Small-angle Neutron Scattering Spectra Research Articles

SANS Spectra with PLUMED: Implementation and Application to Metainference.

Using small-angle scattering with either X-ray or neutron sources has become common in the investigation of soft-matter systems. These experiments provide information about the coarse shape of the scattered objects, but obtaining more-detailed information can usually only be achieved with the aid of molecular simulations. In this Application Note, we report the implementation of an extension in PLUMED to compute the small-angle neutron scattering (SANS), which can be used for data processing as well for enhanced sampling, in particular with the metainference method to bias simulations and sample structures with a resulting spectrum in agreement with an experimental reference. Our implementation includes a resolution function that can be used to smear the SANS intensities according to beamline error sources and is compatible with both all-atom and coarse-grained simulations. Scripts to aid in the calculation of the scattering lengths when the system is coarse-grained and to aid in preparing the inputs are provided. We illustrate the use of the implementation with metainference by performing coarse-grained simulations of beta-octylglucoside and dodecylphosphocholine micelles in water. With different software and different Hamiltonians, we show that the metainference SANS bias can drive micelles to be split and to change shapes to achieve a better agreement with the experimental reference.

Phase behavior and structure of a biocompatible microemulsion based on Tween 20, 2-ethylhexylglycerine and isopropyl palmitate in water

Microemulsions are thermodynamically stable mixtures of water and oil. For many applications, such as in cosmetics, it is important that the components are biocompatible. For the formulation of a new biocompatible microemulsion based on Tween 20 (Tw20) surfactant, the glycerol ether 2-ethylhexylglycerin (EHG) is employed as an alternative to commonly used short- to medium chain alcohols as cosurfactant. Tw20 itself is not capable of any significant solubilization but the addition of EHG allows to enhance the solubilization capacity for the polar oil isopropyl palmitate (IPP) largely. At a Tw20 to EHG molar ratio of 1:12, i.e., very large excess of cosurfactant, the solubilization capacity is increased by nearly three orders of magnitude compared to a pure Tw20 solution. The resulting oil-in-water (O/W) microemulsion is studied using static and dynamic light scattering (SLS/DLS) and small-angle neutron scattering (SANS). The SANS spectra are analyzed with a core–shell ellipsoid model, which indicates formation of oblate droplets at higher EHG content. Dynamic light scattering experiments confirm the presence of microemulsion droplets with hydrodynamic radii increasing from 3.8 to 19.8 nm upon addition of cosurfactant. In summary, this study shows that EHG is a cosurfactant that enhances the solubilization properties of a biocompatible surfactant largely and thereby renders it very useful for formulations.

Shape and Structure Formation of Mixed Nonionic-Anionic Surfactant Micelles.

Aqueous solutions of a nonionic surfactant (either Tween20 or BrijL23) and an anionic surfactant (sodium dodecyl sulfate, SDS) are investigated, using small-angle neutron scattering (SANS). SANS spectra are analysed by using a core-shell model to describe the form factor of self-assembled surfactant micelles; the intermicellar interactions are modelled by using a hard-sphere Percus–Yevick (HS-PY) or a rescaled mean spherical approximation (RMSA) structure factor. Choosing these specific nonionic surfactants allows for comparison of the effect of branched (Tween20) and linear (BrijL23) surfactant headgroups, both constituted of poly-ethylene oxide (PEO) groups. The nonionic–anionic surfactant mixtures are studied at various concentrations up to highly concentrated samples ( ≲ 0.45) and various mixing ratios, from pure nonionic to pure anionic surfactant solutions. The scattering data reveal the formation of mixed micelles already at concentrations below the critical micelle concentration of SDS. At higher volume fractions, excluded volume effects dominate the intermicellar structuring, even for charged micelles. In consequence, at high volume fractions, the intermicellar structuring is the same for charged and uncharged micelles. At all mixing ratios, almost spherical mixed micelles form. This offers the opportunity to create a system of colloidal particles with a variable surface charge. This excludes only roughly equimolar mixing ratios (X≈ 0.4–0.6) at which the micelles significantly increase in size and ellipticity due to specific sulfate–EO interactions.

