Specular Neutron Reflectivity Research Articles

(Invited) Electrochemistry Combined in Situ Neutron Reflectometry: A Powerful Tool for a More Sustainable Environment

Neutrons, having extraordinary characteristics that include penetrating power, sensitivity to lighter elements (such as hydrogen and carbon), and magnetic moment, make neutron reflectometry an ideal candidate for examining surface and interfacial structures of thin films in a non-destructive and non-invasive fashion. Liquids Reflectometer (LR) at the Spallation Neutron Source at Oak Ridge National Laboratory measures specular and off specular neutron reflectivity in a horizontal sample geometry, and tracks changes of layer thickness, scattering length density, and roughness as a function of depth. The exceptional sample environment at LR offers the capability to conduct in situ electrochemistry to probe the morphological and structural changes of solid/liquid interfaces during operation as a function of chemical and electrochemical gradients over time. Furthermore, the time-resolved mode at LR can be carried out continuously over a reduced Q range and binned into 60-second intervals. The improved time resolution of the neutron reflectivity (NR) measurement can help elucidate the kinetic behavior of surface/interfaces under dynamic conditions and provide a unique perspective to the equilibrium study. In this talk, I will give a brief introduction on neutron reflectometry, the LR, and a variety of sample environments at the LR. I will focus on the electrochemical applications coupled with in situ NR measurement. In the end, I will provide a specific example of applications in electrochemically modulated separation for water remediation by using neutron reflectometry. With the rapid growth of human population, it is highly desirable to minimize energy and materials input in selective separations for a more sustainable environment. The study is critical for creating new, more selective separation systems through rationally design of the structure and operating conditions.

Extraction of interaction parameters from specular neutron reflectivity in thin films of diblock copolymers: an "inverse problem".

Diblock copolymers have been shown to undergo microphase separation due to an interplay of repulsive interactions between dissimilar monomers, which leads to the stretching of chains and entropic loss due to the stretching. In thin films, additional effects due to confinement and monomer-surface interactions make microphase separation much more complicated than in that in bulks (i.e., without substrates). Previously, physics-based models have been used to interpret and extract various interaction parameters from the specular neutron reflectivities of annealed thin films containing diblock copolymers (J. P. Mahalik, J. W. Dugger, S. W. Sides, B. G. Sumpter, V. Lauter and R. Kumar, Interpreting neutron reflectivity profiles of diblock copolymer nanocomposite thin films using hybrid particle-field simulations, Macromolecules, 2018, 51(8), 3116; J. P. Mahalik, W. Li, A. T. Savici, S. Hahn, H. Lauter, H. Ambaye, B. G. Sumpter, V. Lauter and R. Kumar, Dispersity-driven stabilization of coexisting morphologies in asymmetric diblock copolymer thin films, Macromolecules, 2021, 54(1), 450). However, extracting Flory-Huggins χ parameters characterizing monomer-monomer, monomer-substrate, and monomer-air interactions has been labor-intensive and prone to errors, requiring the use of alternative methods for practical purposes. In this work, we have developed such an alternative method by employing a multi-layer perceptron, an autoencoder, and a variational autoencoder. These neural networks are used to extract interaction parameters not only from neutron scattering length density profiles constructed using self-consistent field theory-based simulations, but also from a noisy ad hoc model constructed previously. In particular, the variational autoencoder is shown to be the most promising tool when it comes to the reconstruction and extraction of parameters from an ad hoc neutron scattering length density profile of a thin film containing a symmetric di-block copolymer (poly(deuterated styrene-b-n-butyl methacrylate)). This work paves the way for automated analysis of specular neutron reflectivities from thin films of copolymers using machine learning tools.

