Contrast Variation Small-angle Neutron Scattering Research Articles

Salt-Induced Linker Dehydration Modulates Micellar Structure in Ether-Linked Sulfate Surfactants.

Ether-linked surfactants are widely used in formulations such as liquid soaps, but despite their ubiquity, it is unclear how n-ethylene glycol linkers in surfactants, such as sodium lauryl n-(ethylene glycol) sulfate (SLEnS), influence micellar packing in the presence of NaCl. In the present work, we probe the structure and hydration of ether linkers in micelles comprising monodisperse SLEnS surfactants using contrast-variation small-angle neutron scattering (CV-SANS) and small-angle X-ray scattering (SAXS). Using SAXS, changes in micellar structure were observed for SLEnS (n = 1, 2, or 3) arising from the extent of ethoxylation. Scattering profiles indicated a clear transition from elongated cylindrical micelles to shorter ellipsoidal micelles with increasing ethoxylation. With CV-SANS, micellar structure and linker geometries of SLE3S were able to be resolved, indicating that a change in micellar architecture is modulated by dehydration of the tri(ethylene glycol) linker, offering new insights into the role of water and ions in the self-assembly of this key class of surfactants.

A SANS investigation of silk-stabilized aqueous poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester nanoparticle dispersions

Regenerated silk fibroin (RSF) protein comprising both hydrophilic and hydrophobic chain segments offer great potential for interfacial interaction and stabilization of organic nanoparticles (NPs) in aqueous medium. In this work, aqueous dispersion of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) NPs of 1:1 weight ratio were prepared using RSF as surfactant for the first time via the mini-emulsion method. The size, morphology, internal structure, interface and hierarchical organization of RSF-stabilized P3HT:PCBM NPs was investigated using UV–visible spectroscopy, contrast-variation small-angle neutron scattering (CV-SANS) and ultra-small-angle neutron scattering (CV-USANS). The P3HT:PCBM NPs were established to be stabilized in the matrix of RSF colloidal microparticles in water. Unlike the commonly used surfactant sodium dodecyl sulfate which produces core–shell (PCBM–P3HT) NPs, the RSF molecules generated P3HT:PCBM NPs with internal structure that resemble phase-separated solvent-cast films. The as-prepared dispersion has good film-forming ability and has been demonstrated for fabrication of composite films comprising graphene oxide.

Detailed Catalyst Layer Structure of Proton Exchange Membrane Fuel Cells from Contrast Variation Small-Angle Neutron Scattering

Detailed Catalyst Layer Structure of Proton Exchange Membrane Fuel Cells from Contrast Variation Small-Angle Neutron Scattering

Ion Atmosphere of Wormlike Micelles Profiled by Contrast Variation Small-Angle Neutron Scattering.

Structural studies of wormlike micelles have so far mostly focused on the conformational properties of surfactant aggregates. The diffuse ionic atmosphere, which has a profound influence on various micellization phenomena such as thermodynamic stability and structural polymorphism, remains largely unexplored experimentally. In this report a strategy of contrast variation small-angle neutron scattering for this crucial structural study is outlined. Underlined by a general criterion established for unbiasedly identifying the length scale relevant to charge association from the spectral evolution, our analytical framework can provide a quantitative description of counterion distribution in a mathematically tractable manner. Our method can be conveniently extended to facilitate structural studies of complex multicomponent systems using contrast variation neutron scattering.

How the silica determines properties of filled silicone rubber by the formation of filler networking and bound rubber

