Small Wide Angle X-ray Scattering Research Articles

Surfactant-free microemulsions of n-butanol, ethanol, and water

Surfactant-free microemulsions (SFMEs) refer to the microemulsions formed with two immiscible fluids (generally called the oil and water phases) in the presence of an amphi-solvent rather than the traditional surfactants. The understanding of SFMEs is still limited now. In the current work, the phase behavior of the ternary mixture of n-butanol (oil phase), ethanol (amphi-solvent), and water was investigated. The ternary phase diagram shows a single-phase homogeneous region and a multiphase region. The mesoscale structuring was observed within the macroscopically homogeneous single-phase zone near the phase boundary using dynamic light scattering (DLS) and small-wide angle X-ray scattering (SWAXS) techniques. That is, microemulsions are formed in the ternary mixture without the traditional surfactants. In addition, the electrical conductivity and micropolarity of the single-phase mixtures were determined, suggesting the existence of three types of microstructures, i.e., oil-in-water (O/W), bicontinuous (BC), and water-in-oil (W/O), for the SFMEs. The subregions corresponding to the three microstructures were roughly identified in the ternary phase diagram. n-Butanol is one of the extremely weak hydrophobic substances, and the detection of microemulsions in the mixture indicates that the formation of SFMEs is a universal phenomenon. This study provides a better understanding of the phase behavior of ternary liquid mixtures.

Visualizing how inclusion of higher reciprocal space in SWAXS data analysis improves shape restoration of biomolecules: case of lysozyme

Query remains whether use of increased resolution data from X-ray scattering aids in better understanding of the dynamic shape of the biomolecule in solution? To address this, we acquired Small/Wide angle X-ray scattering (SWAXS) data in the q range of 0.008 − 1.72 Å−1 from dilute solutions of lysozyme (0.9 to 5 mg/ml). Samples lacked any interparticulate effect and datasets showed Bragg peaks at q∼0.325, 0.65 and 1.4 Å−1, as reported before by other authors. Considering an averaged profile, we estimated shape parameters and distance distribution profiles of interatomic vectors by gradually increasing input qmax value. Interestingly, use of higher resolution led to emergence of new peaks amongst smaller vectors. Deconvolution of these peaks provided positions of smaller peaks which correlated well with an earlier theoretical work. These peaks arise from secondary structures or due to non-uniform internal motions within the larger shape of this protein. Dummy residue modeling considering uniform density yielded model(s) with holes or cavities when considering higher q values implying limitations of this method. Employing normal mode calculations, we searched for better fitting model of lysozyme using differentially ranged SWAXS data and a crystal structure of lysozyme as starting structure. Comparison of refined models with structures from crystallography and NMR data showed that use of data till mid q region resulted in adjustments near the center of mass of starting structure, and inclusion of higher resolution induced pan-structure adjustments. We conclude that high resolution SWAXS data analysis provides additional dimension towards understanding biomolecular structural dynamics. Communicated by Ramaswamy H. Sarma

Characterizing Water Behavior in α-Gel (α-Type Hydrated Crystal) Formed from Monohexadecyl Phosphate and L-Arginine.

Monohexadecyl phosphate, neutralized by L-arginine (C16MP-Arg), forms an α-gel (α-type hydrated crystal) with water. In this study, we characterized the behavior of water in the C16MP-Arg α-gel system by means of small/wide angle X-ray scattering (SWAXS), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), and 1H nuclear magnetic resonance (NMR) spectroscopy. An increased water concentration resulted in an increase in the d-spacing of the lamellar bilayers as well as an increased wavenumber for the O-H stretching vibration peak. In addition, the melting enthalpy increased with increasing water concentration, while freezing was not observed below a water concentration of 20 wt%. These results suggest that the overall properties of water changed as a function of its concentration in the sample. 1H-NMR spin-spin relaxation time (T2) measurements further suggest that the protons of water and C16MP-Arg can be classified into three components (low-, middle-, and high-T2 components) as a function of the temperature and concentration. The low-T2 component mainly arose from the protons of C16MP-Arg alkyl chains, and its mobility increased with increasing temperature. The high-T2 component arose from the protons of water. The water behaved as "bound water" for the C16MP-Arg headgroups at -30°C and a water concentration of 20 wt%, and the mobility increased with increasing temperature and water concentration. These changes suggest that an increased water concentration results in an increased amount of water being incorporated between the C16MP-Arg lamellar bilayers as well as in spaces surrounded by α-gel domains.

