The structure of water in the hydration shell of cations from x-ray Raman and small angle x-ray scattering measurements

X-ray Raman scattering (XRS) spectroscopy and small angle x-ray scattering (SAXS) are used to study water in aqueous solutions of NaCl, MgCl(2), and AlCl(3) with the particular aim to provide information about the structure of the hydration shells of the cations. The XRS spectra show that Na(+) weakens the hydrogen bonds of water molecules in its vicinity, similar to the effect of increased temperature and pressure. Mg(2+) and Al(3+), on the other hand, cause the formation of short and strong hydrogen bonds between the surrounding water molecules. The SAXS data show that Mg(2+) and Al(3+) form tightly bound hydration shells that give a large density contrast in the scattering data. From the form factors extracted from the SAXS data, we found that Mg(2+) and Al(3+) have, respectively, an equivalent of one and one and a half stable hydration shells that appear as a density contrast. In addition, we estimated that the density of water in the hydration shells of Mg(2+) and Al(3+) is, respectively, ～61% and ～71% higher than in bulk water.     

Structure of star-burst dendrimers: a comparison between small angle x-ray scattering and computer simulation results

We investigated the generation dependent shape and internal structure of star-burst dendrimers under good solvent conditions using small angle x-ray scattering and molecular modeling. Measurements have been performed on poly(amidoamine) dendrimers with generations ranging from g=0 up to g=8 at low concentrations in methanol. We described the static form factor P(q) by a model taking into account the compact, globular shape as well as the loose, polymeric character of dendrimers. Monomer distributions within dendrimers are of special interest for potential applications and have been characterized by the pair correlation function gamma(r), as well as by the monomer and end-group density profile, rho(r) and rho(e)(r), respectively. Monomer density profiles and gamma(r) can be derived from P(q) by modeling and via a model independent approach using the inverse Fourier transformation algorithm first introduced by Glatter. Experimental results are compared with computer simulations performed for single dendrimers of various generations using the cooperative motion algorithm. The simulation gives direct access to gamma(r) and rho(r), allows an independent determination of P(q), and yields in addition to the scattering experiment information about the distribution of the end groups. Excellent qualitative agreement between experiment and simulation has been found.     

Versatile application of indirect Fourier transformation to structure factor analysis: from X-ray diffraction of molecular liquids to small angle scattering of protein solutions

We highlight versatile applicability of a structure-factor indirect Fourier transformation (IFT) technique, hereafter called SQ-IFT. The original IFT aims at the pair distance distribution function, p(r), of colloidal particles from small angle scattering of X-rays (SAXS) and neutrons (SANS), allowing the conversion of the experimental form factor, P(q), into a more intuitive real-space spatial autocorrelation function. Instead, SQ-IFT is an interaction potential model-free approach to the 'effective' or 'experimental' structure factor to yield the pair correlation functions (PCFs), g(r), of colloidal dispersions like globular protein solutions for small-angle scattering data as well as the radial distribution functions (RDFs) of molecular liquids in liquid diffraction (LD) experiments. We show that SQ-IFT yields accurate RDFs of liquid H(2)O and monohydric alcohol reflecting their local intermolecular structures, in which q-weighted structure function, qH(q), conventionally utilized in many LD studies out of necessity of performing direct Fourier transformation, is no longer required. We also show that SQ-IFT applied to theoretically calculated structure factors for uncharged and charged colloidal dispersions almost perfectly reproduces g(r) obtained as a solution of the Ornstein-Zernike (OZ) equation. We further demonstrate the relevance of SQ-IFT in its practical applications, using SANS effective structure factors of lysozyme solutions reported in recent literatures which revealed the equilibrium cluster formation due to coexisting long range electrostatic repulsion and short range attraction between the proteins. Finally, we present SAXS experiments on human serum albumin (HSA) at different ionic strength and protein concentration, in which we discuss the real space picture of spatial distributions of the proteins via the interaction potential model-free route.     

