SAXS Curves Research Articles

New long tail gemini surfactant in aqueous solution: Self-assembly, rheological properties and responsiveness to hydrocarbon

Gemini surfactants are promising to replace conventional monomeric surfactants in various applications. In this paper, a new gemini surfactant with long unsaturated oleyl tails and bis(hydroxyethyl) ammonium head groups linked by butyl spacer was synthesized. Its critical micelle concentration in salt-free water (12 µM) is much lower than for its monomeric analog (220 µM). Without any additives this surfactant can form viscoelastic solutions with the viscosity of up to 1 kPa·s and elastic modulus of 15 Pa at the conditions, when the viscosity of the single-chained monomeric analog is close to that of pure water. These are one of the highest values reported so far for C18-tailed gemini surfactants without additives. The structure of the gemini surfactant solutions was studied by cryo-TEM and SAXS. Cryo-TEM revealed the presence of closed-looped micelles coexisting with linear wormlike micelles as well as the regular spacing between the micelles arising from strong electrostatic repulsion between them. The SAXS curves were well-fitted by a form-factor of core–shell cylinder, which yielded the radius of the core and the thickness of the shell equal to 1.4 nm each. Addition of n-decane induced the drop of viscosity by 6 orders of magnitude accompanied by the transformation of wormlike micelles into microemulsion droplets with 4.4 nm radius. Such gemini surfactants providing high viscoelasticity to aqueous solution that is responsive to added hydrocarbon are promising for the application in oil recovery as thickeners of fracturing fluids that can be cleaned up without any breakers at the backflow of oil.

Amyloid Peptide Mixtures: Self-Assembly, Hydrogelation, Nematic Ordering, and Catalysts in Aldol Reactions.

Morphological, spectroscopic, and scattering studies of the self-assembly and aggregation of mixtures of [RF]4 and P[RF]4 peptides (where R = arginine; F = phenylalanine; P = proline), in solution and as hydrogels, were performed to obtain information about polymorphism. CD data confirmed a β-sheet secondary structure in aqueous solution, and TEM images revealed nanofibers with diameters of ∼10 nm and micrometer lengths. SAXS curves were fitted using a mass fractal-component and a long cylinder shell form factor for the liquid samples, and only a long cylinder shell form factor for the gels. Increasing the P[RF]4 content in the systems leads to a reduction in cylinder radius and core scattering density, suggesting an increase in packing of the peptide molecules; however, the opposite effect is observed for the gels, where the scattering density is higher in the shell for the systems containing higher P[RF]4 content. These compounds show potential as catalysts in the asymmetric aldol reactions, with cyclohexanone and p-nitrobenzaldehyde in aqueous media. A moderate conversion (36.9%) and a good stereoselectivity (69:31) were observed for the system containing only [RF]4. With increasing P[RF]4 content, a considerable decrease of the conversion was observed, suggesting differences in the self-assembly and packing factor. Rheological measurements were performed to determine the shear moduli for the soft gels.

The multi-scale structure, thermal and digestion properties of mung bean starch.

Identification and selection one special variety mung bean for lower GI food is very useful, however, the fundamental study for mung bean starch is still insufficient to meet its demand. In this study, four varieties of mostly planted mung bean in China were selected as model materials. The multi-scale structure of mung bean starch was characterized by SEC, HPAEC, XRD, SAXS, and SEM. SEC and HPAEC give the amylose contents, amylose and amylopectin fine structure of mung bean starch. Mung bean starch from XRD spectrum display CA type semi crystallinity. The crystalline lamellar thickness from SAXS curves were 7.34-7.60 nm. DSC indicated that the peak gelatinization temperature is at 67 °C-68 °C. Resistant starch in mung bean disappears rapidly after cooking, although the amount of slowly digested starch was still more than half of the total starch. Since the gene backgrounds of the mung bean starch samples are very close, there was no obvious difference in their molecular and aggregated state structure, and the digestion properties were similar, too. Unique SEC and HPAEC profiles of starch chain length distribution can be utilized to help find more genetic resources and cultivate variety to meet the needs for starch applications.