Elucidating the effect of additives on the alkyl chain packing of a double tail cationic surfactant

HypothesisSome low molecular weight additives can strongly influence the phase behavior of aqueous surfactant systems, and this offers an important handle to control the properties of surfactant solutions and thus to optimize the stability and performance of various formulations. ExperimentsThe surfactant dioctadecyldimethylammonium chloride (DODAC) self-assembles into two lamellar phases in water, the gel phase (Lβ) and the liquid crystalline phase (Lα). Here, we present approaches to tune the gel-to-liquid crystalline transition temperature (Tm) with the use of additives. The effects of urea, sodium butyrate and butyric acid on the packing behavior of DODAC were determined. The surfactant phases were characterized using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and small/wide angle X-ray scattering (SWAXS). FindingsAll three additives - urea, sodium butyrate and butyric acid yield a single and stable lamellar phase. Urea and sodium butyrate have only minor effects on Tm, butyric acid gives a large decrease as it stabilizes the Lα phase with respect to the Lβ phase. From the bilayer thickness of the gel phase an interdigitated or tilted packing of the surfactant molecules is suggested. The addition of sodium butyrate gives a highly interdigitated gel structure and resulted in the transition from lamellar liquid crystal to an isotropic L3 phase.

Phase Diagram of Monohexadecyl Phosphate Neutralized by L-Arginine: α-Gel Formation Ability.

The aqueous phase diagram of monohexadecyl phosphate neutralized by L-arginine (C16MP-Arg) was studied using analytical techniques including differential scanning calorimetry (DSC) and small/wide-angle X-ray scattering (SWAXS) measurements. We found that the mixture could form an α-gel over wide ranges of concentrations and temperatures. The α-gel was thermodynamically stable at 25°C (below the phase transition temperature for hexagonal or lamellar liquid crystals), and did not transform to coagel for at least one year. This behavior is unique among anionic surfactant systems. Thus, the balance between hydrophilic and hydrophobic volumes is a key factor for determining whether a stable α-gel can be obtained. In this system, the relatively large volume of Arg relaxed the alkyl chains of C16MP and prevented their crystallization.

Role of Solvent and Effect of Substituent on Azobenzene Isomerization by Using Room-Temperature Ionic Liquids as Reaction Media.

The effects of a para substituent, as the electron-donating -OCH3 and -OtBu groups and the electron-withdrawing -Br and -F atoms, on azobenzene isomerization have been investigated in a series of imidazolium ionic liquids (BMIM PF6, BMIM BF4, BMIM Tf2N, EMIM Tf2N, BM2IM Tf2N, and HMIM Tf2N). The thermal cis-trans conversion tends to be improved in the presence of the substituent, as pointed out by the first-order rate constants measured at 25 °C. Both the rotation and the inversion mechanisms occur in BMIM Tf2N, EMIM Tf2N, and HMIM Tf2N, as highlighted by typical V-shape Hammett plots, but only rotation takes place in BMIM PF6, BMIM BF4, and BM2IM Tf2N. The possible interactions between the cation and the anion of the solvent and both the isomers of the azobenzene derivatives have been studied by small-wide-angle X-ray scattering (SWAXS). The calculated cis population in the photostationary state and the hardness parameter η of the trans isomer show that azobenzene and F-azobenzene are the less reactive molecules for the trans-cis conversion in all the investigated ionic liquids.