Structure of poly(propyl ether imine) dendrimer from fully atomistic molecular dynamics simulation and by small angle x-ray scattering

We study the structure of carboxylic acid terminated neutral poly(propyl ether imine) (PETIM) dendrimer from generations 1-6 (G1-G6) in a good solvent (water) by fully atomistic molecular dynamics (MD) simulations. We determine as a function of generation the structural properties such as radius of gyration, shape tensor, asphericity, fractal dimension, monomer density distribution, and end-group distribution functions. The sizes obtained from the MD simulations have been validated by small angle x-ray scattering experiment on dendrimer of generations 2-4 (G2-G4). A good agreement between the experimental and theoretical value of radius of gyration has been observed. We find a linear increase in radius of gyration with the generation. In contrast, Rg scales as approximately Nx with the number of monomers. We find two distinct exponents depending on the generations, x=0.47 for G1-G3 and x=0.28 for G3-G6, which reveal their nonspace filling nature. In comparison with the amine terminated poly(amidoamine) (PAMAM) dendrimer, we find that Rg of Gth generation PETIM dendrimer is nearly equal to that of (G+1)th generation of PAMAM dendrimer as observed by Maiti et al. [Macromolecules 38, 979 (2005)]. We find substantial back folding of the outer subgenerations into the interior of the dendrimer. Due to their highly flexible nature of the repeating branch units, the shape of the PETIM dendrimer deviates significantly from the spherical shape and the molecules become more and more spherical as the generation increases. The interior of the dendrimer is quite open with internal cavities available for accommodating guest molecules, suggesting the use of PETIM dendrimer for guest-host applications. We also give a quantitative measure of the number of water molecules present inside the dendrimer.     

Using small angle scattering (SAS) to structurally characterise peptide and protein self-assembled materials

In the past 20 years protein and peptide self-assembly has attracted material scientists' interest due to the possibility to exploit such molecular mechanism to create novel biomaterials including hydrogels. One of the main challenges when dealing with "soft" biological materials is their structural and morphological characterisation. Small angle scattering (SAS) can be a highly complementary tool to microscopy for the characterisation of such materials as it allows the investigation of samples in their wet-state without the need for any sample preparation such as drying and/or freezing. In this tutorial review we introduce briefly the SAS technique to the non-expert and through selected examples from the literature show how SAS can be readily used thanks to existing analytical approaches developed by a number of authors to extract structural information on the self-assembly of peptide and proteins.     

Trifluoperazine effects on anionic and zwitterionic micelles: a study by small angle X-ray scattering

In this work small angle X-ray scattering (SAXS) studies on the interaction of the phenothiazine trifluoperazine (TFP, 2-10 mM), a cationic drug, with micelles of the zwitterionic surfactant 3-(N-hexadecyl-N,N-dimethylammonium) propane sulfonate (HPS, 30 mM) and the anionic surfactant sodium dodecyl sulfate (SDS, 40 mM) at pH 4.0, 7.0, and 9.0 are reported. The data were analyzed through the modeling of the micellar form factor and interference function, as well as by means of the distance distribution function p(r). For anionic micelles (SDS), the results evidence a micellar shape transformation from prolate ellipsoid to cylinder accompanied by micellar growth and surface charge screening as the molar ratio TFP:SDS increases in the complex for all values of pH. Small ellipsoids with axial ratio nu=1.5+/-0.1 (long dimension of 60 A) grow and reassemble into cylinder-like aggregates upon 5 mM drug incorporation (1 TFP:8 SDS monomers) with a decrease of the micelle surface charge. At 10 mM TFP:40 mM SDS cylindrical micelles are totally screened with an axial ratio nu approximately 4 (long dimension approximately 140 A at pH 7.0 and 9.0). However, at pH 4.0, where the drug is partially diprotonated, 10 mM TFP incorporation gives rise to a huge increase in micellar size, resulting in micelles at least 400 A long, without altering the intramicellar core. For zwitterionic micelles (HPS), the results have shown that the aggregates also resemble small prolate ellipsoids with averaged axial ratio approximately nu=1.6+/-0.1. Under TFP addition, both the paraffinic radius and the micellar size show a slight decrease, giving evidence that the micellar hydrophobic core may be affected by phenothiazine incorporation rather than that observed for the SDS/TFP comicelle. Therefore, our results demonstrate that the axial ratio and shape evolution of the surfactant:TFP complex are both dependent on surfactant surface-charge and drug:surfactant molar ratio. The results are compared with those recently obtained for another phenothiazine drug, chlorpromazine (CPZ), in SDS and HPS micelles (Caetano, Gelamo, Tabak, and Itri, J. Colloid Interface Science 248 (2002) 149).     

