of colloids and polymers, where both components contribute to the scattering. This case, however, is in fact limited to low K. For higher K, SANS or SAXS experiments are needed. As a special feature it is revealed how the thermodynamics-of-mixing properties obtained from scattering can be exactly expressed in the total- and the osmotic compressibilities.

New Synthetic Oligoamide Gelators: Structural Study by X-ray and Neutron Scattering

The nanostructures of fibrillar organogels made from complex triamide and tetramide were investigated by small-angle X-ray and neutron scattering. In the explored concentration c-range and scattering vector q-range, it is mainly the form factors of the fibers that are measured and information about the average shape of their cross section that is gained. The fibers were found to have a cross section of characteristic average size close to 80 Å and a crystalline internal structure. However, they can be considered as homogeneous particles for a spatial resolution of 20 Å, as shown by using the contrast variation method with neutron scattering. Eventually, the q-dependence of the form factors in the Porod limit can be accounted only by introducing anisotropy in the cross section shape. For the tetramer, this anisotropy was in agreement with the crystalline order of the fibers observed in the solid state.

Self-Assembly of Polyelectrolyte Rods in Polymer Gel and in Solution: Small-Angle Neutron Scattering Study

Self-aggregation of rigid-rod poly(sodium p-phenylenesulfonate) in aqueous solution and inside water-swollen polyacrylamide gel was studied by small-angle neutron scattering. It was shown that both inside the hydrogel and in solution polyelectrolyte rods self-assemble into cylindrical aggregates having eight to nine single polymer chains in the cross-section, the chains being aligned parallel to the axis of the aggregate. The length of these aggregates is much higher than the contour length of a single chain. Gels with embedded rods were studied by contrast variation method in order to examine separately the scattering by the gel and by the rods. Two important observations were made. First, it was shown that the ordering of the rods in the gel resembles that in solution. Second, it was shown that the gel itself is more homogeneous in the presence of rods. Most probably, this effect is due to mobile counterions of rods, which counteract the formation of spatial inhomogeneities in the network during synthesis, because in an inhomogeneous network mobile counterions should be also distributed nonuniformly that is associated with significant translational entropy losses.

Evidence of Elongated Polymeric Aggregates in Nafion

Small-angle X-ray and neutron-scattering techniques have been used to probe the structure of swollen Nafion membranes in the range 10−10000 A. From analyzing the scattering data as a function of the polymer volume fraction and using a contrast variation method for the neutron experiments, we suggest a new structural model of Nafion in the hydrated state. It is based on the aggregation of the ionomer chains into elongated polymeric bundles with a diameter on the order of 40 A and a length larger than 1000 A, surrounded by the electrolyte solution.

A New Insight into Nafion Structure

The structure of hydrated Nafion membranes neutralized by N(CH3)4+ ions has been investigated by the small angle neutron scattering technique. First, the spectra show that the counterions are condensed at the interface between hydrophilic and hydrophobic phases. More importantly, by applying a contrast variation method, the normalized scattered intensity of membranes swollen in several D2O/H2O mixtures was analyzed. The N(CH3)4+ counterions were used as a probe to identify the nature of the scattering entity that is polymeric aggregates surrounded by ionic groups and water molecules, contrary to the generally accepted model. This new insight into the Nafion structure will allow one to reconsider the swelling process and the degree of mesoscopic orientation in such perfluorinated systems.

Structure of Physically Adsorbed Polymer Layers Measured by Small-Angle Neutron Scattering Using Contrast Variation Methods

The scattering from poly(ethylene oxide) adsorbed on polystyrene latex particles was measured as a function of the isotopic composition of the aqueous solvent. Oscillations in the data were observed even when the particle was matched in scattering length density to the solvent. These oscillations are a new and important characteristic of the scattering from an adsorbed polymer layer compared to the scattering from free chains. The contrast variation approach allowed the scattering from the interference between the layer and the particle to be measured explicitly. The data have been fitted using a model function to find the volume fraction profile of the adsorbed layer without the complications that can arise from density-fluctuation contributions to the scattering. It was therefore possible to obtain the fluctuation contribution itself by simple subtraction. The fluctuation scattering was found to be in good agreement with scaling predictions and with a new function derived for an exponential volume fract...

Contrast variation methods in small-angle neutron scattering

Contrast variation methods in small-angle neutron scattering

Counterion distribution in a spherical charged sparse brush

The counterion distribution within a spherical polyelectrolyte sparse brush was measured by small-angle X-ray scattering using contrast variation with different counterions by means of ion dialysis. The brush was made by self-association of charged diblock copolymers. Thanks to the contrast variation method, we were able to separate the signal due to the monomers and the signal due to the counterions. At a small length scale, it is demonstrated that the system behaves as independent charged rods whose counterion distribution follows the Poisson-Boltzmann model.