Ionic distribution and polymer conformation, near phase separation, in sodium polyacrylate/divalent cations mixtures: small angle X-ray and neutron scattering

Anomalous small angle X-ray scattering experiments show that before demixion in sodium polyacrylate/cobalt and sodium polyacrylate/calcium mixtures all the divalent counterions are in the close vicinity of the polyacrylate chain. The present results are consistent with previous UV/VIS spectroscopy, which have shown that all cobalt ions are chemically associated with acrylate groups. The chemical association dehydrates the acrylate monomers. However, the hydrophobicity of the complexed monomers is not strong enough to induce a collapse of the polymer chain at small spatial scale before the demixion. Indeed, the scattered intensity (X-ray and neutron scattering) decreases with the scattering vector q as q^-x with for q > 0.1 nm ^-1 which indicates that the local conformation of the chain is Gaussian. Received 21 January 1999     

Neutron diffraction and inelastic scattering from adsorbed molecules

Neutron scattering measurements on the structure and dynamics of adsorbed phases at the gas solid interface have proliferated in the last five years and this paper reports some recently obtained results for the systems methane-graphite and ammonia-graphite, which show BET type I and type III isotherms respectively. These systems clearly illustrate the differences in both structure and dynamics to be expected in other examples where either wetting of the surface or non-wetting behaviour occurs upon adsorption. Evidence for phase transitions in both types of system coming from diffraction is reported, and this is substantiated by measurements using inelastic scattering to follow the molecular dynamics. The neutron inelastic scattering from molecules adsorbed on platinum black and in zeolites reveals some of the virtues of neutron inelastic scattering for the study of chemisorbed species. Finally, some preliminary experiments on neutron small angle scattering at the solid liquid interface in polystyrene latex-water sols are reported.     

Tuning intermicellar potential of Triton X-100-anthranilic acid mixed micelles

Structural parameters of micelles formed by Triton X-100 in the presence of solubilized anthranilic acid at different pH values was investigated using light scattering and small angle neutron scattering. Analysis of the SANS data indicate that micelles are oblate ellipsoidal in nature with little variation in the dimensions, in the investigated pH range (from 0.5 to 6.0). The interaction potential of the micelles shows a minimum closer to the isoelectric point of anthranilic acid. A similar variation is observed in the cloud point of the micelles with pH. The observed variation in the interaction potential with pH of the micellar solution can be explained in terms of the reversal of charge on anthranilic acid due to shift in the acid-base equilibrium. The variation in interaction potential and cloud point with pH is modelled using Coulombic repulsion of charged molecules at the micelle interface.      

Phase separation of a binary liquid in anodic aluminium oxide templates: a structural study by small angle neutron scattering

The radial nanostructure of the binary liquid triethylamine/water confined in 60 nm diameter independent cylindrical pores of anodic aluminium oxide membranes is studied by small angle neutron scattering. It is shown that composition inhomogeneities are present in the confined mixtures well below the bulk critical point. An analysis of the neutron scattering form factor reveals the existence of an adsorbed water layer of a few nanometers at the liquid/alumina interface, coexisting with a TEA-rich phase in the core.     

Phase separation of a binary liquid in anodic aluminium oxide templates A structural study by small angle neutron scattering

The radial nanostructure of the binary liquid triethylamine/water confined in 60 nm diameter independent cylindrical pores of anodic aluminium oxide membranes is studied by small angle neutron scattering. It is shown that composition inhomogeneities are present in the confined mixtures well below the bulk critical point. An analysis of the neutron scattering form factor reveals the existence of an adsorbed water layer of a few nanometers at the liquid/alumina interface, coexisting with a TEA-rich phase in the core.     

Adsorption kinetics in micellar solutions of nonionic surfactants

Standard models of the adsorption kinetics of surfactants at the air-water surface assume that micelles break down into monomers in the bulk solution and that only monomers adsorb. We show here that micelles of the nonionic surfactant C14E8 adsorb to the surface of a liquid jet at a diffusion-controlled rate. Micellar adsorption can be switched off by incorporation of a small amount of ionic surfactant into the micelle and switched on again by addition of salt. More sophisticated models of adsorption processes in micellar solutions are required that permit a kinetic flux of micelles to the air-water interface.     

