Small Angle Neutron Scattering Study Research Articles

A Small Angle Neutron Scattering Study of the Adsorbed Asphaltene Layer in Water-in-Hydrocarbon Emulsions: Structural Description Related to Stability

We have developed a specific protocol to study with SANS measurements, the structure of the interfacial film layer in water-in-oil emulsions stabilized by asphaltene. Using the contrast matching technique available for neutron scattering, we have access to both the composition and the quantity of interface. The results obtained give us a view of the asphaltene aggregates in the interfacial film, which are structured as a monolayer and show a direct correlation between the size of asphaltene aggregates in solution and the thickness of the film layer. The organization of the interface has been studied as a function of several parameters such as the quantity of resins, i.e., the size of aggregates, the pH of the aqueous phase, and the aging time of the emulsions and the consequences of these variations on the macroscopic stability of these emulsions. We show that the key parameter for the stability is the inter-asphaltene aggregate interaction inside the film layer. Changing the attractive/repulsive balance between the aggregates in the film at the microscopic scale, by changing the aggregate's size or the aggregate's ionization, has a direct incidence on the quantity of water recovered after centrifugation: the stronger the attraction between aggregates in the film, the more stable the emulsion is.

Small Angle Neutron Scattering Studies of the Counterion Effects on the Molecular Conformation and Structure of Charged G4 PAMAM Dendrimers in Aqueous Solutions

The structural properties of generation 4 (G4) poly(amidoamine) starburst dendrimers (PAMAM) with an ethylenediamine (EDA) central core in D2O solutions have been studied by small-angle neutron scattering (SANS). Upon the addition of DCl, SANS patterns show pronounced inter-particle correlation peaks due to the strong repulsion introduced by the protonation of the amino groups of the dendrimers. By solving the Ornstein−Zernike integral equation (OZ) with hypernetted chain closure (HNC), the dendrimer−dendrimer structure factor S(Q) is determined and used to fit the experimental data, where Q is the magnitude of the scattering wave vector. Quantitative information such as the effective charge per dendrimer and the radius of gyration, RG, at different pD values is obtained. The results show that RG only changes by about 4% when the pD value varies from 10.25 to 4.97, and significant counterion association/condensation occurs, strongly mediating the inter-dendrimer interaction. The influence of interplay between counterions and molecular protonation of dendrimers imposes a strong effect on the dendrimer conformation and effective interaction. Although the change of RG is very small, careful analyses of the high Q data and fitting parameters indicate a possible internal structure change of a dendrimer when the amino groups are progressively charged.

In situ and real -time small-angle neutron scattering studies of living anionic polymerization process and polymerization-induced self-assembly of block copolymers

We have studied a simultaneous living anionic polymerization process of isoprene and deuterated styrene in deuterated benzene with sec-buthyl lithium as an initiator into polyisoprene- block-poly(styrene- d 8) and the polymerization-induced self-assembling process. This polymerization-induced self-assembling process was directly observed by an in situ and real -time small-angle neutron scattering (SANS) experiment. The time-resolved SANS studies enabled us to explore a time evolution of hierarchical structures induced by a time evolution of the primary structure (linear sequential connection of two monomers).

Polarised Small Angle Neutron Scattering study of microstructural radiation damage in steels for nuclear applications

Polarised SANS has been used to investigate two martensitic steels for fusion reactor technology (MANET and OPTIFER, differing in their initial Cr content), before and after neutron irradiation. The measurements were carried out at the instrument D22 of the ILL, Grenoble, using an ad hoc polarised beam set-up. The results show both the occurrence of Cr redistribution phenomena with correlated changes in the composition of the precipitate phases, and the growth of non-magnetic defects (possibly microvoids), depending on the irradiation conditions and on the original steel composition.

