Scattering And Small-angle Neutron Scattering Research Articles

Colloidal Chemistry in Water Treatment: The Effect of Ca2+ on the Interaction between Humic Acid and Poly(diallyldimethylammonium chloride) (PDADMAC).

The complexation of humic acid (HA), as a major component of natural organic matter (NOM) in raw water, with polycations is a key step in the water treatment process. At sufficiently high addition of a polycation, it leads to neutralization of the formed complexes and precipitation. In this work, we studied the effect of the presence of Ca2+ ions on this process, with poly(diallyldimethylammonium chloride) (PDADMAC) as a polycation. This was done by determining the phase behavior and characterizing the structures in solution by light scattering and small-angle neutron scattering (SANS). We observe that with increasing Ca2+ concentration, the phase boundaries of the precipitation region shift to a lower PDADMAC concentration, which coincides well with a shift of the ζ-potential of the aggregates in solution. Light scattering shows the formation of aggregates of a 120-150 nm radius, and SANS shows that Ca2+ addition promotes a compaction in the size range of 10-50 nm within these aggregates. This agrees well with the observation of more densely packed precipitates by confocal microscopy in the presence of Ca2+. Following the precipitation kinetics by turbidimetry shows a marked speeding up of the process already in the presence of rather small Ca2+ concentrations of 1 mg/L. It can be stated that the presence of Ca2+ during the complexation process of HA with a polycation has a marked effect on phase behavior and precipitation kinetics of the formed aggregates. In general, the presence of Ca2+ facilitates the process largely already at rather low concentrations, and this appears to be linked to a compaction of the formed structures in the mesoscopic size range of about 10-50 nm. These findings should be of significant importance for tailoring the flocculation process in water treatment, which is a highly important process in delivering drinking water of sufficient quality to humans.

Effect of red blood cell shape changes on haemoglobin interactions and dynamics: a neutron scattering study.

By using a combination of experimental neutron scattering techniques, it is possible to obtain a statistical perspective on red blood cell (RBC) shape in suspensions, and the inter-relationship with protein interactions and dynamics inside the confinement of the cell membrane. In this study, we examined the ultrastructure of RBC and protein–protein interactions of haemoglobin (Hb) in them using ultra-small-angle neutron scattering and small-angle neutron scattering (SANS). In addition, we used the neutron backscattering method to access Hb motion on the ns time scale and Å length scale. Quasi-elastic neutron scattering (QENS) experiments were performed to measure diffusive motion of Hb in RBCs and in an RBC lysate. By using QENS, we probed both internal Hb dynamics and global protein diffusion, on the accessible time scale and length scale by QENS. Shape changes of RBCs and variation of intracellular Hb concentration were induced by addition of the Na+-selective ionophore monensin and the K+-selective one, valinomycin. The experimental SANS and QENS results are discussed within the framework of crowded protein solutions, where free motion of Hb is obstructed by mutual interactions.

Ionic dye–surfactant nanoassemblies: interplay of electrostatics, hydrophobic effect, and π–π stacking

The interplay of several non-covalent interaction forces is used as key to supramolecular structures. Combining cationic alkyltrimethylammonium bromide surfactants and the divalent anionic azo dye Acid Red 26 (Ar26) as small building blocks in aqueous solution, electrostatic interactions of the oppositely charged building blocks in combination with hydrophobic effect and π–π interactions play a major role in aggregate formation. Static and dynamic light scattering and small-angle neutron scattering (SANS) revealed different sizes of aggregates in the range of 2 nm ≤ R H ≤ 420 nm depending on surfactant length, concentration and of dye to surfactant loading ratio. A strong relationship of assembly size with surfactant concentration has been found, where initial surfactant monomers and micelles influence the aggregate formation differently. The stability of dye–surfactant aggregates which also shows a dependency on surfactant tail length has been related to ζ-potential measurements. Small-angle neutron scattering elucidated that dye–surfactant aggregates possess cylindrical shapes with different aspect ratios. UV/Vis spectroscopy gave information on the dye–dye π–π stacking geometry and extent, while the thermodynamic parameters for micellization and dye–surfactant binding ΔH, ΔG, and ΔS as well as stoichiometry and binding constant obtained by isothermal titration calorimetry revealed insight into the interplay of interactions.

Solution structure of the 128 kDa enzyme I dimer from Escherichia coli and its 146 kDa complex with HPr using residual dipolar couplings and small- and wide-angle X-ray scattering.