Determining population densities in bimodal micellar solutions using contrast-variation small angle neutron scattering.

Self-assembly of amphiphilic polymers in water is of fundamental and practical importance. Significant amounts of free unimers and associated micellar aggregates often coexist over a wide range of phase regions. The thermodynamic and kinetic properties of the microphase separation are closely related to the relative population density of unimers and micelles. Although the scattering technique has been employed to identify the structure of micellar aggregates as well as their time-evolution, the determination of the population ratio of micelles to unimers remains a challenging problem due to their difference in scattering power. Here, using small-angle neutron scattering (SANS), we present a comprehensive structural study of amphiphilic n-dodecyl-PNIPAm polymers, which shows a bimodal size distribution in water. By adjusting the deuterium/hydrogen ratio of water, the intra-micellar polymer and water distributions are obtained from the SANS spectra. The micellar size and number density are further determined, and the population densities of micelles and unimers are calculated to quantitatively address the degree of micellization at different temperatures. Our method can be used to provide an in-depth insight into the solution properties of microphase separation, which are present in many amphiphilic systems.

Polymer Chain Behavior in Polymer Nanocomposites with Attractive Interactions.

Chain behavior has been determined in polymer nanocomposites (PNCs) comprised of well-dispersed 12 nm diameter silica nanoparticles (NPs) in poly(methyl methacrylate) (PMMA) matrices by Small-Angle Neutron Scattering (SANS) measurements under the Zero Average Contrast (ZAC) condition. In particular, we directly characterize the bound polymer layer surrounding the NPs, revealing the bound layer profile. The SANS spectra in the high-q region also show no significant change in the bulk polymer radius of gyration on the addition of the NPs. We thus suggest that the bulk polymer conformation in PNCs should generally be determined using the high q region of SANS data.

Analysis of organic-coated steels by using small-angle neutron scattering

Small angle neutron scattering (SANS) was applied to analyze the degradation of an organic coating layer on steel and for the non-destructive evaluation of the corrosion behavior of the coated steels. The steel was a modified AISI 1004 steel with 1.2% aluminum and 0.8% chromium to improve corrosion resistance. The organic coating layer was epoxy primer, which was painted with about 250-μm thickness on the steel. The organic-coated specimen was subjected to wet-and-dry cyclic corrosion conditions for 112 days in artificial sea water at 40°C. The corrosion potential and the corrosion rate of the steel were −1.1 VSHE and 8.8 × 10−5 A/cm2, respectively. SANS spectra showed that nano-sized defects were not formed in the organic layers after the 112-day wet-and-dry corrosion test. Artificial cracks of the organic layer by X-cutting made corrosion spots of submicron size after a 2-day wet-and-dry corrosion test, which were monitored by SANS.

Water management in proton exchange membrane fuel cell at sub-zero temperatures: An in operando SANS-EIS coupled study

The water management in a proton exchange membrane fuel cell (PEMFC) was studied in operando at sub-zero temperatures combining Small-Angle Neutron Scattering (SANS) and Electrochemical Impedance Spectroscopy (EIS) techniques. The two main processes which control the water repartition within the fuel cell, namely the electro-osmosis and the back-diffusion of water in the membrane can be separated by the decrease of the operating temperature along with the use of a thick membrane compared to state-of-the-art material. It is shown that an increase of the current load at sub-zero temperature induces reversible membrane dehydration despite a higher production of water. This effect is revealed by the very good correlation between the increase of the membrane resistance measured by EIS and the decrease of water content in the membrane quantified from the SANS spectra. The predominance of the electro-osmosis on the back-diffusion, in addition to water accumulation, most probably in supercooled state, in the channel and in the gas diffusion layer, especially at the cathode side, could explain the difficulty to operate a fuel cell at sub-zero temperature.