Anaklasis: a compact software package for model-based analysis of specular neutron and X-ray reflectometry data sets.

anaklasis constitutes a set of open-source Python scripts that facilitate a range of specular neutron and X-ray reflectivity calculations, involving the generation of theoretical curves and the comparison/fitting of interfacial model reflectivity against experimental data sets. The primary focus of the software is twofold: on one hand to offer a more natural framework for model definition, requiring minimum coding literacy, and on the other hand to include advanced analysis methods that have been proposed in recent work. Particular attention is given to the ability to co-refine reflectivity data and to the estimation of model-parameter uncertainty and covariance using bootstrap analysis and Markov chain Monte Carlo sampling. The compactness and simplicity of model definition together with the streamlined analysis do not present a steep learning curve for the user, an aspect that may accelerate the generation of reproducible, easily readable and statistically accurate reports in future neutron and X-ray reflectivity related literature.

PH-sensitive behavior of the PS-b-PDMAEMA copolymer at the air - water interface

We investigated the behavior of a pH-responsive polystyrene‐b‐poly(2-dimethylamino)ethyl methacrylate (PS‐b‐PDMAEMA) diblock copolymer at the air/water interface. We synthetized different copolymers with a small hydrophobic PS block of constant length attached to polyelectrolyte PDMAEMA blocks of various lengths. We demonstrate that a Langmuir monolayer is formed with the hydrophobic collapsed PS block anchoring the hydrophilic polyelectrolyte at the interface. By combining macroscopic surface pressure measurements and specular neutron reflectivity, we studied the monolayers as a function of surface density σ and pH of the subphase. At pH = 2, the PDMAEMA is fully charged and the system at high σ behaves as a polyelectrolyte brush with chains protruding in water with a gaussian profile. At pH = 10 where the PDMAEMA is neutral, the system exhibits a phase transition between the pancake configuration to the brush configuration, with a threshold that depends on the PDMAEMA chain length, in agreement with theoretical predicted scaling laws.

Hydration State Variation of Polyzwitterion Brushes through Interplay with Ions.

Polyzwitterions have emerged as a new class of antifouling materials alternating poly(ethylene glycol). The exemplary biopassivation and lubrication behaviors are often attributed to the particular chemical structure of zwitterions, which involve a large dipole moment of the charged groups and a neutral net charge, while the hydration state and dynamics also associate with these characteristics. Polymer brushes composed of surface-tethered polyzwitterion chains produced by surface-initiated controlled radical polymerization have been developed as thin films which exhibit excellent antifouling and lubrication properties. In past decades, numerous studies have been devoted to examining the structure and dynamics of polyzwitterion brush chains in aqueous solutions. This feature article provides an overview of recent studies exploring the hydration state of polyzwitterion brushes with specular neutron reflectivity, highlights some newly published work on the nonuniform equilibrium structure, ion concentration dependence, ion specificity, and the effects of charge spacer length in the zwitterions, and discusses future perspective in this field.

Understanding the Role of Oxidative Debris on the Structure of Graphene Oxide Films at the Air-Water Interface: A Neutron Reflectivity Study.

We address here the role of oxidation impurities on the structure of graphene oxide films at the air-water interface by specular neutron reflectivity (SNR). We study films of purified graphene oxide (PGO) and nonpurified graphene oxide in the close-packed state. Nonpurified graphene oxide is constituted by graphene oxide (GO) layers with oxidation impurities adsorbed on the basal plane, while in PGO sheets, impurities are eliminated. SNR measurements show that GO films are formed by well-defined bilayers constituted by 2-3 layers of GO stacked in contact with air and a second layer of impurities submerged in the aqueous subphase. In contrast, PGO films are formed by a single layer in contact with air. We show for the first time that impurities constitute a layer submerged in the aqueous subphase, decrease the elasticity, and favor the collapse of graphene oxide films. Our results allow designing the surface properties of GO trapped at fluid interfaces.

Structuring of colloidal silica nanoparticle suspensions near water-silica interfaces probed by specular neutron reflectivity.