Aiming to unveil the correlation among formula, hierarchical structure, and performance of silica filled silicone rubber foams (SRFs), mechanical and structural investigations are performed on three selected samples. Precipitation (P) and fumed (F) silica filled SRFs with diameters of primary particles around 20 nm (PA), 16 nm (FB), and 14 nm (FC) are used, namely SRPA, SRFB, and SRFC. The hierarchical structures are obtained by contrast variation-small-angle neutron scattering (CV-SANS) with an established three-level unified Guinier-exponential/power-law approach. The sizes of both aggregates (agg) and agglomerates (agl) levels have the correlation of SRPA > SRFB > SRFC. Mass-fractals df, agg of SRPA, SRFB, and SRFC are 2.16, 2.16, and 2.54, while df, agl of the set are 2.90, 2.52, and 2.54, respectively. CV-SANS also indicates that the static thickness of bound rubber (TSBR) of SRPA, SRFB, and SRFC decreases from 10.3 nm to 7.5 nm, and then to 6.5 nm. Interestingly, low-field 1H nuclear magnetic resonance (NMR) indicates that the dynamics thickness of bound rubber (TNBR) of the set increases from 3.33 nm to 4.35 nm, and then to 4.41 nm. The discrepancy between TSBR and TNBR is ascribed to the interface interaction. Mechanically, three samples present different behaviors, while SRPA and SRFB have similar extension modules and compressive relaxations. According to a modified constitutive model fitting, and with combining the structural information, the roles of filler-filler and filler-rubber interactions on the reinforcement of SRFs are discussed.

Temperature-induced shear-thinning in catalyst inks

Temperature is a key parameter for ionomer adsorption on carbon supports in catalyst inks for fuel cells, and thus it affects the properties of the ink. This study aimed to investigate the temperature-induced structural evolution and viscosity transition in a catalyst ink. The effect of temperature on the nanostructures of the catalyst ink fabricated from surface-modified carbon supports without platinum nanoparticles was investigated using contrast-variation small-angle neutron scattering (CV-SANS) analysis, through which the concentration and thickness of the adsorbed ionomer layer on the surface of carbon were evaluated. The CV-SANS analysis revealed that the thickness of the shell ionomer layer at 70 °C was twice that at 25 °C, while at elevated temperatures the catalyst ink exhibited shear-thinning behavior. It was surmised that plausible factors in the shear-thinning behavior were a reduction in electrostatic repulsion and the appearance of bridging attraction. Our findings indicate that temperature is a key parameter in the fabrication of catalyst inks, which may be overlooked during the formulation of catalyst inks and catalyst layers.

Interplay between Interparticle Potential and Adsorption Structure in Nanoparticle Dispersions with Polymer Addition as Displayed by Small-Angle Scattering.

The effect of polymer adsorption on the dispersion stability of particles is an important subject applicable to various manufacturing processes. In this study, small-angle scattering was used to examine the relationship between interparticle potential and polymer adsorption in dispersions of nanoparticles with an 81 Å radius containing two types of polymers. Small-angle X-ray scattering (SAXS) measurements in a silica/polyacrylamide (PAAm) system showed an increase in interparticle attractive interactions as PAAm concentration was increased. In a silica/poly(ethylene oxide) (PEO) system, the correlation between PEO concentration and interparticle potential strength became negligible at higher concentrations. Hence, the contrast variation small-angle neutron scattering (CV-SANS) method was employed to evaluate the interparticle potential and polymer adsorption simultaneously. CV-SANS revealed that PAAm was adsorbed to silica particles with a polymer shell layer thickness of 186 Å. The attractive potential observed in the absorbed layer region can be attributed to bridging PAAm molecules between the silica particles. By contrast, CV-SANS of the silica/PEO system indicated a low-polymer-concentration layer with a thickness of 34 Å around silica particles, indicating weak adsorption of PEO molecules. Negligible interaction between PEO and silica particles was assumed to be the origin of the depletion stabilization from excess polymer addition. Thus, quantitative analyses conducted using SAXS and CV-SANS measurements for the first time clearly demonstrated a difference in the adsorption structure of the polymer, which induces changes in the interaction potential between nanoparticles.

Distinguishing Adsorbed and Deposited Ionomers in the Catalyst Layer of Polymer Electrolyte Fuel Cells Using Contrast-Variation Small-Angle Neutron Scattering.

