小角X射线散射中porod正偏离的校正

当散射体系中除散射体外还存在微电子密度起伏时,实测散射强度将形成对Porod定理的正偏离,从而使散射体的散射失真。提出了一种在长狭缝准直条件下应用模糊强度校正正偏离的方法：作出In[q^3I(q)]～q^2曲线,用公式n[q^3I(q)]=InK'＋σ^2q^2拟合大波矢区直线,求出斜率σ^2,作出In[q^3I(q)]-σ^2q^2～q^2曲线即为无偏离的Porod曲线,由此曲线再还原出无偏离的散射强度,即I'(q)=exp{In[q^3I(q)]-σ^2q^2}/q^3,再以醇热法合成的介孔氧化锆粉体为例进行了讨论。     

Curvature effect on the structure of phospholipid bilayers

High-resolution small-angle X-ray scattering (SAXS), complemented by small-angle neutron scattering (SANS) and dynamic light scattering (DLS) experiments, was used to study the effect of curvature on the bilayer structure of dioleoyl-phosphatidylcholine (DOPC) and dioleoyl-phosphatidylserine (DOPS) unilamellar vesicles (ULVs). Bilayer curvature, as a result of finite vesicle size, was varied as a function of vesicle radius and determined by DLS and SANS measurements. Unilamellarity of large DOPC ULVs was achieved by the addition of small amounts (up to 4 mol %) of the charged lipid, DOPS. A comparison of SANS data over the range of 0.02 < q <0.2 A-1 indicated no change in the overall bilayer thickness as a function of ULV diameter (620 to 1840 A). SANS data were corroborated by high-resolution (0.06 < q <0.6 A-1) SAXS data for the same diameter ULVs and data obtained from planar samples of aligned bilayers. Both the inner and outer leaflets of the bilayer were found to be indistinguishable. This observation agrees well with simple geometric models describing the effect of vesicle curvature. However, 1220-A-diameter pure DOPS ULVs form asymmetric bilayers whose structure can most likely be rationalized in terms of geometrical constraints coupled with electrostatic interactions, rather than curvature alone.     

Applications of small-angle X-ray scattering/small-angle neutron scattering and cryogenic transmission electron microscopy to understand self-assembly of surfactants

The self-assembling structures and dynamics of surfactants determine most of their macroscopic physicochemical properties and performances. Herein, we review recent work on the self-assembly of surfactants by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) in conjunction with cryogenic transmission electron microscopy (Cryo-TEM) from the perspective of researchers having only limited theoretical knowledge of these techniques but expert in surfactants. Emphasis is placed on the structural analysis of typical surfactant aggregates over a wide range of size scales from nanometers up to microns, including spherical and rod-like micelles, wormlike micelles, vesicles, liquid crystals and coacervates, by combining different numerical approaches to the treatment of small-angle scattering data with the direct Cryo-TEM imaging method. Furthermore, the complementarity between SAXS and SANS, and between the scattering techniques and Cryo-TEM, that is, specific contributions of these techniques, is also covered.     

SAXS and SANS studies of polymer colloids

The analysis of latex particles by small-angle scattering (small-angle X-ray scattering, SAXS; small-angle neutron scattering, SANS) is reviewed. Small-angle scattering techniques give information on the radial structure of the particles as well as on their spatial correlation. Recent progress in instrumentation allows to extend SANS and SAXS to the q-range of light scattering. Moreover, contrast variation employed in SANS and SAXS studies may lead to an unambiguous determination of the radial scattering length density of the particles in situ, i.e. in suspension. Hence, these techniques are highly valuable for a comprehensive analysis of polymer colloids as shown by the examples discussed herein.     

Small-angle neutron and x-ray scattering study of swollen nylon-6

Results of swelling and small-angle scattering experiments on samples of nylon-6 swollen with heavy water are discussed on the basis of the lamellar and switchboard models. The small-angle neutron scattering (SANS) intensity is very sensitive to the distribution of water in swollen samples, while the small-angle x-ray scattering (SAXS) data characterize the dry samples. The observed values of the mean-square fluctuation of scattering-density can be explained by a model with assumed inhomogeneous swelling of the amorphous phase.     

A novel application of small-angle scattering techniques: Quality assurance testing of virus quantification technology

Small-angle scattering (SAS) techniques, like small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), were used to measure and thus to validate the accuracy of a novel technology for virus sizing and concentration determination. These studies demonstrate the utility of SAS techniques for use in quality assurance measurements and as novel technology for the physical characterization of viruses.     