Structuring of aqueous suspensions of colloidal silica nanoparticles near an isolated planar silica-water interface is studied by specular neutron reflectivity. The reflectivity data clearly show that the suspensions develop a damped, oscillatory concentration profile in the normal direction to the interface. The wavelengths of these oscillations agree well with those independently determined by direct force measurements in the slit-geometry. The reflectivity data further demonstrate that the oscillatory structure persists over several layers and that the first particle layer is separated from the interface by a particle-free region.

An introduction to neutron reflectometry

Specular neutron reflectivity is a neutron diffraction technique that provides information about the structure of surfaces or thin films. It enables the measurement of the neutron scattering length density profile perpendicular to the plane of a surface or an interface, and thereby gives access to the profile of the chemical composition of the film. The wave-particle duality allows to describe neutrons as waves; at an interface between two media of different refractive indexes, neutrons are partially reflected and refracted by the interface. Interferences can occur between waves reflected at the top and at the bottom of a thin film at an interface, which gives rise to interference fringes in the reflectivity profile directly related to its thickness. The characteristic sizes that can be probed range from 5Å to 2000 Å. Neutron-matter interaction directly occurs between neutron and the atom nuclei, which enable to tune the contrast by isotopic substitution. This makes it particularly interesting in the fields of soft matter and biophysics. This course is composed of two parts describing respectively its principle and the experimental aspects of the method (instruments, samples). Examples of applications of neutron reflectometry in the biological domain are presented by Y. Gerelli in the book section “Applications of neutron reflectometry in biology”.

Experimental Approbation of Reference Layer Method in Resonant Neutron Reflectometry

Abstract—The first application of resonant neutron scattering at Gd atoms for the experimental determination of the phase of the reflection coefficient in neutron reflectometry is presented in this paper. The surface of the sample studied was covered by the reference Gd layer, for which the scattering length of thermal neutrons is highly energy-dependent. After measuring the coefficient of specular reflection of neutrons at three different wavelengths of each impulse, we were able to resolve the phase problem of the neutron reflectometry and determine the absolute magnitude and phase of the complex reflection coefficient from an unknown part of sample structure.

Model-independent recovery of interfacial structure from multi-contrast neutron reflectivity data.

Neutron specular reflectivity at soft interfaces provides sub-nanometre information concerning the molecular distribution of thin films, while the application of contrast variation can highlight the scattering from different parts of the system and lead to an overall reduction in fitting ambiguity. Traditional modelling approaches involve the construction of a trial scattering length density profile based on initial speculation and the subsequent refinement of its parameters through minimization of the discrepancy between the calculated and measured reflectivity. In practice this might produce an artificial bias towards specific sets of solutions. On the other hand, direct inversion of reflectivity data, despite its ability to provide a unique solution, is subject to limitations and experimental complications. Presented here is an integrated indirect Fourier transform/simulated annealing method that, when applied to multiple solvent contrast reflectivity data and within the limits of finite spatial resolution, leads to reliable reconstructions of the interfacial structure without the need for any a priori assumptions. The generality of the method permits its straightforward application in common experimental contrast-variation investigations at the solid/liquid and air/liquid interface.

Structuration of the Surface Layer during Drying of Colloidal Dispersions.

During evaporative drying of a colloidal dispersion, the structural behavior at the air-dispersion interface is of particular relevance to the understanding of the consolidation mechanism and the final structural and mechanical properties of the porous media. The drying interface constitutes the region of initial drying stress that, when accumulated over a critical thickness, leads to crack formation. This work presents an experimental study of top-down drying of colloidal silica dispersions with three different sizes (radius 5, 8, and 13 nm). Using specular neutron reflectivity, we focus on the structural evolution at the free drying front of the dispersion with a macroscopic drying surface and demonstrate the existence of a thick concentrated surface layer induced by heterogeneous evaporation. The reflectivity profile contains a strong structure peak due to scattering from particles in the interfacial region, from which the interparticle distance is deduced. A notable advantage of these measurements is the direct extraction of the corresponding dispersion concentration from the critical total reflection edge, providing a straightforward access to a structure-concentration relation during the drying process. The bulk reservoir of this experimental configuration renders it possible to verify the evaporation-diffusion balance to construct the surface layer and also to check reversibility of particle ordering. We follow the structural evolution of this surface layer from a sol to a soft wet-gel that is the precursor of a fragile skin and the onset of significant particle aggregation that precedes formation of the wet-crust. Separate complementary measurements on the structural evolution in the bulk dispersion are also carried out by small-angle neutron scattering, where the particle concentration is also extracted directly from the experimental curves. The two sets of data reveal similar structural evolution with concentration at the interface and in the bulk and an increase in the degree of ordering with the particle size.