Aggregation Properties of Amide Bearing Cleavable Gemini Surfactants by Small Angle Neutron Scattering and Conductivity Studies

The micellar aggregation of different amide bearing cleavable gemini surfactants with varying methylene spacer chain length (m = 2, 4, 6, 8, and 12) along with the corresponding monomeric surfactant in aqueous media has been investigated by conductometric and small angle neutron scattering (SANS) studies. The critical micellar concentration (CMC) values of gemini surfactants were found to be very low (CMC = 0.08-0.19 mM) and were 23-55 times lesser than the corresponding monomeric analogue (CMC = 4.4 mM). With increase in the spacer chain length, CMC was found to decrease whereas the degree of ionization was found to increase. SANS data have been analyzed by considering the screened coulombic interactions between the micelles to compute the interparticle structure factor S(Q). The extent of micellar growth and the variation of shapes of the micelles formed by these new surfactants in aqueous solution have been found to depend strongly on the spacer chain length. It was observed that the extent of micellar growth and variation of micellar shapes are more pronounced for surfactants with short spacer chain length (m ≤ 4), whereas the surfactants with a long spacer chain length (m ≥ 6) showed slight variation of these properties in aqueous solution. The effects of the variation of the concentration and temperature on the SANS spectra (and hence on the microstructure) of the gemini surfactant (m = 4) were also examined. With an increase in concentration the aggregation number (N) and size of the micelles (the ratio of semimajor axis (a) to semiminor axis (b = c)) increased whereas opposite phenomena was observed with an increase in temperature.

The kinetics of water sorption in Nafion membranes: a small-angle neutron scatteringstudy

The optimization of the water management in proton exchange membrane fuel cells is amajor issue for the large-scale development of this technology. In addition to the operatingconditions, the membrane water sorption and transport processes obviously control thewater management. The main objective of this work is to provide new experimentalevidence based on the use of the small-angle neutron scattering (SANS) technique in orderto allow a better understanding of water sorption processes. SANS spectra were recordedfor membranes equilibrated with either water vapor or liquid. Sorption kinetics data weredetermined and the SANS spectra were analyzed using the method developed forextracting water concentration profiles across the membrane in operating fuelcells. The water concentration profiles across the membrane are completely flat,which indicates that the water diffusion within the membrane is not the limitingprocess. This result provides new insight into the numerous data published on theseproperties. For the first time, the swelling kinetics of a Nafion membrane immersed inliquid water is studied and a complete swelling is obtained in less than 1 min.

Non-Destructive Evaluation of Nano-Sized Structure of Thin Film Devices by Using Small Angle Neutron Scattering

Nano-sized multi-layers copper-doped SrZrO3, platinum (Pt) and silicon oxide (SiO2) on silicon substrates were prepared by dense plasma focus (DPF) device with the high purity copper anode tip and analyzed by using small angle neutron scattering (SANS) to establish a reliable method for the non-destructive evaluation of the under-layer structure. Thin film was well formed at the time-to-dip of 5 microsec with stable plasma of DPF. Several smooth intensity peaks were periodically observed when neutron beam penetrates the thin film with multi-layers perpendicularly. The platinum layer is dominant to intensity peaks, where the copper-doped SrZnO3 layer next to the platinum layer causes peak broadening. The silicon oxide layer has less effect on the SANS spectra due to its relative thick thickness. The SANS spectra shows thicknesses of platinum and copper-doped SrZnO3 layers as 53 and 25 nm, respectively, which are well agreement with microstructure observation.

Time-resolved small-angle neutron scattering as a lamellar phase evolves into a microemulsion

Time-resolved small-angle neutron scattering (SANS) was used to follow the formation of a microemulsion after rapid stopped-flow mixing of a lamellar phase (Lα), comprising the non-ionic surfactant TX-100 and water, with toluene. Initially after mixing, an oil-swollen Lα phase was formed. Between 4 and 160 s the Lα system evolved into a dense water-in-oil (w/o) microemulsion, during which time the two phases coexisted. After 160 s, the system was a pure single-phase microemulsion. SANS spectra were fitted using a linear combination of the phenomenological Teubner–Stey microemulsion and the Nallet lamellar models. The characteristic repeat distance of water domains in the microemulsion was seen to decrease over the first 70 s of the transformation as more of the microemulsion formed; conversely the domain correlation length appeared to increase.