Small-Angle Neutron Scattering Study of Concentrated Colloidal Dispersions: The Electrostatic/Steric Composite Interactions between Colloidal Particles

Small-angle neutron scattering was used to investigate the interactions in concentrated colloidal dispersions containing silica or polystyrene latex with adsorbed polyethyleneoxide (PEO). In these dispersions of charged particles, both electrostatic and steric repulsions are present. The PEO layer was made invisible to neutrons through contrast matching. The effect of the interparticle repulsion was clearly shown in the scattering spectra by the appearance of a peak at low Q. The effective potentials can be well described by the Hayter-Penfold/Yukawa (HPY) potential. In the silica dispersions studied, the layer thickness is small, hence the electrostatic potential dominates and the potential has a lower concentration dependence. In the dispersions of polystyrene latex, the adsorbed layer is thicker; consequently, the electrostatic potential dominates at low volume fraction (the potential has a lower concentration dependence), and the steric potential dominates at higher volume fraction (the potential has a higher concentration dependence). This study also suggests that when more than one potential is present the stronger one has a dominant influence in determining the structure factor. This finding makes it possible to describe the multicomponential interactions by a single function.

Small-angle neutron-scattering studies of reentrant spin-glass behavior in Fe–Al alloys

Fe–Al alloys around the concentration of 30 at. % Al show very interesting magnetic properties. Alloys with this concentration present magnetic inhomogeneities and their evolution with temperature is also interesting because they show a reentrant spin-glass behavior at low temperatures. Small-angle neutron-scattering measurements were performed in Fe69.5Al30.5,Fe70Al30, and Fe70.5Al29.5 alloys in the temperature range of 4–300 K. Even though the change in Fe content is very small, the response variation is quite large. The results show that the samples have magnetic inhomogeneities that evolve with temperature. Only in the Fe-richest sample the random-field model can be applied.

Magnetic properties of self-assembled ferritin-core arrays

We studied the magnetic properties of a ferritin-core array, deposited on a Si wafer and encapsulated in silica. The array was formed by a simple evaporation-induced self-assembly mechanism of ferritin proteins, and it consists of 6 nm ferritin cores crystallized in a two-dimensional hexagonal lattice with a core-core distance of about 10.5 nm. Magnetization studies indicate that the array undergoes a magnetic transition around 210 K. Our small-angle neutron-scattering studies provide evidence that huge net magnetic moments are stabilized in each core and that long-range magnetic order between the core moments occurs in the array.

Measuring and Predicting the Dynamics of Linear Monodisperse Entangled Polymers in Rapid Flow through an Abrupt Contraction. A Small Angle Neutron Scattering Study

Small-angle neutron scattering measurements on a series of monodisperse linear entangled polystyrene melts in nonlinear flow through an abrupt 4:1 contraction have been made. Clear signatures of melt deformation and subsequent relaxation can be observed in the scattering patterns, which were taken along the centerline. These data are compared with the predictions of a recently derived molecular theory. Two levels of molecular theory are used: a detailed equation describing the evolution of molecular structure over all length scales relevant to the scattering data and a simplified version of the model, which is suitable for finite element computations. The velocity field for the complex melt flow is computed using the simplified model and scattering predictions are made by feeding these flow histories into the detailed model. The modeling quantitatively captures the full scattering intensity patterns over a broad range of data with independent variation of position within the contraction geometry, bulk flow rate and melt molecular weight. The study provides a strong, quantitative validation of current theoretical ideas concerning the microscopic dynamics of entangled polymers which builds upon existing comparisons with nonlinear mechanical stress data. Furthermore, we are able to confirm the appreciable length scale dependence of relaxation in polymer melts and highlight some wider implications of this phenomenon.