Structure and dynamics of water in nonionic reverse micelles: A combined time-resolved infrared and small angle x-ray scattering study

We study the structure and reorientation dynamics of nanometer-sized water droplets inside nonionic reverse micelles (water/Igepal-CO-520/cyclohexane) with time-resolved mid-infrared pump-probe spectroscopy and small angle x-ray scattering. In the time-resolved experiments, we probe the vibrational and orientational dynamics of the O-D bonds of dilute HDO:H(2)O mixtures in Igepal reverse micelles as a function of temperature and micelle size. We find that even small micelles contain a large fraction of water that reorients at the same rate as water in the bulk, which indicates that the polyethylene oxide chains of the surfactant do not penetrate into the water volume. We also observe that the confinement affects the reorientation dynamics of only the first hydration layer. From the temperature dependent surface-water dynamics, we estimate an activation enthalpy for reorientation of 45 ± 9 kJ mol(-1) (11?±?2 kcal mol(-1)), which is close to the activation energy of the reorientation of water molecules in ice.     

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.     

Surface versus confinement induced morphology transition in triblock copolymer films: a grazing incidence small angle neutron scattering investigation

The internal nanostructure resulting from microphase separation in triblock copolymer films of polyparamethylstyrene-block-polystyrene-block-polyparamethylstyrene, P(pMS-b-Sd8-b-pMS), has been investigated with grazing incidence small angle neutron scattering (GISANS). X-ray reflectivity, grazing incidence small-angle X-ray scattering (GISAXS), optical microscopy and atomic force microscopy (AFM) complement the investigation. The influence of two limiting interfaces present in confinement is compared to the presence of only one surface. GISANS allows for the detection of structures in the very limited sample volume of confined films as well as for a depth sensitivity to probe the near free surface part of bulk films. With respect to the surface a perpendicular oriented lamella is observed. In contrast to the shrinkage of the characteristic lamellar spacing in confinement at the free surface, a slight increase is determined.     

Interaction strength between proteins and polyelectrolyte brushes: a small angle X-ray scattering study

We present an investigation of β-lactoglobulin adsorption onto spherical polyelectrolyte brushes (SPBs) by small angle X-ray scattering (SAXS). The SPB consists of a polystyrene core onto which long chains of poly(styrene sulfonate) are grafted. The amount and the distribution of proteins adsorbed in the brush layer at low ionic strength can be derived from SAXS. The analysis of the SAXS data reveals additionally that some of the protein molecules form aggregates of about six monomers in the adsorbed state. Furthermore, the position and the amount of slightly bound protein can be detected by the combination of the SAXS results and the SPB loading after extensive ultrafiltration. The total amount of adsorbed protein is compared to data derived from isothermal titration calorimetry. The comparison of both sets of data demonstrates that the protein molecules in the inner layers of the spherical polyelectrolyte brush are firmly bound. Proteins located in the outer layers are only weakly bound and can be washed out by prolonged ultrafiltration.     

Increasing correlation length in bulk supercooled H2O, D2O, and NaCl solution determined from small angle x-ray scattering

Using small angle x-ray scattering, we find that the correlation length of bulk liquid water shows a steep increase as temperature decreases at subzero temperatures (supercooling) and that it can, similar to the thermodynamic response functions, be fitted to a power law. This indicates that the anomalous properties of water are attributable to fluctuations between low- and high-density regions with rapidly growing average size upon supercooling. The substitution of H(2)O with D(2)O, as well as the addition of NaCl salt, leads to substantial changes of the power law behavior of the correlation length. Our results are consistent with the proposed existence of a liquid-liquid critical point in the deeply supercooled region but do not exclude a singularity-free model.     