Membrane interaction of off-pathway prion oligomers and lipid-induced on-pathway intermediates during prion conversion: A clue for neurotoxicity

Soluble oligomers of prion proteins (PrP), produced during amyloid aggregation, have emerged as the primary neurotoxic species, instead of the fibrillar end-products, in transmissible spongiform encephalopathies. However, whether the membrane is among their direct targets, that mediate the downstream adverse effects, remains a question of debate. Recently, questions arise from the formation of membrane-active oligomeric species generated during the β-aggregation pathway, either in solution, or in lipid environment. In the present study, we characterized membrane interaction of off-pathway oligomers from recombinant prion protein generated along the amyloid aggregation and compared to lipid-induced intermediates produced during lipid-accelerated fibrillation. Using calcein-leakage assay, we show that the soluble prion oligomers are the most potent in producing leakage with negatively charged vesicles. Binding affinities, conformational states, mode of action of the different PrP assemblies were determined by thioflavin T binding-static light scattering experiments on DOPC/DOPS vesicles, as well as by FTIR-ATR spectroscopy and specular neutron reflectivity onto the corresponding supported lipid bilayers. Our results indicate that the off-pathway PrP oligomers interact with lipid membrane via a distinct mechanism, compared to the inserted lipid-induced intermediates. Thus, separate neurotoxic mechanisms could exist following the puzzling intermediates generated in the different cell compartments. These results not only reveal an important regulation of lipid membrane on PrP behavior but may also provide clues for designing stage-specific and prion-targeted therapy.

Effect of electrochemical control function on the internal structure and composition of electrodeposited polypyrrole films: A neutron reflectometry study

Electrodeposited conducting polymer films derived from aromatic monomers are known to possess properties that depend significantly on the deposition protocol, particularly the electrochemical control function employed. This study explores the underlying reasons for this common observation for the specific case of polypyrrole films deposited from aqueous media onto gold electrodes under potentiostatic, potentiodynamic and galvanostatic control. Although the control functions impose different conditions, the control parameters (potential, potential range and scan rate, and current) were selected so as generate films at comparable rates; this avoids inappropriate attribution of structural and compositional variations to different thickness regimes, irrespective of how they were generated. In each case, film deposition was periodically interrupted and the film characterised by specular neutron reflectivity measurements. By using d4-pyrrole monomer in H2O solvent, the isotopic selectivity of neutron reflectivity was used to extract polymer and solvent concentration profiles as a function of distance from the electrode/film interface. Spatial integration of these profiles was used to quantify total film solvent populations; these are expressed as solvent volume fractions. Films grown under the three different control regimes have measurably distinct solvent volume fraction profiles and there is evolution of these profiles with increasing thickness. Ultimately, for the conditions employed, the order of increasing porosity (i.e. solvent content) by control function was potentiostatic < potentiodynamic < galvanostatic. At the end of the deposition process, the films were transferred to monomer-free electrolyte and redox cycled. This resulted in an overall increase in film solvation, but little difference in solvation with redox state (doping level). We conclude that film structure and associated solvation level do retain some memory of deposition protocol, but also respond to the medium of exposure.

Controlling Foam Stability with the Ratio of Myristic Acid to Choline Hydroxide.