Self-Assembly of a Nonionic Photoresponsive Surfactant under Varying Irradiation Conditions: A Small-Angle Neutron Scattering and Cryo-TEM Study

We have used small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) to determine the structure of aggregates formed by the photoresponsive surfactants diethylene glycol mono(4',4-butyloxy, butyl-azobenzene) (C4AzoOC4E2) and diethylene glycol mono(4',4-hexyloxy, butyl-azobenzene) (C4AzoOC6E2) under different illumination conditions. At high concentrations, the self-assembly behavior of these surfactants changes remarkably in response to different radiation conditions. The trans isomers assemble into bilamellar (C4AzoOC4E2) and unilamellar (C4AzoOC6E2) vesicles, while the cis isomers (under UV light) form bicontinuous phases. These light-induced structural changes are attributed to a change in the sign of the Gaussian rigidity, which is the direct result of azobenzene photoisomerization.

Small-angle neutron-scattering study on a structure of microemulsion mixed with polymer networks

The structure of a microemulsion mixed with polymer networks was investigated by means of small-angle neutron scattering (SANS). The system consists of nonionic surfactant, polymer network, oil, and water. The microemulsion and the polymer network employed in this work are known to undergo temperature-induced structural transition and volume phase transition, respectively. Polymer solutions and gels were made by polymerizing monomer solutions in the presence of microemulsion droplets. In the case of a mixture of an N-isopropylacrylamide (NIPA) monomer solution and a microemulsion, the NIPA monomer was found to behave as a cosurfactant. However, polymerization resulted in a phase separation to polymer-rich and -poor phases. Interestingly, SANS results indicated that a well-developed ordered structure of oil domains was formed in polymer network and the structure was very different from its parent systems. Furthermore, the system underwent two different types of structural transitions with respect to temperature. One was originated from the structural transition of microemulsion due to the change of the spontaneous curvature and the other from the volume phase transition of the NIPA gel.

Structural Changes and Chain Radius of Gyration in Cold-Drawn Polyethylene after Annealing: Small- and Wide-Angle X-ray Scattering and Small-Angle Neutron Scattering Studies

Samples made from linear polyethylene were drawn at room temperature and subsequently annealed at high temperatures below the melting point. The structural changes of the crystalline lamellae and lamellar superstructures as well as the single chain radius of gyration were studied by means of combined small- and wide-angle X-ray scattering and small-angle neutron scattering (SANS). After drawing, the polymeric chain segments in the crystalline phase are preferentially oriented along the drawing direction with a high degree of orientation whereas the lamellae in the samples are found to be slightly sheared exhibiting oblique surfaces as evidenced by X-ray scattering. SANS indicates that the chains are highly elongated along the drawing direction. Annealing the deformed samples at temperatures where the mechanical alpha-process of polyethylene is active leads to a thickening of both crystalline lamellae and amorphous layers. The chains in the crystalline phase retain their high degree of orientation after annealing while the lamellae are sheared to a larger extent. In addition, there is also lateral growth of the crystalline lamellae during high-temperature annealing. Despite the structural changes of the crystalline and amorphous regions, there is no evidence for global chain relaxation. The global anisotropic shape of the chains is preserved even after prolonged annealing at high temperatures. The results indicate that the mobility of polyethylene chains-as seen, e.g., by 13C NMR-is a local phenomenon. The results also yield new insight into mechanical properties of drawn PE, especially regarding stress relaxation and creep mechanisms.

Physical Properties of Aqueous Solutions of a Thermo-Responsive Neutral Copolymer and an Anionic Surfactant: Turbidity and Small-Angle Neutron Scattering Studies