The solution structures of free Enzyme I (EI, ∼128 kDa, 575 × 2 residues), the first enzyme in the bacterial phosphotransferase system, and its complex with HPr (∼146 kDa) have been solved using novel methodology that makes use of prior structural knowledge (namely, the structures of the dimeric EIC domain and the isolated EIN domain both free and complexed to HPr), combined with residual dipolar coupling (RDC), small- (SAXS) and wide- (WAXS) angle X-ray scattering and small-angle neutron scattering (SANS) data. The calculational strategy employs conjoined rigid body/torsion/Cartesian simulated annealing, and incorporates improvements in calculating and refining against SAXS/WAXS data that take into account complex molecular shapes in the description of the solvent layer resulting in a better representation of the SAXS/WAXS data. The RDC data orient the symmetrically related EIN domains relative to the C(2) symmetry axis of the EIC dimer, while translational, shape, and size information is provided by SAXS/WAXS. The resulting structures are independently validated by SANS. Comparison of the structures of the free EI and the EI-HPr complex with that of the crystal structure of a trapped phosphorylated EI intermediate reveals large (∼70-90°) hinge body rotations of the two subdomains comprising the EIN domain, as well as of the EIN domain relative to the dimeric EIC domain. These large-scale interdomain motions shed light on the structural transitions that accompany the catalytic cycle of EI.

Dilute Solution Properties and Surface Attachment of RAFT Polymerized 2-Vinyl-4,4-dimethyl Azlactone (VDMA)

We report the controlled radical polymerization of 2-vinyl-4,4-dimethyl azlactone (VDMA), a 2-alkenyl-2-oxazolin-5-one monomer that contains a polymerizable vinyl moiety and a highly reactive, pendant azlactone, as well as dilute solution properties and surface attachment and functionalization. Reversible addition−fragmentation chain transfer (RAFT) was used to polymerize VDMA in benzene at 65 °C using either 2-(2-cyanopropyl) dithiobenzoate (CPDB) or 2-dodecylsulfanylthiocarbonylsulfanyl-2-methylpropionic acid (DMP) as RAFT chain transfer agents (CTAs). The pseudo-first-order kinetics and resultant well-defined polymers of low polydispersity indicate that both CTAs afford control over the RAFT polymerization of VDMA. Dynamic and static light scattering and small-angle neutron scattering (SANS) were performed to determine the weight-average molecular weight, radius of gyration, and second virial coefficient of VDMA homopolymers in THF. Additionally, well-defined polymers of VDMA containing carboxyl end gr...

Surface aggregate structure of nonionic surfactants on silica nanoparticles

The self-assembly of two nonionic surfactants, pentaethylene glycol monododecyl ether (C12E5) and n-dodecyl-{\ss}-maltoside ({\ss}-C12G2), in the presence of a purpose-synthesized silica sol of uniform particle size (diameter 16 nm) has been studied by adsorption measurements, dynamic light scattering and small-angle neutron scattering (SANS) using a H2O/D2O mixture matching the silica, in order to highlight the structure of the surfactant aggregates. For C12E5 strong aggregative adsorption onto the silica beads, with a high plateau value of the adsorption isotherm above the CMC was found. SANS measurements were made at a series of loadings, from zero surfactant up to maximum surface coverage. It is found that the spherical core-shell model nicely reproduces the SANS data up to and including the local maximum at q = 0.42 nm-1 but not in the Porod region of high q, indicating that the surface area of the adsorbed surfactant is underestimated by the model of a uniform adsorbed layer. A satisfactory representation of the entire scattering profiles is obtained with the model of micelle-decorated silica beads, indicating that C12E5 is adsorbed as spherical micellar aggregates. This behaviour is attributed to the high surface curvature of the silica which prevents an effective packing of the hydrophobic chains of the amphiphile in a bilayer configuration. For the maltoside surfactant {\ss}-C12G2 very weak adsorption on the silica beads was found. The SANS profile indicates that this surfactant forms oblate ellipsoidal micelles in the silica dispersion, as in the absence of the silica beads.

Adsorbed Anthranilic Acid Molecules Cause Charge Reversal of Nonionic Micelles

The effect of anthranilic acid, an aromatic amino acid, on the structural characteristics of nonionic micelles of Triton X-100 at different pH values was investigated by light scattering and small-angle neutron scattering (SANS) measurements. The scattered light intensity decreases as pH is increased or decreased on either side of the isoelectric point (IEP = 3.4) of the amino acid. Analysis of the SANS data using a sticky hard-sphere model shows that the micelles are oblate ellipsoids with an axial ratio of approximately 2.3. No significant change could be observed in the size of the micelles with a change in the pH, while the stickiness parameter (tau), which is related to the interaction potential (u(0)) increases on either side of the IEP. As tau increases, u(o) becomes less negative, indicating a decrease in the attractive interaction between nonionic micelles. This can be explained in terms of the changes in the surface charge of the micelles resulting from a shift in the acid-base equilibrium of the adsorbed amino acid. The variation of the intermicellar interaction as calculated from the stickiness parameter is consistent with the picture of reversal of charge of amino acids with pH. This is further supported by the observed variation of the cloud point of the solutions at different pH values. The change in the interparticle interaction is also reflected in the diffusion coefficient of the micelles measured by dynamic light scattering.