Density fluctuations in amorphous pharmaceutical solids. Can SAXS help to predict stability?

The analytical potential of X-ray small-angle scattering (SAXS) combined with simultaneous wide-angle diffraction (WAXS) has been explored on the example of three active pharmaceutical ingredients, (desvenlofaxine, simvastatin, and sulfamerazine, resp.) with the aim of identifying quantitative parameters obtained from SAXS that allow to describe the nano-structural characteristics of different amorphous forms and to monitor the processes of amorphisation and ageing. Cryo-milling, co-milling with polymer, melting and melt-quenching have been used for amorphisation of initially crystalline powders. In parallel to SAXS, the WAXS patterns have been obtained to fingerprint the crystalline state. The SAXS results demonstrate strong, systematic nanostructure variations in amorphous samples obtained by different milling conditions, or by melt-quenching. It has been found that the mean-square density fluctuation, directly obtained from the SAXS invariant, is a sensitive and robust parameter to characterize the degree of nano-heterogeneity, which is related to entropy and hence thermodynamic stability. The SAXS curves also allow estimates of amorphous domain sizes of different density. The propensity to recrystallize or to remain amorphous, respectively, upon ageing has been found to depend on the existence of domains in the starting amorphous materials.

Surfactant-free synthesis of silica aerogel microspheres with hierarchically porous structure

In this work, we developed a new method to synthesize silica aerogel microspheres via ambient pressure drying (APD) process without applying any surfactants and mechanical stirring. An ethanol solution of partially hydrolyzed, partially condensed silica (CS) was used as precursor in the synthesis, the water-repellent n-Heptane as solvent, while the water-soluble ammonia gas (NH3) as catalyst. After a fast sol-gel process and APD process, aerogel microspheres were obtained in the form of white powder with packing density ranged from 62 mg/cm3 to 230 mg/cm3 for different samples. The SEM images exhibited fine spherical morphology for these aerogel microparticles, and their statistical average particle diameter ranged from 0.8 μm to 1.5 μm. Besides, according to the analysis of N2 adsorption-desorption isotherms, the BET surface area of these aerogel microspheres was in the range of 800–960 m2/g, and a considerable volume of micropores was detected along with the abundant mesospores in these microspheres, which was further confirmed by the TEM image and SAXS curve. Based on the very limited solubility of NH3 in the reaction system, a non-emulsion formation mechanism was proposed to illustrate the formation of these aerogel microspheres.

Structural Effects of High Hydrostatic Pressure on Human Low Density Lipoprotein Revealed by Small Angle X-ray and Neutron Scattering

Low-density lipoprotein (LDL) is the principal cholesterol transporter in the human blood circulation. The quasi-spherical LDL particles (∼20 nm) are made up of a complex combination of various lipids and a large single amphipathic protein moiety, named apolipoprotein B-100. LDL has a hydrophobic core and an amphiphilic shell. Each particle has a specific phase transition temperature (Tm) corresponding to the melting of the core lipids from an ordered liquid crystalline to a disordered fluid phase.The structural impact of high hydrostatic pressure (HHP) was studied with different types of LDL (native, oxidized and triglyceride rich) with SANS (PSI, Switzerland) and SAXS (Elettra, Italy). The HHP ranged from 50 to 3000 bar. Temperature points were chosen below, on and above Tm.The pair distance distribution functions p(r) of the SAS curves revealed pressure dependent changes of the particle structure seen in the low q-region (∼0.25 nm−1), also reflected by a decrease of Radii of Gyration (Rg) with increasing pressure.Especially the p(r) functions from the SAXS data do not only show an overall particle change but also a highly pressure sensitive inner organization. A triple-peak feature indicating the lamellar lipid organization below Tm was induced by raising the pressure up to 3000 bar. These pressure sensitive lipid layers were observed by scattering intensity changes in SAXS curves at q=1.7 nm−1.The shape alterations could be evidenced by fitting an ellipsoidal model to the SANS curves resulting in a decrease of the ellipsoidal radii under pressure. A new LDL model considering the cross-sectional electron density profile is developed and applied to the SAS data to get a more detailed insight into pressure dependent behavior.