Aqueous mixtures of the anionic sodium dodecyl sulfate (SDS) surfactant and thermo-responsive poly(N-vinylcaprolactam) chains grafted with omega-methoxy poly(ethylene oxide) undecyl alpha-methacrylate (PVCL-g-C11EO42) have been characterized using turbidimetry and small-angle neutron scattering (SANS). Turbidity measurements show that the addition of SDS to a dilute aqueous copolymer solution (1.0 wt %) induces an increase of the cloud point (CP) value and a decrease of the turbidity at high temperatures. In parallel, SANS results show a decrease of both the average distance between chains and the global size of the objects in solution at high temperatures as the SDS concentration is increased. Combination of these findings reveals that the presence of SDS in the PVCL-g-C11EO42 solutions (1.0 wt %) promotes the formation of smaller aggregates and, consequently, leads to a more homogeneous distribution of the chains in solution upon heating of the mixtures. Moreover, the SANS data results show that the internal structure of the formed aggregates becomes more swollen as the SDS concentration increases. On the other hand, the addition of moderate amounts of SDS (up to 4 mm) to a semidilute copolymer solution (5.0 wt %) gives rise to a more pronounced aggregation as the temperature rises; turbidity and SANS studies reveal in this case a decrease of the CP value and an increase of the scattered intensity at low q. The overall picture that emerges from this study is that the degree of aggregation can be accurately tuned by varying parameters such as the temperature, level of surfactant addition, and polymer concentration.

Small-Angle Neutron Scattering Study of Concentrated Colloidal Dispersions: The Interparticle Interactions between Sterically Stabilized Particles

Small-angle neutron scattering (SANS) was used to investigate the interparticle interactions in concentrated dispersions of colloidal silica stabilized either by steric or by electrostatic repulsive interactions. In 10 mM NaCl, an adsorbed PEO layer is required to prevent flocculation, and particles are stabilized by steric repulsions. The adsorbed layer was made invisible to neutrons by contrast matching with the aqueous continuous phase. Dispersions of the same particles at the same concentrations but in the absence of added salt and adsorbed PEO were also studied. In both cases, the SANS spectra of concentrated dispersions show a peak at low Q, which is due to interparticle interactions: a structure factor. The SANS data can be described rather well by a homogeneous spherical form factor and a structure factor based on the Hayter-Penfold/Yukawa potential model. The steric potential was compared to the electrostatic potential obtained by fitting the SANS data of the bare silica dispersions. The steric potential shows a greater dependence on the particle volume fraction, which we ascribe to the penetration and compression of the adsorbed PEO layer as the particles approach.

Small Angle Neutron Scattering Study of Demixing in Micellar Solutions Containing CTAC and a Partially Fluorinated Cationic Surfactant

Demixing of fluorocarbon and hydrocarbon surfactants to form coexisting fluorocarbon-rich and hydrocarbon-rich micelles has been studied by small angle neutron scattering in aqueous solution, using an equimolar mixture of cetyltrimethylammonium chloride and the partially fluorinated cationic surfactant N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride, with a deuterated pyridinium headgroup. Measurements have been performed under varying experimental conditions: in both pure aqueous solutions and with salt (0.10 M NaCl), at several contrasts for neutrons obtained by varying the H(2)O/D(2)O ratio, mainly at 25 degrees C but also at 60 degrees C to promote mixing of the surfactants. The experiments show that a substantial residual scattering is retained at the solvent composition where the average scattering length density of mixed micelles would match that of the solvent. It is moreover observed that, in solutions without added salt, a prominent correlation peak observed in 100% D(2)O disappears at the match point. These observations are in accordance with a very broad composition distribution, possibly composed of two populations of mixed micelles of similar sizes but different compositions, but would not result from micelles with merely a highly inhomogeneous internal structure. Increasing the temperature from 25 to 60 degrees C reduces substantially the scattered intensity at zero angle at the match point, as expected for a less broad population of mixed micelles. In the numerical analysis, the scattering data for scattering vector q > or = 0.02 A(-1) were analyzed by the indirect Fourier transform method to give the scattering at zero angle. From these data, the average micelle aggregation number was obtained as 76 at 25 degrees C and 54 at 60 degrees C. The contrast variation results for the intensity at zero angle give a measure of the width of the micelle distribution, which is obtained as sigma = 0.33 at the lower temperature and sigma = 0.20 at 60 degrees C. The result at the low temperature is compatible with the formation of two populations that are polydisperse (sigma = 0.07) and centered around 18 and 82%; other broad distributions cannot be excluded.