Surface esterification of cellulose fibers: characterization by DRIFT and contact angle measurements

The surface chemical modification of microcrystalline cellulose and cellulose fibers obtained from different sugar cane bagasse pulping processes, viz. Kraft, organosolv ethanol/water and organosolv/supercritical carbon dioxide, were studied in heterogeneous conditions using modest amounts of octadecanoyl and dodecanoyl chloride. The ensuing surfaces acquired a non-polar character, suitable for incorporating these fibers as reinforcing agents in composite materials based on polymeric matrices. The success of these chemical modifications was assessed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, elemental analysis, scanning electron microscopy (SEM) and contact angle measurements. In particular, the dynamic and equilibrium contact angle measurements, before and after the treatments, revealed that the value of the polar component (gamma(s)p) of the surface energy had decreased very considerably following the modification.     

Conformation, directed self-assembly and engineered modification: some recent near surface structure determinations by grazing incidence small angle X-ray and neutron scattering

Some recent developments in grazing incidence small angle scattering (GISAS) technique are reviewed. The emphasis is on the application of GISAS to elucidating the effects of geometrical surface constraint on self-assembled systems and the effect of modification of interfaces or molecular subassemblies to direct the formation of more complex structures.     

Associative phase separation of beta-lactoglobulin/pectin solutions: a kinetic study by small angle static light scattering

Interpolymer complexation between beta-lactoglobulin (beta-lg) and pectin led to phase separation. Small angle static light scattering and phase contrast microscopy were used to monitor the phase separation of beta-lg/low-methoxyl or high-methoxyl-pectin (LM- or HM-pectin) dispersions as they were slowly acidified from pH 7 to 4 with glucono-delta-lactone (GDL). The monotonic decrease in scattered light intensity with the wave vector was associated with a nucleation and growth phase separation mechanism. Microscopic observations and turbidity measurements showed the increase of complex amounts with lower pH and at higher beta-lg/pectin ratios. The formation of intrapolymer complexes was initiated at pH 6.4 with the LM-pectin and at pH 5.0 with the HM-pectin. Local ordering with increasing amounts of small complexes was observed as scattered light intensity increased at intermediate q values. The beta-lg/LM-pectin complexes at the 5:1 and the 2:1 weight ratios and the beta-lg/HM-pectin complexes at 5:1 weight ratio have fractal structures. The formation of large amounts of small assemblies and sedimentation would be responsible for the decrease in the number and volume mean diameters and fractal dimension of beta-lg/LM-pectin complexes over time.     

Surface structure of amorphous PMMA from SPM: auto‐correlation function and fractal analysis

The surface topography of amorphous poly(methyl methacrylate) has been investigated by scanning probe microscopy technique and analysed using an auto‐correlation function approach. Spatial correlations in the arrangement of sub‐macromolecular fragments on the surface are studied depending on preparation conditions (whether the surface was free, i.e. in contact with air, or confined with the silicon substrate). The correlation length of the roughness distribution on the surface of amorphous polymers corresponds to the internal dimensions of unperturbed polymer chains (the radius of gyration). The scaling behaviour of the roughness is analysed with the Hurst approach, and found to be different for the free surface and for the confined one. As a result of annealing at a temperature above the glass transition temperature of the bulk polymer, both surfaces converge to similar fractal dimensionalities and correlation lengths.     

Structure and shear orientation of a side chain liquid crystal polymer studied by small angle X-ray scattering

We have studied the structure and shear flow behaviour of a side chain liquid crystal polymer, namely P₁₂, in the smectic (SmA_d) phase. Simultaneous oscillatory shear and small angle X-ray scattering experiments showed that, as expected, the shearing of P₁₂ at high frequency and large strain amplitudes leads to the alignment of the smectic layer normals parallel to the neutral direction. It was also found that, on erasing the memory of the prior shear alignment through melting, P₁₂ can unexpectedly orient with the smectic layers perpendicular to the shear flow direction (b-configuration). We speculate that the b-configuration is adopted due to a counter-balance between the rather long spacer in P₁₂ which decouples mesogens from the backbone, and the oscillatory character of the shear applied to P₁₂.