Structural discrimination of nanosilica particles and mixed-structure silica by multivariate analysis applied to SAXS profiles in combination with FT-IR spectroscopy

New methodology to quickly identify changes in the structural properties of mesoporous silica materials through simultaneous multivariate analyses applied to techniques with different principles as SAXS curves and FT-IR spectra.

Exploring free energy landscapes of large conformational changes: molecular dynamics with excited normal modes.

Proteins are found in solution as ensembles of conformations in dynamic equilibrium. Exploration of functional motions occurring on micro- to millisecond time scales by molecular dynamics (MD) simulations still remains computationally challenging. Alternatively, normal mode (NM) analysis is a well-suited method to characterize intrinsic slow collective motions, often associated with protein function, but the absence of anharmonic effects preclude a proper characterization of conformational distributions in a multidimensional NM space. Using both methods jointly appears to be an attractive approach that allows an extended sampling of the conformational space. In line with this view, the MDeNM (molecular dynamics with excited normal modes) method presented here consists of multiple-replica short MD simulations in which motions described by a given subset of low-frequency NMs are kinetically excited. This is achieved by adding additional atomic velocities along several randomly determined linear combinations of NM vectors, thus allowing an efficient coupling between slow and fast motions. The relatively high-energy conformations generated with MDeNM are further relaxed with standard MD simulations, enabling free energy landscapes to be determined. Two widely studied proteins were selected as examples: hen egg lysozyme and HIV-1 protease. In both cases, MDeNM provides a larger extent of sampling in a few nanoseconds, outperforming long standard MD simulations. A high degree of correlation with motions inferred from experimental sources (X-ray, EPR, and NMR) and with free energy estimations obtained by metadynamics was observed. Finally, the large sets of conformations obtained with MDeNM can be used to better characterize relevant dynamical populations, allowing for a better interpretation of experimental data such as SAXS curves and NMR spectra.

Small-angle scattering simulations for suspensions of nanocrystals

Suspensions of nanocrystals which possess large lattice spacings d(hkl) and only a small number of unit cells along the direction of the reciprocal lattice vector G(hkl) can feature broadened Bragg peaks in small-angle scattering (SAS) patterns. The scattering from molecules located at the interface between the nanocrystals and the dispersion medium which stabilize and functionalize the nanocrystals can interfere with the scattering of the nanocrystals and affect the shape and position of their Bragg peaks. This allows to study how these molecules arrange on the surface of the nanocyrstals. As an example we study suspensions of lecithin stabilized β-tripalmitin nanocrystals which adopt a platelet-like shape. Their SAS patterns exhibit a broadened 001 Bragg peak (cf. SAXS curves in the graphical abstract). With the x-ray and neutron powder pattern simulation analysis (XNPPSA) we have demonstrated that the SAXS and SANS patterns of dilute tripalmitin (3 wt%) suspensions can be simultaneously reproduced on an absolute scale [1,2]. Thereby, powder averaged SAS diffractograms are computed for an ensemble of nanocrystals which are embedded in a dispersion medium. The crystallographic structure of the nanocrystals (CIF-file) and their geometry are taken into account and the amphiphilic lecithin molecules which cover the nanocrystals are modelled with two shells (cf. model in the right inset). From the analysis of the fitted shell thicknesses and scattering length densities it turns out that the lecithin molecules arrange rather flatly and densely packed on the surface of the nanocyrstals. Moreover, the XNPPSA method allows a reliable determination of the thickness distribution of the nanocrystals with molecular resolution [1,2]. With rising tripalmitin concentration the platelets form self-assembled stack-like structures [1,3] and finally nematic liquid-crystalline domains. The XNPPSA allows to investigate the structure and amount of such stacks in the suspensions.