Small-Angle Neutron Scattering Studies of Charged Carboxyl-Terminated Dendrimers in Solutions

Small-angle neutron scattering was used to characterize the solution behavior of charged carboxylic acid terminated "cascade" dendrimers (Z-Cascade/methane [4]/3-oxo-6-oxa-2-azaheptylidyne/3-oxo-2-azaheptylidyne/propanoic acids) of third (G3) and fifth (G5) generations as a function of dendrimer concentration, pH, and ionic strength. An increase in dendrimer concentration leads to a single broad peak in the scattering profile arising from interdendrimer interaction. The dissociation of terminal carboxylate groups also gives rise to an interdendrimer interaction peak, which could be suppressed by the addition of excess salt. The results of contrast matching measurements indicate the accumulation of an excess concentration of tetramethylammonium counterions around the surface of these highly charged particles, and the thickness of these counterions lies somewhere between 4 and 6 A. This conclusion is consistent with our previous potentiometric titration (Zhang, H.; et al. J. Phys. Chem. B 1997, 101, 3494) and capillary electrophoresis (Huang, Q. R.; et al. J. Phys. Chem. B 2000, 104, 898) data.

Facile Syntheses of a Class of Supramolecular Gelator Following a Combinatorial Library Approach: Dynamic Light Scattering and Small-Angle Neutron Scattering Studies

Following a combinatorial library approach, 60 organic salts have been prepared by reacting 5 bile acids and 12 secondary amines. Gelation tests with various organic and aqueous solvents reveal that 16 salts are supramolecular gelators of which 6 salts are found to be ambidextrous displaying the ability to form gels with organic as well as aqueous solvents. The salt didodecylammonium cholate (1a7b) is the most versatile gelator, displaying gelation ability with a maximum number of solvents. AFM images of the xerogels of 1a7b in organic and aqueous solvents reveal the presence of fibrous aggregate including helical fibers. Dynamic and morphological behaviors of the aggregates in the gel state have been probed by dynamic light scattering (DLS) and small-angle neutron scattering (SANS) experiments on an aqueous gel (1:1 DMSO/H2O) of 1a7b. It is revealed that the gelation takes place via the formation of flexible fibrous clusters followed by physical cross-linking. DLS and SANS results also disclose that (1) ...

Effect of Sol?Gel Encapsulation on Lipase Structure and Function: A Small Angle Neutron Scattering Study

The application of small angle neutron scattering (SANS) to the characterisation of sol–gel hosts containing biomolecules offers the opportunity to explore the relationship between gel structure and catalyst. A model system involving the immobilisation of Candida antarctica lipase B (CALB) was investigated. Gels were produced by fluoride-catalysed hydrolysis of fixed ratios of tetramethylorthosilicate (TMOS) and methyltrimethoxysilane (MTMS). Phase separation between the enzyme and the evolving sol–gel matrix was minimised by incorporating glycerol into the sol–gel precursor solution. The potential stabilising effect of the NaF catalyst upon the enzyme was also investigated. Scattering studies were conducted on both immobilised lipase, and lipase in free solution. Scattering studies on free enzyme provided evidence of multiple populations of enzyme aggregates and showed that choice of solvent affected the degree of aggregation. Both NaF and glycerol affected neutron scattering, indicating changes in lipase conformation. Increasing glycerol concentration increased the degree of aggregation and produced differences in solvent packing on the surface of protein molecules. Initial evidence from SANS data indicated that the presence of the enzyme during gel formation conferred structural changes on the gel matrix. Modelling the effect of sol–gel encapsulation on lipase requires comparison of data from free enzyme to the immobilised form. Removal of the enzyme from the sol–gel structure, post gelation, is necessary to better characterise the modified matrix. This methodological problem will be the subject of future investigations.