Structure and interactions of block copolymer micelles of Brij 700 studied by combining small-angle X-ray and neutron scattering

Spherical micelles of the diblock copolymer/surfactant Brij 700 (C(18)EO(100)) in water (D(2)O) solution have been investigated by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). SAXS and SANS experiments are combined to obtain complementary information from the two different contrast conditions of the two techniques. Solutions in a concentration range from 0.25 to 10 wt % and at temperatures from 10 to 80 degrees C have been investigated. The data have been analyzed on absolute scale using a model based on Monte Carlo simulations, where the micelles have a spherical homogeneous core with a graded interface surrounded by a corona of self-avoiding, semiflexible interacting chains. SANS and SAXS data were fitted simultaneously, which allows one to obtain extensive quantitative information on the structure and profile of the core and corona, the chain interactions, and the concentration effects. The model describes the scattering data very well, when part of the EO chains are taken as a "background"contribution belonging to the solvent. The effect of this becomes non-negligible at polymer concentrations as low as 2 wt %, where overlap of the micellar coronas sets in. The results from the analysis on the micellar structure, interchain interactions, and structure factor effects are all consistent with a decrease in solvent quality of water for the PEO block as the theta temperature of PEO is approached.     

Formation and structure of self-assembled silica nanoparticles in basic solutions of organic and inorganic cations

The phase behavior of silica solutions containing organic and inorganic cations was studied at room temperature using conductivity, pH, and small-angle scattering experiments. A critical aggregation concentration (cac) was observed at approximately 1:1 ratio of SiO(2)/OH(-) for all cation solutions from conductivity and pH studies. From this cac, a phase diagram of the system was developed with three distinct phase regions in pseudoequilibrium: a monomer/oligomer region (I), a monomer/oligomer/nanoparticle region (II), and a gel region (III). Small-angle X-ray and neutron scattering (SAXS and SANS) on solutions of region II formed with tetrapropylammonium hydroxide (TPAOH) revealed that the nanoparticles have a core-shell structure. Structure analysis of the SAXS and SANS data was best fit by a core-shell oblate ellipsoid model. A polydisperse set of core-shell spheres also fit the data well although with lower agreement factors. Similar nanoparticle morphologies were found in solutions of TMAOH, CsOH, and NaOH.     

In situ investigations on organic foam films using neutron and synchrotron radiation

We report on small-angle neutron scattering (SANS) and X-ray scattering (SAXS) investigations of foam films stabilized by sodium dodecyl sulfate. Previous measurements on dry foams (Axelos, M. A. V.; Boue, B. Langmuir 2003, 19, 6598) have shown the presence of spikes in the two-dimensional scattering data which suggest that the incident beam is reflected on some film surfaces. The latter interpretation is confirmed by new neutron studies performed on ordered ("bamboo") foams which allow selection of single films. In the first case, we show that the spikes of the scattered intensity can be obtained by reflection on two parts of the foam, namely, the films and the Plateau borders. With synchrotron radiation, first observations of distinct interference fringes have allowed an accurate determination of the film thickness. A comparison with X-ray and neutron data is made, opening a general discussion about the capabilities of small-angle scattering techniques for studying the microscopic properties of foam films.     

Structures of Fibrous Supramolecular Assemblies Constructed by Amino Acid Surfactants: Investigation by AFM, SANS, and SAXS

Aqueous gel-like solutions of N-acyl-L-aspartic acids (C(n)Asp, n=14, 16, 18) and N-dodecanoyl-beta-alanine (C(12)Ala) were prepared at pH 5-6 at room temperature. Structures of supramolecular assemblies in the solutions were investigated by atomic force microscopy (AFM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). The cross-sectional radii, 22-30 ?, of helical, fibrous assemblies were obtained from analysis of SANS for 1% gel-like C(n)Asp solutions. Three Bragg spacings were observed in a SANS spectrum for a 6% C(16)Asp solution. C(n)Asp molecules are associated into the unit chain of a helical bilayer strand with a diameter of 50-60 ?. Unit chains where linear bilayers twist form a double strand with helical sense of approximately 650-? pitch. It was confirmed from AFM images that cylindrical fibers in a gel-like C(12)Ala solution had a circular cross-section. The SAXS spectrum showed characteristic Bragg spacings. Cylindrical C(12)Ala fibers consist of multilamellar layers of period approximately 34-?. The fibers are laterally organized with period 365-380 ?. Copyright 2000 Academic Press.     

Synthesis and characterization of polystyrene chains on the surface of silica nanoparticles: comparison of SANS, SAXS, and DLS results

An extensive characterization of well-defined polystyrene (PS)-grafted silica nanoparticles is reported. Bare SiO₂ particles (diameter 50 nm) were functionalized with a suitable initiator for the surface-initiated anionic polymerization of styrene. Both grafted and free PS chains were characterized and compared by size-exclusion chromatography (SEC). PS-grafted particles were characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), small-angle x-ray scattering (SAXS), small-angle neutron scattering (SANS), and dynamic light scattering (DLS). The thickness of the grafted PS chains was obtained by SANS and DLS and scaled with $M_{\mathrm {w}}^{0.6}$ displaying similar behavior with free PS chains in the same solvent used, tetrahydrofuran (THF). Grafting densities obtained from SANS data and TGA were found to be small, and the thickness of the grafted PS chains determined by SANS was found to be similar to $2R_{\mathrm {g}}$ of free PS chains in THF. Both results are consistent with a “coil-like” conformation of the grafted PS chains.     

