Implementation of a self-consistent slab model of bilayer structure in the SasView suite

Slab models are simple and useful structural descriptions which have long been used to describe lyotropic lamellar phases, such as lipid bilayers. Typically, slab models assume a midline symmetry and break a bilayer structure into three pieces, a central solvent-free core and two symmetric outer layers composed of the soluble portion of the amphiphile and associated solvent. This breakdown matches reasonably well to the distribution of neutron scattering length density and therefore is a convenient and common approach for the treatment of small-angle scattering data. Here, an implementation of this model within the SasView software suite is reported. The implementation is intended to provide physical consistency through the area per amphiphile molecule and number of solvent molecules included within the solvent-exposed outer layer. The proper use of this model requires knowledge of (or good estimates for) the amphiphile and solvent molecule volume and atomic composition, ultimately providing a self-consistent data treatment with only two free parameters: the lateral area per amphiphile molecule and the number of solvent molecules included in the outer region per amphiphile molecule. The use of this code is demonstrated in the fitting of standard lipid bilayer data sets, obtaining structural parameters consistent with prior literature and illustrating the typical and ideal cases of fitting for neutron scattering data obtained using single or multiple contrast conditions. While demonstrated here for lipid bilayers, this model is intended for general application to block copolymers, surfactants, and other lyotropic lamellar phase structures for which a slab model is able to reasonably estimate the neutron scattering length density/electron-density profile of inner and outer layers of the lamellae.

Carbonaceous Materials Investigated by Small-Angle X-ray and Neutron Scattering

Carbonaceous nanomaterials have become important materials with widespread applications in battery systems and supercapacitors. The application of these materials requires precise knowledge of their nanostructure. In particular, the porosity of the materials together with the shape of the pores and the total internal surface must be known accurately. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) present the methods of choice for this purpose. Here we review our recent investigations using SAXS and SANS. We first describe the theoretical basis of the analysis of carbonaceous material by small-angle scattering. The evaluation of the small-angle data relies on the powerful concept of the chord length distribution (CLD) which we explain in detail. As an example of such an evaluation, we use recent analysis by SAXS of carbide-derived carbons. Moreover, we present our SAXS analysis on commercially produced activated carbons (ACN, RP-20) and provide a comparison with small-angle neutron scattering data. This comparison demonstrates the wealth of additional information that would not be obtained by the application of either method alone. SANS allows us to change the contrast, and we summarize the main results using different contrast matching agents. The pores of the carbon nanomaterials can be filled gradually by deuterated p-xylene, which leads to a precise analysis of the pore size distribution. The X-ray scattering length density of carbon can be matched by the scattering length density of sulfur, which allows us to see the gradual filling of the nanopores by sulfur in a melt-impregnation procedure. This process is important for the application of carbonaceous materials as cathodes in lithium/sulfur batteries. All studies summarized in this review underscore the great power and precision with which carbon nanomaterials can be analyzed by SAXS and SANS.

Small-angle scattering from polydisperse particles with a diffusive surface

Particles with a diffusive surface, characterized by a deviation from the Porod power-law asymptotic behavior in small-angle scattering towards an exponent below −4, are considered with respect to the polydispersity problem. The case of low diffusivity is emphasized, which allows the description of the scattering length density distribution within spherically isotropic particles in terms of a continuous profile. This significantly simplifies the analysis of the particle-size distribution function, as well as the change in the scattering invariants under contrast variation. The effect of the solvent scattering contribution on the apparent exponent value in power-law-type scattering and related restrictions in the analysis of the scattering curves are discussed. The principal features and possibilities of the developed approach are illustrated in the treatment of experimental small-angle neutron scattering data from liquid dispersions of detonation nanodiamond. The obtained scattering length density profile of the particles fits well with a transition of the diamond states of carbon inside the crystallites to graphite-like states at the surface, and it is possible to combine the diffusive properties of the surface with the experimental shift of the mean scattering length density of the particles compared with that of pure diamond. The moments of the particle-size distribution are derived and analyzed in terms of the lognormal approximation.

Characterization of the pore structure of metakaolin-derived geopolymers by neutron scattering and electron microscopy

The pore–solid structure of selected high-compressive-strength metakaolin geopolymers has been characterized to facilitate quantitative prediction of their physical properties. Geopolymers are multiphase materials with pore widths ranging from subnanometre to several tenths of a millimetre. Ultramicrotoming of resin-embedded grains was found to be an effective method for producing electron-transparent sections. Scanning and transmission electron microscopy showed the existence of a bi-level pore system and heterogeneity of the pore morphology. Ultra-small-angle neutron scattering, of sufficiently thin specimens, was found to be useful in detecting the length scales on which statistically significant structural changes occur as the geopolymer chemical composition is varied. Contrast variation experiments confirmed that the small-angle neutron scattering from an Si:Al:Na = 2.5:1:1.2 geopolymer before and after dehydration was dominated by scattering from pores. These experiments suggested the presence of closed (under current experimental conditions) pores in the dehydrated geopolymer. A three-phase analysis was developed for this system, and the scattering of the solid, open pore and closed pore phases was determined as a function of scattering length density ρ. The scattering from all three phases had the sameqdependence over the range of likely ρ within the uncertainties. A lower limit of 4.21 (6) × 1010 cm−2was determined for the scattering length density ρwof the nondehydrated geopolymer by assuming the pore fluid to be water. This scattering length density is significantly higher than the expected value of approximately 3.4 × 1010 cm−2. Small-angle neutron scattering from the dehydrated and nondehydrated Si:Al:Na = 2.5:1:1.2 geopolymer showed that dehydration does not cause a severe change in morphology of the nanoporosity on the length scale probed.

Small-angle neutron scattering analysis of a water-based magnetic fluid with charge stabilization: contrast variation and scattering of polarized neutrons

Structure analysis of a magnetic fluid (nanoparticles of maghemite dispersed in water with charge stabilization and without surfactant) by means of small-angle neutron scattering is presented. A combination of the contrast variation technique and scattering of polarized neutrons was applied. In the first case, the scattering curves obtained for the unmagnetized fluid with variation of the heavy water content in the carrier are treated in terms of the basic functions approach. The almost homogeneous character of the nanoparticles with respect to the nuclear scattering length density makes it possible to separate information about their characteristic nuclear and magnetic radii. Polarized neutrons are then used to separate and analyze independently the nuclear and magnetic scattering contributions for the fully magnetized fluid. Both methods reveal a significant excess of the apparent nuclear size over the magnetic one, which is explained by a difference in the nonmagnetic and magnetic interactions in the system. The results indicate that from the viewpoint of magnetic interaction the studied fluid behaves under a magnetic field as a purely superparamagnetic system of independent particles. The magnetic scattering length density of the maghemite nanoparticles is found to be ∼25% less than the bulk value, which is in agreement with the data of the magnetization analysis.

Characterization of Thin Layer SnO2/Glass by Neutrons Reflectometry

The thermal annealing behavior of the SnO2 thin films elaborated by sol-gel method has been studied by the neutrons reflectivity technique. From the fit of the experimental data using Parratt32 software program developed at HMI, Berlin, scattering length density, thickness and roughness are extracted. The obtained results show that the film thickness increases with the increasing annealing temperature, and the roughness is higher at 500 °C. Whereas, approximately, the same scattering length density is obtained after each annealing temperature.