Characterization of polybutadiene-poly(ethyleneoxide) aggregates in aqueous solution: A light-scattering and small-angle neutron-scattering study

For diblockcopolymers of polybutadiene-poly(ethyleneoxide) (PB-PEO) type, water is a selective solvent. In dependence of the length of both blocks and the block length ratio, these polymers form a multitude of self-assembled structures in solution. In the present work scattering methods are used to investigate the water-soluble polymer PB125-PEO155 . It is found to form long rodlike micelles, which are characterized with respect to the aggregate length L , the cross sectional radius RCS , the radial scattering length density profile sigma r, and the radial aggregation number Nrad. Model-independent as well as model-based approaches are used for the scattering data analysis. From dynamic light scattering (DLS) and static light scattering (SLS) experiments the weight averaged length Lw of these stiff elongated aggregates is determined to Lw=350 nm . Small angle neutron scattering (SANS) reveals a cross sectional radius of RCS=17 nm and in combination with results from the SLS the radial aggregation number is found to be Nrad=70 .

The effect of electrolyte on the morphology of vesicles composed of the dialkyl polyoxyethylene ether surfactant 2C 18E 12

Small-angle neutron-scattering (SANS) studies were performed on vesicles composed of 1,2-di- O-octadecyl- rac-glyceryl-3-(ω-methoxydodecaethylene glycol), in deuterium oxide (D 2O) solutions with various ionic strengths of LiCl, NaCl and NaI. Gross vesicle morphologies, examined using freeze-fracture electron microscopy, showed that NaCl promoted the formation of multilamellar vesicles. Model fitting of the SANS data showed changes in bilayer parameters such as thickness and repeat spacings, in response to the presence of ions in the bulk solution. 2C 18E 12 vesicles in D 2O are shown to exist as predominantly unilamellar structures with a bilayer thickness of ∼51 Å. Vesicles in increasing concentrations of LiCl and NaCl exhibit decreased layer thickness and increased lamelarity. Little change was observed for vesicles formed in NaI solutions. We suggest that these changes result from intrusion of E 12 headgroups into the alkyl chain region of the vesicle bilayers, in response to the increase in concentration of ions present and their charge density.

Aggregation Properties of Novel Cationic Surfactants with Multiple Pyridinium Headgroups. Small-Angle Neutron Scattering and Conductivity Studies

Novel single-chain cationic surfactants bearing one, two, and three pyridinium headgroups have been synthesized. These surfactants upon solubilization in water produced micelles or large lamellar structures depending on the number of headgroups they possess. The critical micellar concentrations (cmc) of these surfactants were determined by measuring the specific conductance of the aqueous solution of the surfactants. The cmc values were found to increase with every increase in the number of headgroups in the surfactant. Detailed small-angle neutron scattering (SANS) studies were carried out with their aqueous solutions (D2O) to study their aggregation properties. The data have been analyzed using the Hayter and Penfold model for macro-ion solution to compute the interparticle structure factor S(Q), taking into account the screened Coulomb interactions between the micelles. SANS analysis clearly indicated that the nature of the aggregate structure of these cationic micelles depends on the number of the headgroups present in the surfactants. It was found that a single-headed new pyridinium amphiphile formed the lamellar structure, whereas surfactants with double and triple headgroups formed micelles in water. The aggregates shrank in size with an increase in the number of headgroups in the surfactants. The aggregation number (N) continually decreased and the fractional charge (α) increased with a greater number of headgroups on the surfactants. The semimajor axis (a) and semiminor axis (b = c) of the micelle also decreased with the increase in the number of headgroups in the surfactants. It indicates that hydrocarbon chains in such micelles prepared from multiheaded surfactants adopt a bent conformation and no longer stay in extended conformation. The effects of variation of the concentration and temperature on aggregation properties for all the surfactants have also been investigated. In the case of h = 1, the lamellar structure remains approximately the same with the variation of concentration, but with an increase in temperature there was transition from lamellar to rodlike micelles. For other surfactants, with an increase in concentration the aggregation number and the size of the micelles increased. The micelles became progressively smaller in size, the aggregation number decreased, and the fractional charge increased with an increase in temperature.