Micropore formation process during stretching of polypropylene casting precursor film

The micropore formation process during stretching of polypropylene casting precursor film was followed by DSC, SEM, AFM, SAXS and WAXD. DSC and SAXS results prove that annealing improves the lamellae periodicity and new crystals are formed. The increase of lamellae thickness is also observed by SEM and AFM. Preferentially oriented crystalline grain is along the direction perpendicular to the MD. After cold stretching, the newly formed crystals between the lamellae disappear and the voids appear in SEM and AFM. The hot stretching makes the lamellae separated step by step and many connecting bridge crystals appear between the adjacent lamellae, at the same time pores are formed. Their crystallisation behaviour is proved by the higher temperature shoulder in DSC curves and the broadening of second order peak in SAXS curves. During stretching, the long period decreases and the lamellae become twisted. More crystal grains are preferentially oriented along the direction of (110) plane.

Partial Aggregation of Silver Nanoparticles Induced by Capping and Reducing Agents Competition

It is well known that nanomaterials properties and applications are dependent on the size, shape, and morphology of these structures. Among nanomaterials, silver nanoparticles (AgNPs) have attracted attention since they have considerably versatile properties, such as a variable surface area to volume ratio, which is very useful for many biomedical and technological applications. Within this scenario, small nanoparticle aggregates can have their properties reduced due to the increased size and alterations in their shape/morphology. In this work, silver nanoparticles aggregation was studied through chemical reduction of silver nitrate in the presence of sodium borohydride (reducing agent) and sodium citrate (capping agent). By changing the amount of reducing agent along the reaction, unaggregated and partially aggregated samples were obtained and characterized by UV-vis, zeta potential, and SAXS techniques. pH was measured in every step of the reaction in order to correlate these results with those obtained from structural techniques. Addition of the reducing agent first causes the reduction of Ag(+) to silver nanoparticles. For higher concentrations of sodium borohydrate, the average AgNPs size is increased and NPs aggregation is observed. It was found that zeta potential and pH values have a strong influence on AgNPs formation, since reducing agent addition can induce partial removal of citrate weakly associated on the AgNPs surface and increase the ionic strength of the solution, promoting partial aggregation of the particles. This aggregation state was duly identified by coupling SAXS, zeta potential and pH measurements. In addition, the SAXS technique showed that aggregates formed along the process are elongated-like particles due to the exponential decay evidenced through SAXS curves.

Modeling of Polymer Structure with the Use of SAXSDAT Computer Program

The most important structural parameters of semicrystalline polymers can be determined by modeling of their structure and best fitting of a theoretical intensity distribution calculated for the assumed model to the experimental SAXS curve. This paper presents a module of a computer program SAXSDAT dedicated to such calculations.

Solution Structures of Flexible RNA Molecules in Mono- and Divalent Salt

Cells transmit and process genetic information using single-stranded RNA (ssRNA); these molecules realize their function through dynamic structural changes such as folding, association with binding partners, and conformational switching. Disordered, un-basepaired states of RNA play a key role in these dynamic structural transitions, yet most relevant measurements of single-stranded oligonucleotides have focused on DNA. Thus, we measure and compare the conformations of chemically similar ssRNA and ssDNA oligonucleotides lacking secondary structure. SAXS curves and smFRET efficiencies of rU40 and dT40 in 100mM NaCl are well-described by a simple wormlike chain model. We detect subtle but significant differences between the contour and persistence lengths of rU40 and dT40 that agree with predictions based on relative sugar pucker preferences of the two nucleotides. To compare the polyelectrolyte properties of ssRNA and ssDNA, we report persistence lengths derived from smFRET data acquired over a wide range of ionic conditions. We find that the apparent charge screening efficiency of divalent magnesium is anomalously large compared to monovalent sodium in both ssRNA and ssDNA. This strong interaction between divalent ions and disordered RNA presents an important problem for polyelectrolyte theory and challenges our understanding of how ions influence RNA folding.