Small-angle neutron scattering study of structural evolution of different phases in protein solution

Small-angle neutron scattering (SANS) has been used to study the structural evolution of different phases in protein solution leading to crystallization, denaturation and gelation. The protein solution under crystallization mostly consists of monomers and dimers, and higher-mers are not observed as they are perhaps formed in very small numbers. The onset and the rate of crystallization strongly depend on the salt concentration. Protein denaturation on addition of surfactant occurs due to the formation of micelle-like clusters along the unfolded polypeptide chains of the protein. The structure of such protein-surfactant complex is found to be independent of the size of the micelles in their pure surfactant solutions. The structure of temperature-induced protein gels shows a fractal structure. Rheology of these gels shows a strong dependence on varying pH or protein concentration, whereas the structure of such gels is found to be similar.      

Small angle neutron scattering studies on the interaction of cationic surfactants with bovine serum albumin

The structure of the protein-surfactant complex of bovine serum albumin (BSA) and cationic surfactants has been studied by small angle neutron scattering. At low concentrations, the CTAB monomers are observed to bind to the protein leading to an increase in its size. On the other hand at high concentrations, surfactant molecules aggregate along the unfolded polypeptide chain of the protein resulting in the formation of a fractal structure representing a necklace model of micelle-like clusters randomly distributed along the polypeptide chain. The fractal dimension as well as the size and number of micelles attached to the complex have been determined.      

Monte Carlo simulations of an amphiphilic polymer at a hydrophobic/hydrophilic interface

Monte Carlo simulations have been carried out for an off-lattice model of an amphiphilic polymer at a hydrophobic/hydrophilic interface. The model system consists of a polynorbornene backbone with poly(ethylene oxide) (PEO) grafts modelled atomistically at an idealized interface between hydrophobic and hydrophilic regions, which are represented by external potentials. Results are presented for the distribution of PEO chain ends, and the density of PEO segments perpendicular to the surface. The latter is used to provide predictions for neutron reflectivity profiles normal to the surface as a function of the lateral confinement of the PEO grafts. At low surface coverage the simulation results are found to be in good agreement with experimental neutron scattering results from similar polymers studied at the water/air interface.     

Structural evidence of charge renormalization in semi-dilute solutions of highly charged polyelectrolytes

We show experimentally that Manning counterion condensation also leads to a renormalization of the charge density at high concentrations of highly charged, flexible, hydrophilic polyelectrolytes. Investigations by small angle neutron and X-ray scattering of semi-dilute solutions of poly(acrylamide-co-sodium-2-acrylamido-2-methylpropane sulfonate) at different charge densities above the condensation threshold, show that the scattering function is invariant with the charge density. Received 16 June 1998     

Time of flight grazing incidence small angle neutron scattering

Grazing incidence small angle neutron scattering (GISANS) overcomes the limitations of conventional small angle scattering with respect to extremely small sample volumes in the thin film geometry. In time of flight (TOF) mode neutrons with a broad range of wavelengths are used simultaneously and recorded as a function of their respective times of flight. The combination of both, TOF-GISANS, enables the simultaneous performance of several GISANS measurements, which differ in wavelength. As a consequence, within one measurement a full set of GISANS pattern related to different scattering vectors, different scattering depths and resolutions result. This allows the detection of nanostructures with a chemical sensitivity. The possibilities of TOF-GISANS are demonstrated by the simple example of polymer nano-dots located on top of a silicon surface. As probed with atomic force microscopy (AFM) the nano-dots exhibit a large characteristic nearest neighbour distance of 545 nm and a surface coverage of 28%. From the analysis of the wavelength dependent data in combination with AFM the mass density of the polymer nano-dots is determined to be equal to the bulk value. A comparison to common single wavelength GISANS experiments is shown.     