The ionomers distributed on carbon particles in the catalyst layer of polymer electrolyte fuel cells (PEFCs) govern electrical power via proton transport and oxygen permeation to active platinum. Thus, ionomer distribution is a key to PEFC performance. This distribution is characterized by ionomer adsorption and deposition onto carbon during the catalyst-ink coating process; however, the adsorbed and deposited ionomers cannot easily be distinguished in the catalyst layer. Therefore, we identified these two types of ionomers based on the positional correlation between the ionomer and carbon particles. The cross-correlation function for the catalyst layer was obtained by small-angle neutron scattering measurements with varying contrast. From fitting with a model for a fractal aggregate of polydisperse core–shell spheres, we determined the adsorbed-ionomer thickness on the carbon particle to be 51 Å and the deposited-ionomer amount for the total ionomer to be 50%. Our technique for ionomer differentiation can be used to optimally design PEFC catalyst layers.

SANS quantification of bound water in water-soluble polymers across multiple concentration regimes.

Contrast-variation small-angle neutron scattering (CV-SANS) is a widely used technique for quantifying hydration water in soft matter systems, but it is predominantly applied in the dilute regime or for systems with a well-defined structure factor. Here, CV-SANS was used to quantify the number of hydration water molecules associating with three water-soluble polymers with different critical solution temperatures and types of water-solute interactions in dilute, semidilute, and concentrated solution through the exploration of novel methods of data fitting and analysis. Multiple SANS fitting workflows with varying levels of model assumptions were evaluated and compared to give insight into SANS model selection. These fitting pathways ranged from general, model-free algorithms to more standard form and structure factor fitting. In addition, Monte Carlo bootstrapping was evaluated as a method to estimate parameter uncertainty through simulation of technical replicates. The most robust fitting workflow for dilute solutions was found to be form factor fitting without CV-SANS (i.e. polymer in 100% D2O). For semidilute and concentrated solutions, while the model-free approach can be mathematically defined for CV-SANS data, the addition of a structure factor imposes physical constraints on the optimization problem, suggesting that the optimal fitting pathway should include appropriate form and structure factor models. The measured hydration numbers were consistent with the number of tightly bound water molecules associated with each monomer unit, and the concentration dependence of the hydration number was largely governed by the chemistry-specific interactions between water and polymer. Polymers with weaker water-polymer interactions (i.e. those with fewer hydration water molecules) were found to have more bound water at higher concentrations than those with stronger water-polymer interactions due to the increase in the number of forced water-polymer contacts in the concentrated system. This SANS-based method to count hydration water molecules can be applied to polymers in any concentration regime, which will lead to improved understanding of water-polymer interactions and their impact on materials design.

An alternative analysis of contrast-variation neutron scattering data of casein micelles in semi-deuterated milk.

Contrast-variation small-angle neutron scattering (CV-SANS) is an excellent way to determine the structure of complex, hierarchical colloids, including self-assembled biological systems. In these experiments, the scattering length density of solvents is changed (by varying the ratio of water or [Formula: see text] and heavy water or [Formula: see text]) to highlight or mask scattering from different components in the system. This approach has been used with synthetic colloids, but it is also increasingly being used in the biological and food sciences. Perhaps the most studied food colloid is the "casein micelle," a self-assembled nanometer-scale colloid of the structure-forming casein protein in milk. CV-SANS data available in the literature are typically analyzed using approximations, which may be invalid for casein micelles, as they have been shown to be sticky spheres. To assess the applicability of this approximate approach, a comprehensive set of CV-SANS data from casein micelles in diluted milk was reanalyzed using a model-based approach, where the casein micelles were formally treated as interacting spheres. In general, the conclusions of the previous study are reproduced, but this new approach makes it more straightforward to distinguish the different components in milk and can be applied to any dairy sample with known form of interparticle interactions, which offers the possibility of studying semi-deuterated milk at its native concentration.

Disruption of Cationic/Anionic Viscoelastic Surfactant Micellar Networks by Hydrocarbon as a Basis of Enhanced Fracturing Fluids Clean-Up.