Nanostructure and dynamic mechanical properties of silane-functionalized montmorillonite/epoxy nanocomposites

In this work sodium montmorillonite (Na-MMT) was functionalized with N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane and the corresponding silylated clay was used to modify epoxy matrix cured with triethylenetetramine. The grafting/intercalation of the aminosilane inside the clay galleries were followed by infrared spectroscopy, X-ray diffraction, thermogravimetric analysis and 29Si cross-polarization magic-angle-spinning nuclear magnetic-resonance (CP/MAS NMR) spectroscopy. Epoxy-based nanocomposites were prepared with different amounts of silylated clay or commercial organoclay, Cloisite 30B, whose intercalating agent consists of a methyl, tallow, bis-2-hydroxyethyl quaternary ammonium salt. The degree of intercalation/exfoliation was estimated by X-ray diffraction experiments and confirmed by small angle X-ray scattering. Nanocomposites prepared with silylated clay displayed no peak in both XRD and SAXS curves whereas those prepared with Cloisite 30B exhibited a clear interference peak corresponding to an interlayer spacing d 001 of 4.1 nm. The former also presented a better dispersion, with a high proportion of tactoids smaller than 2 nm, as estimated by SAXS. From the results of dynamic mechanical analysis it was observed that most of the nanocomposites display higher storage modulus mainly at temperatures above the glass transition temperature. The glass transition temperature is similar or higher than the neat epoxy network for nanocomposites containing 1 wt.% of silylated clay or higher.

SAXS and WAXD, Time Resolved Investigations of the Morphology of Polyethylenes

This work shortly describes the morphology of polymers and presents the possibilities of SAXS method in the investigations of the structure of polymers referring to our studies on the structure of homogeneous copolymers ethylene-1-octene. Besides, we describe our computer program SAXSDAT designed for the analysis of SAXS curves of polymers.

Effects of stretching on crystalline phase structure and morphology of hard elastic PVDF fibers

AbstractThe phase characteristics and morphology of stretched hard elastic poly(vinylidene fluoride) (PVDF) fibers were investigated by X‐ray diffraction (XRD) and wide‐angle and small‐angel X‐ray scattering (WAXS and SAXS). It was indicated that α and β phases coexisted in stretched PVDF fibers, stretching assisted in α to β phase transformation. The β/α ratios of stretched PVDF fibers were affected by stretching temperature, rate, and ratio. The β phase content of stretched PVDF fibers had an abrupt increase when stretched near 70°C, and then it decreased with increasing stretching temperature. Besides, the β/α ratio of PVDF fibers increased with stretching rate and ratio. The total crystallinity of PVDF fibers did not change much even on different stretching conditions. WAXS results indicated that the unstretched and stretched PVDF fibers all exhibited three strong equatorial streaks, with d‐spacing (0.964, 0.488, and 0.439 nm) and (0.946, 0.494, and 0.480 nm), which suggested that PVDF fibers still remained the crystalline reflections of c‐axis orientation even after being stretched. The long periods of stretched PVDF fibers, calculated from SAXS curves, increased from 19.04 to 39.75nm. On the basis of these results, the β transformation mechanism of stretched PVDF fibers was also discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2254–2259, 2007

Investigation of the Thermotropic Behavior of Isomer Mixtures of Diacyl Arginine-Based Surfactants. Comparison of Polarized Light Microscopy, DSC, and SAXS Observations