Capillary condensation in a fractal porous medium

Small-angle x-ray and neutron scattering are used to characterize the surface roughness and porosity of a natural rock which are described over three decades in length scales and over nine decades in scattered intensities by a surface fractal dimension D = 2.68+/-0.03. When this porous medium is exposed to a vapor of a contrast-matched water, neutron scattering reveals that surface roughness disappears at small scales, where a Porod behavior typical of smooth interfaces is observed instead. Water-sorption measurements confirm that such interface smoothing is due predominantly to the water condensing in the most strongly curved asperities rather than covering the surface with a wetting film of uniform thickness.     

Small angle neutron scattering studies of mixed micelles of sodium cumene sulphonate with cetyl trimethylammonium bromide and sodium dodecyl sulphate

The aqueous solutions of sodium cumene sulphonate (NaCS) and its mixtures with each of cetyl trimethylammonium bromide (CTAB) and sodium dodecyl sulphate (SDS) are characterized by small angle neutron scattering (SANS). NaCS when added to CTAB solution leads to the formation of long rod-shaped micelles with a dramatic increase in the CTAB aggregation number. Its addition to SDS on the other hand results in the formation of smaller mixed micelles where part of SDS molecules in the micelle is replaced by NaCS molecules.      

Unraveling Complexity: A Strategy for the Characterization of Anisotropic Core Multishell Nanoparticles

In this work, a widely applicable routine to characterize the core, surface, stability, and optical properties of CdSe/CdS/ZnS core–shell–shell nanorods after multiple growth steps is established. First, size, shape, and shell thickness of the nanorods are characterized by transmission electron microscopy (TEM), analytical ultracentrifugation (AUC), and small angle X-ray/neutron scattering (SAXS/SANS). In the next step, Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), and SANS measurements are applied to determine the surface species of nanorods. Then, the colloidal stability of the nanorods is investigated by UV–vis spectroscopy and dynamic light scattering (DLS) after different washing cycles. Finally, photoluminescence quantum yield (PLQY) of the nanorods during washing and sample storage is determined. With this highly complementary routine for particle characterization, the core, surface, stability, and optical properties of nanorods after multiple growth steps are resolved. The results demonstrate the importance of the developed toolbox to characterize such highly complex, anisotropic nanorods for a technical environment. This is of major importance for the handling of colloidal quantum materials and their quality control in industrial applications.     

Salt-free aqueous solutions of polyelectrolytes: Small angle X-ray and neutron scattering characterization

We give an introduction to the application of small angle X-ray and neutron scattering techniques to the study of the structure of polyelectrolyte solutions. The goal is not to be exhaustive and to describe the latest progress in the field of polyelectrolytes but rather to present the potential of the technique. We aim to give an overview on the main features of the scattering curves, and discuss the structural information that can be drawn. We will first give the basics of small angle scattering and highlight the role played by neutron scattering and isotopic labeling. Thus, we will discuss the significance of the monomer and counterion partial scattering functions according to the scattering length densities of both components, and will emphasize the unique possibility to separate out the intra- and intermolecular correlations. Then, some examples will show how the dispersion state and the average conformation of macroions can be addressed. We will however focus on salt-free aqueous solutions of linear flexible hydrophilic polyelectrolytes in the semidilute regime.     

Small angle neutron scattering study of disordered and crystalline iron nanoparticle assemblies

Monodisperse surfactant-coated iron nanoparticles are used to form both disordered nanoparticle assemblies and ordered face-centered cubic nanoparticle crystals. The structural order is probed by small angle X-ray scattering, and the magnetic scattering is studied using small angle neutron scattering. The magnetic scattering corresponding to different length scales is interpreted in terms of collective correlations among the particles within the assemblies.     