Studies of the effects produced by the solubilization of hydrophobic substances by micellar aggregates in water medium are quite important for applications of viscoelastic surfactant solutions for enhanced oil recovery (EOR), especially in hydraulic fracturing technology. The present paper aims at the investigation of the structural transformations produced by the absorption of an aliphatic hydrocarbon (n-decane) by mixed wormlike micelles of cationic (n-octyltrimethylammonium bromide, C8TAB) and anionic (potassium oleate) surfactants enriched by C8TAB. As a result of contact with a small amount (0.5 wt%) of oil, a highly viscoelastic fluid is transformed to a water-like liquid. By small-angle neutron scattering (SANS) combined with cryo-TEM, it was shown that this is due to the transition of long wormlike micelles with elliptical cross-sections to ellipsoidal microemulsion droplets. The non-spherical shape was attributed to partial segregation of longer- and shorter-tail surfactant molecules inside the surfactant monolayer, providing an optimum curvature for both of them. As a result, the long-chain surfactant could preferably be located in the flatter part of the aggregates and the short-chain surfactant—at the ellipsoid edges with higher curvature. It is proven that the transition proceeds via a co-existence of microemulsion droplets and wormlike micelles, and upon the increase of hydrocarbon content, the size and volume fraction of ellipsoidal microemulsion droplets increase. The internal structure of the droplets was revealed by contrast variation SANS, and it was shown that, despite the excess of the cationic surfactant, the radius of surfactant shell is controlled by the anionic surfactant with longer tail. These findings open a way for optimizing the performance of viscoelastic surfactant fluids by regulating both the mechanical properties of the fluids and their clean-up from the fracture induced by contact with hydrocarbons.

Nanostructure of a Catalyst Ink Investigated by Contrast-Variation Small-Angle Neutron Scattering

One of the major aims when investigating catalyst inks for polymer electrolyte fuel cell applications is to understand the relationship between their macroscale physical properties and nanostructure. Catalyst inks are complex colloidal dispersions consisting of catalyst particles, ionomers, and water–alcohol mixtures. Some of the ionomer is adsorbed onto the surface of the catalyst particles. Due to the electrostatic repulsion observed between the particles covered with the ionomer, the ink possesses high dispersion stability. Meanwhile, the role of the remaining ionomer in the dispersion is unclear to date. To clarify the effect of the non-adsorbed ionomer on the physical properties of a catalyst ink, we have established a method to elucidate the amount of non-adsorbed ionomer in the catalyst ink using contrast-variation small-angle neutron scattering (CV-SANS). CV-SANS measurements revealed that the volume fraction of the non-adsorbed ionomer in catalyst ink monotonically increases upon increasing the weight ratio of ionomer/carbon. It was found that the non-adsorbed ionomer in the catalyst ink acts as a typical ionomer dispersion, which can be described by the Huggins equation at high shear viscosity. Our findings may serve as useful guidelines for the preparation of catalyst inks, which is a crucial step for the construction of rationally designed membrane electrode assemblies used for polymer electrolyte fuel cell applications.

Nanostructure of a Catalyst Ink Investigated by Contrast-Variation Small-Angle Neutron Scattering

One of the major aims when investigating catalyst inks for polymer electrolyte fuel cell applications is to understand the relationship between their macroscale physical properties and nanostructure. Catalyst inks are complex colloidal dispersions consisting of catalyst particles, ionomers, and water–alcohol mixtures. Due to the electrostatic repulsion observed between the particles covered with the ionomer, the ink possesses high dispersion stability. Meanwhile, the role of the remaining ionomer in the dispersion is unclear to date. We have established a method to elucidate the amount of non-adsorbed ionomer in the catalyst ink using contrast-variation small-angle neutron scattering (CV-SANS). CV-SANS measurements revealed that the volume fraction of the non-adsorbed ionomer in the catalyst ink monotonically increases upon increasing the weight ratio of ionomer/carbon. It was found that the non-adsorbed ionomer in the catalyst ink acts as a typical ionomer dispersion, which can be described by the Huggins equation at high shear viscosity.

Revealing the Influence of Salts on the Hydration Structure of Ionic SDS Micelles by Contrast-Variation Small-Angle Neutron Scattering.