The interfacial and foam properties of a model system based on the mixture between myristic acid and choline hydroxide have been investigated as a function of the molar ratio ( R) between these two components and temperature. The aim of this study was to obtain insight on the links between the self-assemblies in bulk and in the foam liquid channels, the surfactant packing at the interface, and the resulting foam properties and stability. A multiscale approach was used combining small angle neutron scattering, specular neutron reflectivity, surface tension measurements, and photography. We highlighted three regimes of foam stability in this system by modifying R: high foam stability for R < 1, intermediate at R ∼ 1, and low for R > 1. The different regimes come from the pH variations in bulk linked to R. The pH plays a crucial role at the molecular scale by setting the ionization state of the myristic acid molecules adsorbed at the gas-liquid interface, which in turn controls both the properties of the monolayer and the stability of the films separating the bubbles. The main requirement to obtain stable foams is to set the pH close to the p Ka in order to have a mixture of protonated and ionized molecules giving rise to intermolecular hydrogen bonds. As a result, a dense monolayer is formed at the interface with a low surface tension. R also modifies the structure of self-assembly in bulk and therefore within the foam, but such a morphological change has only a minor effect on the foam stability. This study confirms that foam stability in surfactant systems having a carboxylic acid as polar headgroup is mainly linked to the ionization state of the molecules at the interface.

Grazing incidence SANS and reflectometry combined with simulation of adsorbed microgel particles

Absrtract Adsorbed ethylene glycol based microgel particles on a silicon surface were studied. Neutron reflectometry (NR) and grazing incidence small-angle neutron scattering (GISANS) were performed to investigate their internal structure. Scattering experiments on soft matter systems such as adsorbed microgels often give only partial information about the inner structure of the polymer system. In this contribution, we discuss how the detailed inner structure of adsorbed microgel particles can be reconstructed by a combination of the specular Neutron Reflectivity (NR), Grazing Incidence Small Angle Neutron Scattering (GISANS), Atomic Force Microscopy (AFM) and a simulation in the framework of the Distorted Wave Born Approximation.

Towards optimised drug delivery: structure and composition of testosterone enanthate in sodium dodecyl sulfate monolayers.

Surface tension and specular neutron reflectivity measurements have been used, for the first time to systematically study both the interfacial structure and composition of monolayers of the soluble surfactant, sodium dodecyl sulfate containing a low-dose, poorly water soluble drug, testosterone enanthate. Modelling of the specular neutron reflectivity data suggests that the hydrophobic testosterone enanthate was adsorbed in the C12 hydrophobic tail region of the surfactant monolayer, regardless of the concentration of surfactant at the interface and whether or not additional drug was added to the interface. The location of the hydrophobic drug in the tail region of the surfactant monolayer is supported by the results of classical, large-scale molecular dynamics simulations. The thickness of the surfactant monolayer obtained, in the presence and absence of drug, using molecular dynamics simulations was in good agreement with the corresponding values obtained from the specular neutron reflectivity measurements. The stoichiometry of surfactant:drug at the air-water interface at sodium dodecyl sulfate concentrations above the critical micelle concentration was determined from specular neutron reflectivity measurements to be approximately 3 : 1, and remained constant after the spreading of further testosterone enanthate at the interface. Significantly, this stoichiometry was the same as that obtained in the micelles from bulk solubilisation studies. Important insights into the preferred location of drug in surfactant monolayers at the air-water interface as well as its effect on the structure of the monolayer have been obtained from our combined use of experimental and simulation techniques.

Neutron reflectivity for soft matter

Specular neutron reflectivity is a technique enabling the measurement of coherent neutron scattering length density profile perpendicular to the plane of a surface or interface, and thereby the profile of chemical composition. The characteristic sizes that are probed range from around 5Å up 5000 Å. It is a scattering technique that averages information over the entire surface and it is therefore not possible to obtain information on correlations in the plane of the interface. The specific properties of neutrons (possibility of tuning the contrast by isotopic substitution, negligible absorption, low energy of the incident neutrons) makes it particularly interesting in the fields of soft matter and biophysics. This course is composed of three parts describing respectively its principle, the experimental aspects (diffractometers, samples), and some scientific examples of neutron reflectometry focusing on the use of contrast variation to probe polymeric systems.