Transition from rigid to flexible structures in micelles of Undecylammonium chloride in aqueous solutions of NaCl

Small angle neutron scattering (SANS) data for Undecylammonium chloride (UAC) in heavy water in the presence of NaCl 0.0428 and 0.3422 M are consistent with the presence of elongated micelles. This micellar shape has been adopted to analyze viscosity data of UAC in water in the presence of NaCl. The results obtained from this last technique are consistent with the increase of the micelle aggregation number with increasing the surfactant concentration. Micelles change from prolate ellipsoidal shape to cylindrical and wormlike shapes by increasing the added NaCl concentration and surfactant concentrations. The differences between results for the micelle aggregation number calculated from viscosity, SANS and light scattering data have been attributed to the solvent effect on micelle formation as well as changes in the size, shape and flexibility of the micelle. Viscosity data provide qualitative information on the effect of the added NaCl concentration and surfactant concentration on the size, shape, flexibility of the micelles in diluted solutions.     

Structure Analysis of Solid Lipid Nanoparticles for Drug Delivery: A Combined USANS/SANS Study

Suspensions of solid lipid nanoparticles (SLNs) stabilized with emulsifiers have been extensively investigated (since the 1990s) as drug carriers, although details of their ultrastructure are poorly defined. Previously, a novel microwave‐assisted microemulsion‐based technique to prepare SLNs was reported. To understand the detailed internal structure of these SLNs, ultra‐small angle neutron scattering (USANS) and small angle neutron scattering (SANS) experiments are conducted on suspensions of hydrogenated stearic acid SLNs stabilized with hydrogenated Tween 20 surfactant in D₂O. Together, SANS and USANS gives a combined Q range of 0.000047 to 0.6 Å^?1 (corresponding to a size range of ≈1 nm–15 µm). This extended Q range allows a comprehensive understanding of the hierarchical structure of SLNs. The data are consistent with the multi‐length scale structure of SLNs having polydispersed large particles with roughened surfaces at the microscale level. At the nanoscale level, the results are consistent with the SLNs having an ellipsoidal shape intermediate between spheres and rods, with a crossover from mass fractals to surface fractals. The elucidation of this structure is particularly important given that the structure influences the stability and drug release properties of the nanoparticles. These results assist in the development of systems with desired shape and properties.     

Dynamic templating role of polynaphtalene sulphonate in the formation of silver nanocrystals in aqueous solution

We have used time resolved small angle X-ray scattering (SAXS) for investigating the early stages of the formation of crystalline colloidal Ag particles by AgNO₃ reduction with ascorbic acid in an aqueous acidic solution of a polynaphthalene sulphonate polymer (Daxad). In order to exploit the polymer role in silver crystallization, the measurements have been performed at different polymer concentrations. Data analysis suggests that the nucleation and growth of primary particles of about 20 nm in diameter, occurring in a surface-fractal arrangement of the dynamic polymeric template, is assisted by the development of long range spatial correlations. These correlations could be a sign of polymer-induced liquid fluctuations as intermediate precursors of Ag crystallization.     

Molecular simulation of sodium butyl benzene sulfonate at air–water interface and in aqueous solution

Molecular mechanics(MM) calculations for interfacial behaviour of sodium n-butyl benzene sulfonate (NaNBBS), sodium iso-butyl benzene sulfonate (NaIBBS) and sodium tert-butyl benzene sulfonate (NaTBBS) show a significant effect of the butyl group geometry on the surface area occupied by these molecules at the air–water interface. NaNBBS, in comparison with NaIBBS and NaTBBS, shows a closer molecular packing at the interface. The simulation predicts minimum hydrotrope concentration of each hydrotrope to reach surface saturation and molecular surface area at the interface match with good accuracy. The shape, size and charge of the hydrotrope aggregates obtained by molecular dynamics simulation also match well with the results of small angle neutron scattering experiments on the same hydrotrope. The simulation shows non-regular and ellipsoidal hydrotropes aggregates with substantial charge on the surface. The aggregates are also more open structures as compared to surfactant micelles. The water accessible surface area of a NaNBBS aggregate was 25% lower in comparison to that of NaTBBS aggregate, indicating closer packing of NaNBBS molecules. The fractional charge on the NaNBBS aggregate decreases with the increase in the number of NaNBBS molecules in the aggregate indicating more counter-ion association.