Structural and Relaxometric Characterization of Peptide Aggregates Containing Gadolinium Complexes as Potential Selective Contrast Agents in MRI

The structural and relaxometric characterization of a novel class of supramolecular aggregates, as potential tumor-specific contrast agents in magnetic resonance imaging (MRI), is reported. The aggregates are based on a new monomer with an upsilon shape (MonY) that contains, in the same molecule, all three fundamental tasks that are required: 1) a hydrophobic moiety that allows the formation of supramolecular aggregates; 2) the bioactive CCK8 peptide for target recognition; and 3) a chelating agent able to give stable gadolinium complexes. As indicated by dynamic light scattering and small-angle neutron scattering (SANS) measurements, MonY and its gadolinium complex MonY(Gd) aggregate in aqueous solution to give ellipsoidal micelles with a ratio between the micellar axes of approximately 1.7 and an aggregation number N(agg) of approximately 30. There are no differences in the aggregation behavior of MonY and MonY(Gd), which indicates that the presence of metal ions, and therefore the reduction of the net charge, does not influence the aggregation behavior. When MonY or MonY(Gd) are blended with dioleoyl phosphatidylcholine (DOPC), the aggregation behavior is dictated by the tendency of DOPC to give liposomes. Only when the amount of MonY or MonY(Gd) is higher than 20 % is the coexistence of liposomes and micelles observed. The thickness d of the bilayer is estimated by SANS to be approximately 35-40 A, whereas cryogenic transmission electron microscopy images show that the diameter of the liposomes ranges from approximately 50 to 150 nm. Self-assembling micelles of MonY(Gd) present high relaxivity values (r(1p)=15.03 mM(-1) s(-1)) for each gadolinium complex in the aggregate. Liposomes containing MonY(Gd) inserted in the DOPC bilayer at a molar ratio of 20:80 present slightly lower relaxivity values (r(1p)=12.7 mM(-1) s(-1)), independently of their internal or external position in the liposome.

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.

Thermodynamic interaction parameters in polymer solutions and gels

AbstractPolymer‐solvent interaction parameters are reported for poly(vinyl acetate)‐acetone, poly(vinyl acetate)‐toluene, and poly(dimethyl siloxane)‐toluene systems using different techniques. Results obtained by osmotic deswelling are compared with those from quasi‐elastic light scattering and small‐angle neutron scattering (SANS). In gels, the latter techniques involve separation of the time‐dependent from the static component of the scattered radiation+ Separation is achieved in quasi‐elastic light scattering through the heterodyning properties of the light, and in SANS by subtracting an appropriate static structure factor. The interaction parameters obtained by different separation procedures are consistent with measurements using the osmotic method. © 1995 John Wiley & Sons, Inc.

SANS and light scattering of nonylammonium chloride micelles in aqueous solutions of sodium chloride

Aqueous solutions of nonylammonium chloride (NAC) containing sodium chloride were studied by using light scattering and small-angle neutron scattering (SANS) techniques. It was found that the aggregation number of NAC is independent of the surfactant concentration and increases smoothly with increasing the ionic strength. Micelles of NAC in D2O with [NaCl]=0.3422 have prolate ellipsoidal shape and their average aggregation number is greater than that found in H2O at the same ionic strength. A possible explanation of this difference may arise from a solvent effect.

Reentrant spin-glass properties of a-(FexCr1−x)75P15C10

The magnetic excitations and instantaneous spatial correlations have been studied in amorphous (FexCr1−x)75P15C10 using neutron inelastic scattering and small-angle neutron scattering (SANS). We report here the results for the sample with x=0.7, which is in the reentrant spin-glass region (RSG) of the magnetic phase diagram. As in other materials displaying RSG properties, we find conventional spin-wave behavior for temperatures down to about half the Curie temperature (TC=134 K), but decreasing excitation energies and lifetimes as the temperature is further lowered. We have studied the critical scattering near TC with SANS, and find that the transverse correlation length ‘‘diverges’’ provided that the data analysis includes the longitudinal fluctuations. The longitudinal correlation length decreases rapidly below TC as expected in a conventional ferromagnet, and the amplitudes for transverse and longitudinal scattering have the expected 2:1 ratio. The RSG temperature regime is characterized by the appearance of a Lorentzian-squared term in the scattering function with a ‘‘nondiverging’’ correlation length, and an amplitude proportional to the RSG order parameter.