Alkylimidazolium Based Ionic Liquids: Impact of Cation Symmetry on Their Nanoscale Structural Organization

Aiming at evaluating the impact of the cation symmetry on the nanostructuration of ionic liquids (ILs), in this work, densities and viscosities as a function of temperature and small-wide angle X-ray scattering (SWAXS) patterns at ambient conditions were determined and analyzed for 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (asymmetric) and 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide (symmetric) series of ionic liquids. The symmetric IL series, [CN/2CN/2im][NTf2], presents lower viscosities than the asymmetric [CN-1C1im][NTf2] counterparts. For ionic liquids from [C1C1im][NTf2] to [C6C6im][NTf2], an odd-even effect in the viscosity along the cation alkyl side chain length was observed, in contrast with a linear increase found for the ones ranging between [C6C6im][NTf2] and [C10C10im][NTf2]. The analysis of the viscosity data along the alkyl side chain length reveals a trend shift that occurs at [C6C1im][NTf2] for the asymmetric series and at [C6C6im][NTf2] for the symmetric series. These results are further supported by SWAXS measurements at ambient conditions. The gathered data indicate that both asymmetric and symmetric members are characterized by the occurrence of a distinct degree of mesoscopic structural organization above a given threshold in the side alkyl chain length, regardless the cation symmetry. The data also highlight a difference in the alkyl chain dependence of the mesoscopic cluster sizes for symmetric and asymmetric cations, reflecting a different degree of interdigitation of the aliphatic tails in the two families. The trend shift found in this work is related to the structural segregation in the liquid after a critical alkyl length size (CALS) is attained and has particular relevance in the cation structural isomerism with higher symmetry.

Solution Structure Analysis of Cytoplasmic Domain of Podocyte Protein Neph1 Using Small/Wide Angle X-ray Scattering (SWAXS)

Neph1 is present in podocytes, where it plays a critical role in maintaining the filtration function of the glomerulus, in part through signaling events mediated by its cytoplasmic domain that are involved in actin cytoskeleton organization. To understand the function of this protein, a detailed knowledge of the structure of the Neph1 cytoplasmic domain (Neph1-CD) is required. In this study, the solution structure of this domain was determined by small/wide angle x-ray scattering (SWAXS). Analysis of Neph1-CD by SWAXS suggested that this protein adopts a global shape with a radius of gyration and a maximum linear dimension of 21.3 and 70 Å, respectively. These parameters and the data from circular dichroism experiments were used to construct a structural model of this protein. The His-ZO-1-PDZ1 (first PDZ domain of zonula occludens) domain that binds Neph1-CD was also analyzed by SWAXS, to confirm that it adopts a global structure similar to its crystal structure. We used the SWAXS intensity profile, the structural model of Neph1-CD, and the crystal structure of ZO-1-PDZ1 to construct a structural model of the Neph1-CD·ZO-1-PDZ1 complex. Mapping of the intermolecular interactions suggested that in addition to the C-terminal residues Thr-His-Val, residues Lys-761 and Tyr-762 in Neph1 are also critical for stabilizing the complex. Estimated intensity values from the SWAXS data and in vivo and in vitro pull-down experiments demonstrated loss of binding to ZO-1 when these residues were individually mutated to alanines. Our findings present a structural model that provides novel insights into the molecular structure and function of Neph1-CD.

Modeling the Hydration Layer around Proteins: Applications to Small- and Wide-Angle X-Ray Scattering

Small-/wide-angle x-ray scattering (SWAXS) experiments can aid in determining the structures of proteins and protein complexes, but success requires accurate computational treatment of solvation. We compare two methods by which to calculate SWAXS patterns. The first approach uses all-atom explicit-solvent molecular dynamics (MD) simulations. The second, far less computationally expensive method involves prediction of the hydration density around a protein using our new HyPred solvation model, which is applied without the need for additional MD simulations. The SWAXS patterns obtained from the HyPred model compare well to both experimental data and the patterns predicted by the MD simulations. Both approaches exhibit advantages over existing methods for analyzing SWAXS data. The close correspondence between calculated and observed SWAXS patterns provides strong experimental support for the description of hydration implicit in the HyPred model.