Small angle neutron scattering and dynamic mechanical thermal analysis of dimethacrylate/epoxy IPNs

Dynamic mechanical thermal analysis (DMTA) and small angle neutron scattering (SANS) have been performed on a number of dimethacrylate (or mono-methacrylate)/diepoxy (or mono-epoxy) interpenetrating polymer networks (IPNs) and semi-IPNs to probe their phase structure. The DMTA behaviour ranged from IPNs that produce one tan δ peak, indicative of a single-phase system, to systems that are clearly phase separated, showing two tan δ peaks. These results were correlated with the SANS data – samples that showed two tan δ peaks also showed scattering in the SANS spectrum. Fitting of the scattering data to the Debye–Bueche scattering model for a phase-separated structure gave a scale to this phase separation of about 180Å. DMTA analysis of the rubbery region of the semi-IPNs revealed that they either had a co-continuous morphology or a matrix phase that was crosslinked.

Transition from Unilamellar to Bilamellar Vesicles Induced by an Amphiphilic Biopolymer

We report some unusual structural transitions upon the addition of an amphiphilic biopolymer to unilamellar surfactant vesicles. The polymer is a hydrophobically modified chitosan and it embeds its hydrophobes in vesicle bilayers. We study vesicle-polymer mixtures using small-angle neutron scattering (SANS) and cryotransmission electron microscopy (cryo-TEM). When low amounts of the polymer are added to unilamellar vesicles of ca. 120 nm diameter, the vesicle size decreases by about 50%. Upon further addition of polymer, lamellar peaks are observed in the SANS spectra at high scattering vectors. We show that these spectra correspond to a co-existence of unilamellar and bilamellar vesicles. The transition to bilamellar vesicles as well as the changes in unilamellar vesicle size are further confirmed by cryo-TEM. A mechanism for the polymer-induced transitions in vesicle morphology is proposed.

Molecular dynamics of confined glucose solutions

Silica gels containing solutions of glucose in heavy water at different concentrations have been prepared by a sol-gel method. Dynamical studies with quasielastic neutron scattering, compared with previous results on bulk solutions, show that the dynamics of the glucose molecules are not appreciably affected by the confinement, even though the gels behave macroscopically as solid materials. Small-angle neutron-scattering spectra on the same systems, fitted with a fractal model, yield a correlation length that decreases from 20 to 2.5 nm with increasing glucose concentration, suggesting a clustering of glucose molecules in concentrated solutions that is consistent with the dynamical measurements. These two sets of results imply that 20 nm is an upper limit for the scale at which the dynamics of glucose molecules in solution are affected by confinement.

Combination of small angle neutron scattering data and mesoscopic simulation techniques as a tool for the structural characterization and prediction of properties of bi-phasic media

A powerful, versatile methodology based on the combination of small angle neutron scattering (SANS) with mesoscopic simulation techniques is developed in an effort to relate in a sufficiently accurate manner the structure of biphasic media to their macroscopic physicochemical (equilibrium and dynamic) properties. Appropriate mathematical transformation of SANS spectra provides valuable information about the disordered geometry of a wide range of biphasic materials spanning from typical porous materials (Vycor porous glass, silica gel, alumina membrane) to complex biological tissues (stratum corneum), in terms of the corresponding autocorrelation function. Digital reconstruction (stochastic or process-based) is employed to generate two- or three-dimensional binary images of the respective structures, on the basis of the statistical properties obtained from the SANS spectra of the original materials. The resulting reconstructed domains are further used for the simulation of equilibrium (sorption) and dynamic (diffusion and permeation) processes, addressing the most common applications of the materials under consideration.