Dilauroylglycerol arginine conjugates constitute a novel class of specific biocompatible surfactants, which can be considered analogues of partial glycerides and phospholipids. During their preparation, spontaneous intramolecular acyl-migration reactions are observed and both possible regioisomers are obtained: 1,2-dilauroyl-rac-glycero-3-(Nα-acetyl-l-arginine) (1212RAc) and 1,3-dilauroylglycero-2-(Nα-acetyl-l-arginine) (12RAc12). To study the influence of the presence of both regioisomers on the physicochemical properties, the phase behavior in the dry state of pure 1,2-dilauroyl-rac-glycero-3-(Nα-acetyl-l-arginine) and two mixtures of both regioisomers were studied by differential scanning calorimetry and small-angle X-ray diffraction complemented by polarized light microscopy. The experiments were performed for a sequence of heating, cooling and reheating scans. The results have shown that all samples investigated display thermotropic liquid crystallinity. The transition temperatures have been observed to be independent of the particular technique. The first heating scan is different from the cooling and reheating scans with respect to both thermal and structural parameters. The SAXS curves have shown the coexistence of two orders of lamellar spacing in the first heating scan. This coexistence is lost in subsequent scans. This suggests the presence of microphase separation of isomers in the lyophilized samples with miscibility after melting. The lamellar distance obtained for the third scan was used to study the molecular conformation of 1212RAc and 12RAc12 compounds.

Change of pH indicator's pKa value via molecular imprinting.

The pKa value of the indicator is significantly changed up to several orders of magnitude by the imprinting synthesis; this fine-tuning of the reactivity for pH indicators via the imprinting synthesis may find novel applications in fabricating pH sensors with tailored pKa values.

Hydrolysis of Titanium Alkoxides: Thermochemical, Electron Microscopy, Saxs Studies

Hydrolysis of Ti(OR)4 (R = Et, iPr, nBu) at various concentrations of titanium alkoxides and ratios h = [H2O] /[Ti(OR)4] is studied in alcoholic medium by means of calorimetry, electron microscopy, SAXS, and chemical analysis. The measured values for heat of hydrolysis of Ti(OR)4 by excess water (−Δ Hh) at 298.15 K comprise 14.2, 64.9, 19.3 kJ/mol for R = Et, R = Et, iPr, nBu respectively. −Δ Hh increases drastically in the region of 0<h<1 and demonstrates practically no changes with further increase of h ratio. In the solid hydrolysis product with the composition TiOx(OR)4−2x·y ROH, both x and y increase with increase of Ti(OR)4 concentration in solution. Bushy network first formed in solution as a result of hydrolysis gradually structures with formation of well-shaped spherical particles with diameters ≥0.2μm. SAXS curves analysis in the range of scattering vector values s = 0.07–4.26 nm−1 for Ti(OBu)4 hydrolysis products allows us to suggest their multilevel nature. Speculations on the structure of titanium oxobutoxide were made on the basis of the well-known structural data for crystalline first hydrolysis products of Ti(OEt)4 and Ti(Oi Pr)4. It is suggested to perform hydrolysis of Ti(OBu)4 with addition of water in two steps which allows us to decrease the rate of the solid precipitate formation, to regulate particles morphology in a wide range and to obtain well-shaped spherical species more than one micron in size. The influence of the powder size distribution on the grain growth during ceramic sintering is discussed.

Structural temperature transformation of the cellulose–water system using time-resolved SAXS

The main goal of the work was to examine the structural changes induced by water (D2O and H2O) in the dense cellulose film, Tomophan I, using temperature time-resolved SAXS and WAX measurements. We found that dry cellulose and water-saturated cellulose contains hydrophobic microregions at room temperature. At increasing temperature microphase separation was observed, as a formation of a broad peak on the SAXS curves, with Tc of about 55 and 60°C, depending on the kind of water. Further heating, up to 85°C, causes an appearance of a set of very small peaks, generated by the cubic structure, in SAXS. The structural changes of the system are typical for the thermo-associative process, observed in the copolymer–water systems.