Morphology and intermolecular dynamics of 1-alkyl-3-methylimidazoliumbis{(trifluoromethane)sulfonyl}amide ionic liquids: structural and dynamic evidence ofnanoscale segregation

Here we report on the structural and dynamical properties of a series of room temperatureionic liquids, namely 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([Cnmim][NTf2]), with varying alkylchain lengths (1≤n≤10) at ambient temperature, where all the salts are stable liquids. Using small-wide anglex-ray scattering (SWAXS), three major diffraction peaks are found: two high-Q peaks that show little dependence on the alkyl chain length (n) anda low-Q peak that strongly depends both in amplitude and position onn. Thislow-Q peak is the signature of the occurrence of nanoscale structural heterogeneities whose sizesdepend on the length of the alkyl chain and are related to chain segregation intonano-domains. Using optical heterodyne-detected Raman-induced Kerr effect spectroscopy, weaccess intermolecular dynamic features that suggest that chain aggregation only occurs forn≥3, in agreement with the SWAXS data. Moreover, the increase in the frequencyand width of the main band of the optical Kerr effect spectra in going fromn = 2 to 3is consistent with stiffening of the intermolecular potential due to chain segregation. Multicomponentline shape analysis suggests that there are least three modes that underlie the main band in the 0–200 cm−1 region of the optical Kerr effect spectra of these ionic liquids. Given the similarity of ionicliquids to other complex fluid systems, we assign the low-frequency component to a fastβ-relaxation mode and the intermediate- and high-frequency components to librationalmodes.

Effect of Cation Symmetry and Alkyl Chain Length on the Structure and Intermolecular Dynamics of 1,3-Dialkylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquids

In this article, the structure and intermolecular dynamics of 1,3-alkylmethylimidazolium bis(trifluoromethanesulfonyl)amides [C(n)mim][NTf(2)] with n = 2-5 are compared to those of 1,3-dialkylimidazolium bis(trifluoromethanesulfonyl)amides [(C(n))(2)im][NTf(2)] with n = 2-5. The structures of these room-temperature ionic liquids (RTILs) were studied by small-wide-angle X-ray scattering (SWAXS), and their intermolecular dynamics were studied by optical Kerr effect (OKE) spectroscopy. The SWAXS measurements indicate that, on a microscopic scale, the liquid structure of RTILs with symmetric cations is similar to that of RTILs with asymmetric cations. The OKE measurements indicate that the intermolecular dynamics of RTILs with symmetric cations are higher in frequency than those of RTILs with asymmetric cations. These results suggest that the local structure of RTILs with symmetric cations is more solid-like than that of RTILs with asymmetric cations. Further evidence for this difference in local structure on a mesoscopic spatial scale is that the width of the low-Q peak in the SWAXS data is narrower for [(C(5))(2)im][NTf(2)] than for [C(5)mim][NTf(2)]. Moreover, the structure and intermolecular dynamics of the RTILs with ethyl-substituted cations appear to be quite different from those of other RTILs within a given series. This difference is evidenced by a clear change in the dependence of the spectral parameters of the intermolecular part of the OKE spectrum on the alkyl chain length in going from n = 2 to n = 3. The dependence of the SWAXS and OKE data on alkyl chain length is discussed within the context of the nanoscale heterogeneities of RTILs.

Equilibrium, FTIR, scanning electron microscopy and small wide angle X-ray scattering studies of chromates adsorption on modified bentonite

The study presents a discussion about the adsorption mechanism of chromate anions on bentonite modified by hexadecyltrimethylammonium bromide (HDTMA-Br). The formation of alkylammonium chromates: HDTMAHCrO 4, (HDTMA) 2Cr 2O 7 and to the lesser extent (HDTMA) 2CrO 4 at the water–bentonite interface is examined based on the Scanning Electron Microscopy and surface tension measurements. The histograms of HDTMA/Cr(VI) molar ratio on the bentonite surface, found from Scanning Electron Microscopy (SEM) measurements, show that for the majority of points of bentonite surface the value of this ratio is in 1–2 range. FTIR spectra of modified bentonite samples show the change from gauche to trans conformation in the surfactant arrangement in the clay interlayer accompanying its concentration increase. In turn Small Wide Angle X-Ray Scattering (SWAXS) patterns evidently suggest incorporation of chromate anions into the interlamellar space of bentonite structure.