Evolution of self-assembled silica-tetrapropylammonium nanoparticles at elevated temperatures

The time evolution of silica nanoparticles in solutions of tetrapropylammonium (TPA) has been studied using a combination of small-angle scattering, conductivity, and pH measurements to provide the first comprehensive analysis of nanoparticle structural and compositional changes at elevated temperatures. We have found that silica-TPA nanoparticles subjected to hydrothermal treatment (70-90 degrees C) grow via an Ostwald ripening mechanism with growth rates that depend on both pH and temperature. Small-angle X-ray (SAXS) and neutron (SANS) scattering confirm that the core-shell structure of the particles, initially present at room temperature, is maintained during heating, but an evolution toward sphericity is evidenced especially at high values of pH. SAXS absolute intensity calculations were utilized to calculate the changes in nanoparticle composition and concentration over time. These changes along with the conductivity and pH measurements and SANS contrast matching studies indicate that, upon heating, TPA becomes embedded in the core of nanoparticles giving rise to more zeolitic-looking nanomaterials.     

The structure of metallomicelles

The morphology of micelles formed by two novel metallosurfactants has been studied by small-angle neutron scattering (SANS) and small-angle-X-ray scattering (SAXS). The two surfactants both contain a dodecyl chain as the hydrophobic moiety, but differ in the structure of the head group. The surfactants are Cu(II) complexes of monopendant alcohol derivatives of a) the face-capping macrocycle 1,4,7-triazacyclanonane (tacn), and b) an analogue based upon the tetraazamacrocycle 1,4,7,10-tetraazacyclododecane. Here, neutron scattering has been used to study the overall size and shape of the surfactant micelles, in conjunction with X-ray scattering to locate the metal ions. For the 1,4,7,10-tetraazacyclododecane-based surfactant, oblate micelles are observed, which are smaller to the prolate micelles formed by the 1,4,7-triazacyclononane analogue. The X-ray scattering analysis shows that the metal ions are distributed throughout the polar head-group region, rather than at a well-defined radius; this is in good agreement with the SANS-derived dimensions of the micelle. Indeed, the same model for micelle morphology can be used to fit both the SANS and SAXS data.     

Surface morphology of nanostructured polymer-based activated carbons

Complementary techniques, including nitrogen adsorption, small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM), have been utilized to characterize the surface features of highly microporous carbon materials prepared from highly aromatic polymers. Nitrogen adsorption measurement interpreted by BET, DR, HK, and NLDFT methods reveals these nanostructured activated carbons exhibit a high surface area of up to 4000 m2/g, a micropore volume up to approximately 1.75 mL/g, and an average pore size of approximately 10-20 angstroms. A modified equation, based on Porod's law, the Debye-Bueche equation, and fractal dimension theories, has been proposed and successfully applied to analyze the SAXS spectra and to extract the porous texture of these unique activated carbons. AFM 3D imaging combined with the Fourier transform technique has been applied to statistically quantify pore sizes on the carbon surface.     

Interactions between block copolymers and single-walled carbon nanotubes in aqueous solutions: a small-angle neutron scattering study

The amphiphilic copolymers of the Pluronic family are known to be excellent dispersants for single-walled carbon nanotubes (SWCNT) in water, especially F108 and F127, which have rather long end-blocks of poly(ethylene oxide) (PEO). In this study, the structure of the CNT/polymer hybrid formed in water is evaluated by measurements of small-angle neutron scattering (SANS) with contrast variation, as supported by cryo-transmission electron microscopy (cryo-TEM) imaging. The homogeneous, stable, inklike dispersions exhibited very small isolated bundles of carbon nanotubes in cryo-TEM images. SANS experiments were conducted at different D(2)O/H(2)O content of the dispersing solvent. The data for both systems showed surprisingly minimal intensity values at 70% D(2)O solvent composition, which is much higher than the expected value of 17% D(2)O that is based on the scattering length density (SLD) of PEO. At this near match point, the data exhibited a q(-1) power law relation of intensity to the scattering vector (q), indicating rodlike entities. Two models are evaluated, as extensions to Pederson's block copolymer micelles models. One is loosely adsorbed polymer chains on a rodlike CNT bundle. In the other, the hydrophobic block is considered to form a continuous hydrated shell on the CNT surface, whereas the hydrophilic blocks emanate into the solvent. Both models were found to fit the experimental data reasonably well. The model fit required special considerations of the tight association of water molecules around PEO chains and slight isotopic selectivity.     