The influence of lithium chloride (LiCl) on the hydration structure of anionic micelles of sodium dodecyl sulfate (SDS) in water was studied using the contrast-variation small-angle neutron scattering (SANS) technique. In the past, extensive computational studies have shown that the distribution of invasive water plays a critical role in the self-organization of SDS molecules and the stability of the assemblies. However, in past scattering studies the degree of the hydration level was not examined explicitly. Here, a series of contrast-variation SANS data was analyzed to extract the intramicellar radial distributions of invasive water and SDS molecules from the evolving spectral lineshapes caused by the varying isotopic ratios of water. By addressing the intramicellar inhomogeneous distributions of water and SDS molecules, a detailed description of how the counterion association influences the micellization behavior of SDS molecules is provided. The extension of our method can be used to provide an in-depth insight into the micellization phenomenon, which is commonly found in many soft matter systems.

Small-Angle Neutron Scattering Study of Bicelles and Proteobicelles with Incorporated Mitochondrial Cytochrome c Oxidase

The structural investigations of a model membrane system, bicelles, and the aggregation state of isolated and purified bovine heart cytochrome c oxidase (CcO) in bicelles have been performed using small-angle neutron scattering (SANS), SANS contrast variation, and complemented by various biophysical and biochemical techniques. The average size of bicelles prepared from long-chain 1,2-dimyristoyl-sn-glycero-3-phosphocholine and short-chain 1,2-dihexanoyl-sn-glycero-3-phosphocholine was found to be about 22 nm with a thickness of about 4 nm. Enzyme in bicelles was remained active and structurally unaltered. The estimated volume of protein in bicelles of 240 nm3 corresponded well to the monomeric form of CcO. The ab initio modeling supports the experimental data and suggests that CcO in bicelles is a homogeneous monomeric complex incorporated into bicelles.

The Multilevel Structure of Sulfonated Syndiotactic-Polystyrene Model Polyelectrolyte Membranes Resolved by Extended Q-Range Contrast Variation SANS.

Membranes based on sulfonated synditoactic polystyrene (s-sPS) were thoroughly characterized by contrast variation small-angle neutron scattering (SANS) over a wide Q-range in dry and hydrated states. Following special sulfonation and treatment procedures, s-sPS is an attractive material for fuel cells and energy storage applications. The film samples were prepared by solid-state sulfonation, resulting in uniform sulfonation of only the amorphous phase while preserving the crystallinity of the membrane. Fullerenes, which improve the resistance to oxidation decomposition, were incorporated in the membranes. The fullerenes seem to be chiefly located in the amorphous regions of the samples, and do not influence the formation and evolution of the morphologies in the polymer films, as no significant differences were observed in the SANS patterns compared to the fullerenes-free s-sPS membranes, which were investigated in a previous study. The use of uniaxially deformed film samples, and neutron contrast variation allowed for the identification and characterization of different structural levels with sizes between nm and μm, which form and evolve in both the dry and hydrated states. The scattering length density of the crystalline regions was varied using the guest exchange procedure between different toluene isotopologues incorporated into the sPS lattice, while the variation of the scattering properties of the hydrated amorphous regions was achieved using different H2O/D2O mixtures. Due to the deformation of the films, the scattering characteristics of different structures can be distinguished on specific detection sectors and at different detection distances after the sample, depending on their size and orientation.

Evolution of the Interfacial Structure of a Catalyst Ink with the Quality of the Dispersing Solvent: A Contrast Variation Small-Angle and Ultrasmall-Angle Neutron Scattering Investigation.