Small-Angle Neutron Scattering of Ionic Perfluoropolyether Micellar Solutions: Role of Counterions and Temperature

This paper reports a small-angle neutron scattering (SANS) characterization of perfluoropolyether (PFPE) aqueous micellar solutions with lithium, sodium, cesium and diethanol ammonium salts obtained from a chlorine terminated carboxylic acid and with two perfluoroisopropoxy units in the tail (n(2)). The counterion and temperature effects on the micelle formation and micellar growth extend our previous work on ammonium and potassium salts n(2) micellar solutions. Lithium, sodium, cesium and diethanol ammonium salts are studied at 0.1 and 0.2 M surfactant concentration in the temperature interval 28-67 degrees C. SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multicomponent system reduced to a generalized one component macroions system (GOCM). At 28 degrees C, for all the salts, the micelles are ellipsoidal with an axial ratio that increases from 1.6 to 4.2 as the counterion volume increases. The micellar core short axis is 13 A and the shell thickness 4.0 A for the alkali micelles, and 14 and 5.1 A for the diethanol ammonium micelles. Therefore, the core short axis mainly depends on the surfactant tail length and the shell thickness on the carboxylate polar head. The bulky diethanol ammonium counterion solely influences the shell thickness. Micellar charge and average aggregation number depend on concentration, temperature and counterion. At 28 degrees C, the fractional ionization decreases vs the counterion volume (or molecular weight) increase at constant concentration for both C = 0.1 M and C = 0.2 M. The increase of the counterion volume leads also to more ellipsoidal shapes. At C = 0.2 M, at 67 degrees C, for sodium and cesium micelles the axial ratio changes significantly, leading to spherical micelles with a core radius of 15 A, lower average aggregation number, and larger fractional ionization.

A small angle neutron scattering study of mica based glass-ceramics with applications in dentistry

We are currently developing machinable and load-bearing mica-based glass-ceramics for use in restorative dental surgery. In this paper we present the results of an ambient temperature small angle neutron scattering (SANS) study of several such ceramics with chemical compositions chosen to optimise machinability and strength. The SANS spectra are all dominated by scattering from the crystalline–amorphous phase interface and exhibit Q −4 dependence (Porod scattering) indicating that, on a 100 Å scale, the surface of the crystals is smooth.

Investigation of radiation damage in VVER-440 reactor vessel steels by SANS

The effect of neutron irradiation on the microstructure of VVER-440 type reactor pressure vessel steels was studied by small-angle neutron scattering (SANS). The SANS spectra were fitted by a model of non-ferromagnetic particles in saturated ferromagnetic matrix, which permitted us to evaluate apparent volume fractions (weighted by scattering contrast), size distributions and mean ratio of magnetic to total scattering cross-sections for small ( R<3 nm) irradiation-induced precipitates. Close correlation between the apparent volume fraction of the damaged domains and ductility transition temperature was observed. Differences in the volume fractions obtained from magnetic and nuclear scattering indicated that chemical composition varied with increasing fluence and could be explained by increasing concentrations of solute atoms in the damaged domains. This coarsening effect is also manifested on the size distributions by slight growth of mean domain radius.

Colloidal Stabilization by Adsorbed Gelatin

The stabilizing effect of adsorbed polyampholyte on colloidal dispersions is quantified through small angle neutron scattering measurements of the structure of concentrated dispersions. Gelatin is adsorbed onto colloidal acrylic latex particles of like net charge to provide both steric and electrostatic stabilization. The extent and structure of the adsorbed gelatin corona is measured using dynamic light scattering (DLS), small angle neutron scattering (SANS) and dilution viscometry (DV). The SANS spectra from concentrated dispersions of bare and gelatin-coated colloidal particles are modeled via integral equation theory using pair potentials that superimpose an electric double layer and a simple model of steric repulsion, where all the parameters are determined a priori. Our results demonstrate for the first time that the stabilizing forces arising from the adsorbed gelatin can be predicted quantitatively from a simple combination of Derjaguin−Landau−Verwey−Overbeek (DLVO) theory and a model for the adsorbed polymer brush. These results agree with previous studies of the surface forces of gelatin adsorbed onto mica cylinders and are important for understanding colloidal stabilization imparted by adsorbed polyampholytes.

Investigation of lipid organization on stratum corneum by water absorption in conjunction with neutron scattering

The lipid arrangement of stratum corneum (SC) was investigated by water absorption in conjunction with small-angle neutron scattering (SANS). The SANS spectra are in reasonable agreement with previous SAXS measurements. In the lower Q region, however, there is an indication of the presence of a lipid phase with a periodicity of 32.2 nm. The position of the shoulder does not change upon hydration indicating that no swelling occurs.