Experimental evidences for molecular origin of low-Q peak in neutron/x-ray scattering of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquids

Short- and long-range liquid structures of [C(n)mIm(+)][TFSA(-)] with n = 2, 4, 6, 8, 10, and 12 have been studied by high-energy x-ray diffraction (HEXRD) and small-angle neutron scattering (SANS) experiments with the aid of MD simulations. Observed x-ray structure factor, S(Q), for the ionic liquids with the alkyl-chain length n > 6 exhibited a characteristic peak in the low-Q range of 0.2-0.4 A?(-1), indicating the heterogeneity of their ionic liquids. SANS profiles I(H)(Q) and I(D)(Q) for the normal and the alkyl group deuterated ionic liquids, respectively, showed significant peaks for n = 10 and 12 without no form factor component for large spherical or spheroidal aggregates like micelles in solution. The peaks for n = 10 and 12 evidently disappeared in the difference SANS profiles ΔI(Q) [=I(D)(Q) - I(H)(Q)], although that for n = 12 slightly remained. This suggests that the long-range correlations originated from the alkyl groups hardly contribute to the low-Q peak intensity in SANS. To reveal molecular origin of the low-Q peak, we introduce here a new function; x-ray structure factor intensity at a given Q as a function of r, S(Q) (peak)(r). The S(Q) (peak)(r) function suggests that the observed low-Q peak intensity depending on n is originated from liquid structures at two r-region of 5-8 and 8-15 A? for all ionic liquids examined except for n = 12. Atomistic MD simulations are consistent with the HEXRD and SANS experiments, and then we discussed the relationship between both variations of low-Q peak and real-space structure with lengthening the alkyl group of the C(n)mIm.     

Small-angle scattering studies of nafion membranes

The physical structure of Nafion membranes has been investigated by small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS). Samples in the acid form may exhibit two scattering peaks. The first, observed by SANS at an angle corresponding to a Bragg spacing of 180 Å, is shown to arise from structures in crystalline regions. A second peak at larger scattering angles is shown to arise from ion-containing regions which may be swollen with water. Salt-form samples made by soaking the acid form in an aqueous salt solution can also exhibit the same two scattering signals. But in amorphous salt-form samples produced by quenching from the melt the first peak is absent. This permits a more accurate study of the second peak by SAXS, which shows that the second scattering component is present as a maximum over a wide range of water contents but is absent in a sample dried at 200°C. The position of the peak shifts to lower scattering angles (or larger spacings) at higher water contents. Possible structural models that might give rise to the maximum are discussed. A calculation of the SAX invariant is made and results are consistent with a phase separation of a large fraction of the water.     

Microscopic inhomogeneities in weakly-charged temperature sensitive polymer gels and solutions

A comparison of small-angle neutron scattering (SANS) intensity functions, I(q), was made between gels and solutions of poly(N-isopropyl acrylamide-co-acrylic acid) (NIPA/AAc), where q is the magnitude of the scattering vector. I(q)'s were strongly dependent on polymer concentration as well as temperature. At low temperatures, I(q)'s for both gels and solutions were similar to each other and were monotonous decreasing functions. However, at high temperatures above θ, the so-called theta temperature of poly-NIPA in aqueous solution, both had a distinct peak and a significant difference appeared in I(q) due to the presence or absence of crosslinks. Origins of the peak and the differences in I(q) are discussed.     

Synthesis and characterization of nanogels of poly(N-isopropylacrylamide) by a combination of light and small-angle X-ray scattering

Thermo-responsive crosslinked nanogels of N-isopropylacrylamide (NIPAM) were synthesized by emulsion polymerization and the size was varied using different concentrations of surfactant (sodium dodecyl sulfate, SDS) in the polymerization process. The collapse behavior of the nanogels at the lower critical solution temperature at around 32 °C was investigated by dynamic light scattering, and by combined static light scattering (SLS) and small-angle X-ray scattering (SAXS). The combined data from SLS and SAXS were analyzed by a model for the nanogels which at intermediate temperatures included a central core and a more diffuse outer layer describing pending polymer chains with a low degree of cross linking. In the expanded state, the particles were modeled with a single component with a broad graded surface. In the collapsed state the nanogels were modeled as homogeneous and relatively compact particles. The amount of surfactant used had a profound effect on the final size of the nanogels owing to the phenomenon of colloidal stabilization of the emulsion droplets during polymerization. The combination of SLS and SAXS as applied to the nanogels is an attractive method for particle characterization as it spans a very large range of scattering vector from q = 0.0004 to 0.22 ?(-1).