Coupled Effects of Spreading Solvent Molecules and Electrostatic Repulsions on the Behavior of PS-b-PAA Monolayers at the Air-Water Interface.

We describe the surface behavior of PS-b-PAA monolayers at the air/water interface using N,N-dimethyformamide (DMF) as spreading solvent. At low pH, when the PAA blocks are neutral, the surface pressure versus molecular area isotherm shows a pseudoplateau associated with the presence of remaining spreading solvent molecules in the monolayer, as we described in a former study (Guennouni et al., Langmuir, 2016). We show here that the width of the plateau decreases when increasing pH up to its complete disappearance at high pH, when PAA blocks are fully charged, although two regimes of compressibilities on the isotherm still exist. A refined structural study at pH 9 combining specular neutron reflectivity (SNR), grazing-incidence small-angle X-ray scattering (GISAXS), and atomic force microscopy (AFM) in liquid measurements shows that (i) PAA blocks are stretched in solution, as expected from polyelectrolyte brushes in the osmotic regime; (ii) the system undergoes a spinodal decomposition during deposit at the air/water interface in the presence of DMF. Upon compression, the Qxy* position of the peak associated with the spinodal structure remains almost constant but its intensity evolves strongly and passes through a maximum at intermediate pressures. This reveals two operating processes in the system: strong electrostatic repulsions between chains that prevent in-plane reorganizations and force such reorganizations to occur from the surface to the volume and progressive expulsion of the DMF molecules from the monolayer. These processes have antagonist effects on the intensity of the peak: the increase of the repulsions makes it more pronounced, whereas the expulsion of solvent makes it vanish due to the loss of contrast.

Nanoroughness Strongly Impacts Lipid Mobility in Supported Membranes.

In vivo lipid membranes interact with rough supramolecular structures such as protein clusters and fibrils. How these features whose size ranges from a few nanometers to a few tens of nanometers impact lipid and protein mobility is still being investigated. Here, we study supported phospholipid bilayers, a unique biomimetic model, deposited on etched surfaces bearing nanometric corrugations. The surface roughness and mean curvature are carefully characterized by AFM imaging using ultrasharp tips. Neutron specular reflectivity supplements this surface characterization and indicates that the bilayers follow the large-scale corrugations of the substrate. We measure the lateral mobility of lipids in both the fluid and gel phases by fluorescence recovery after patterned photobleaching. Although the mobility is independent of the roughness in the gel phase, it exhibits a 5-fold decrease in the fluid phase when the roughness increases from 0.2 to 10 nm. These results are interpreted with a two-phase model allowing for a strong decrease in the lipid mobility in highly curved or defect-induced gel-like nanoscale regions. This suggests a strong link between membrane curvature and fluidity, which is a key property for various cell functions such as signaling and adhesion.

Multilayering of Calcium Aerosol-OT at the Mica/Water Interface Studied with Neutron Reflection: Formation of a Condensed Lamellar Phase at the CMC.

Using specular neutron reflection, the adsorption of sodium and calcium salts of the surfactant bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT or AOT) has been studied at the mica/water interface at concentrations between 0.1 and 2 CMC. The pH dependence of the adsorption was also probed. No evidence of the adsorption of Na(AOT) was found even at the critical micelle concentration (CMC) while the calcium salt was found to adsorb significantly at concentrations of 0.5 CMC and above. This interesting and somewhat unexpected finding demonstrates that counterion identity may be used to tune the adsorption of anionic surfactants on anionic surfaces. At the CMC, three condensed bilayers of Ca(AOT)2 were adsorbed at pH 7 and 9 and four bilayers adsorbed at pH 4. Multilayering at the CMC of Ca(AOT)2 on the mica surface is an unusual feature of this surfactant/surface combination. Only single bilayer adsorption has been observed at other surfaces at the CMC. We suggest this arises from the high charge density of mica which must provide an excellent template for the surfactant.