The electrocatalyst layer (ECL) of the proton-exchange membrane fuel cell (PEMFC) is commonly fabricated from colloidal catalyst ink containing carbon-supported catalyst nanoparticles (NPs), ionomer stabilizer, and dispersion medium (DM). The structure, stability, and aggregate size distribution of fuel cell catalyst ink are critically dependent on the quality of DM. However, understanding of the influence of the quality of DM on the hierarchical structure of the ECL is lacking. This work presents a systematic investigation of the effects of reducing alcohol content in isopropyl alcohol/water (IPA/H2O) binary mixtures as DM on the structural evolution of water-rich (green) catalyst ink using contrast-variation small-angle and ultrasmall-angle neutron scattering techniques. Both qualitative and quantitative information are extracted from the data to obtain information about the size, structure, and organization of the catalyst ink using different model functions fit to the experimental data. The catalyst ink prepared using 70% IPA (commonly employed in industry and extensively reported in the literature) is shown to consist of randomly distributed globular carbon aggregates (mean radius of gyration of ∼178.9 nm) stabilized by an ionomer mass fractal shell (thickness of ∼13.0 nm), which is dispersed in the matrix of rodlike (∼1.3 nm radius and ∼35.0 nm length) negatively surface-charged ionomer NPs. These well characterized baseline data are then compared and contrasted with DM formulations of lower IPA content. A sequential reduction in IPA content of DM shows a progressive increase in the ionomer NP radius and electrostatic repulsion, concomitantly with the decrease in the carbon aggregate size and ionomer shell thickness of the catalyst ink. Therefore, the changes in the interfacial structure via adjustments of the DM composition can be used as a controlling parameter to tailor the hierarchical structure of the colloidal fuel cell catalyst ink and to further optimize the performance of the ECL.

Morphology and porous structure of standalone aromatic polyamide films as used in RO membranes – An exploration with SANS, PALS, and SEM

This report presents a study about the morphology of a cross-linked aromatic polyamide (PA) film in its dry and wet state using small-angle neutron scattering (SANS), positron annihilation lifetime spectroscopy (PALS), and scanning electron microscopy (SEM). PA is used as active skin layer for reverse osmosis membranes and determines the selectivity of the membrane with respect to water and salt molecules. This selectivity of PA is largely determined from its morphology. The PA film scatters the neutrons much stronger and shows a different profile as expected from randomly distributed nanoscopic large pores. SANS contrast variation, using supercritical CO2 as contrast medium, confirms that pores are the only scattering centers. The analysis shows that interconnected pores of an average radius of about 16 Å determine the scattering. The pores are formed as a network of channels showing a fractal structure. PALS determines pores of ∼6 Å diameter representing the diameter of such channels. The volume fraction of the pores is estimated as ∼10% which after water up-take increases by ∼30% due to slightly swelling of the pores.

Mesoscopic Investigation of an “Immiscible” Cyclohexane and Water Micro-mixture in Carbon Micropores by Contrast Variation Small-angle Neutron Scattering

Microspaces can simultaneously capture “immiscible” molecules such as cyclohexane and water due to fluid-surface interactions. We investigated the mesoscopic structure of cyclohexane and water co-adsorbed in carbon micropores using contrast variation small-angle neutron scattering (CV-SANS). The results show that cyclohexane tends to adsorb near the carbon surface and that both molecules form relatively small domains (<1 nm).

Parameterization of silica-filled silicone rubber morphology: A contrast variation SANS and TEM study

The hierarchical structures of silica-filled silicone rubber have been investigated by using contrast variation small-angle neutron scattering (CV-SANS) and transmission electron microscopy (TEM). A series of scattering intensities of swollen samples were measured with varying ratio of hydrogenated toluene/deuterated toluene solvent. By using CV-SANS method, the partial scattering functions, self-terms of silica SSS(q) and silicone rubber SRR(q), as well as cross-term between silica and silicone rubber SRS(q), are successfully extracted. The fine parameterization of morphology is characterized and elucidated by adopting unified Guinier-exponential/power-law equations, the fitted results indicate: (i) the volume fraction of silicone rubber in bound rubber and matrix regions, are respectively 0.80 and 0.31. Such spatial inhomogeneous of swelling ratio suggests that there is existence of the immobilized layer; (ii) the average aggregation size is ca. 143 nm, and there are about 24–97 silica particles in each aggregation; (iii) the thickness of the bound rubber is estimated as in the range of ca. 7.2–8.5 nm. A mimic collection and observation of aggregations and bound rubber with TEM confirm the CV-SANS results. Current work provided a scenario to evaluate the reinforcement effect of the silica on the silicone rubber even before any mechanical testing.