G. D. Wignall

Extraction of a hydrophilic compound from water into liquid CO2 using dendritic surfactants

Dendrimers are well defined, highly branched polymers that adopt a roughly spherical, globular shape in solution. Their cores are relatively loosely packed and can trap guest molecules, and by appropriate functionalization of the branch tips the macromolecules can act as unimolecular micelle-like entities. Here we show that dendrimers with a fluorinated shell are soluble in liquid carbon dioxide and can transport CO2-insoluble molecules into this solvent within their cores. Specifically, we demonstrate the extraction of a polar ionic dye, methyl orange, from water into CO2 using these fluorinated dendrimers. This observation suggests possible uses of such macromolecules for the remediation of contaminated water, the extraction of pharmaceutical products from fermentation vessels, the selective encapsulation of drugs for targeted delivery and the transport of reagents for chemical reactions (such as polymerization) in liquid and supercritical CO2 solvents.

Design of Nonionic Surfactants for Supercritical Carbon Dioxide

Interfacially active block copolymer amphiphiles have been synthesized and their self-assembly into micelles in supercritical carbon dioxide (CO2) has been demonstrated with small-angle neutron scattering (SANS). These materials establish the design criteria for molecularly engineered surfactants that can stabilize and disperse otherwise insoluble matter into a CO2 continuous phase. Polystyrene-b-poly(1,1-dihydroperfluorooctyl acrylate) copolymers self-assembled into polydisperse core-shell-type micelles as a result of the disparate solubility characteristics of the different block segments in CO2. These nonionic surfactants for CO2 were shown by SANS to be capable of emulsifying up to 20 percent by weight of a CO2-insoluble hydrocarbon into CO2. This result demonstrates the efficacy of surfactant-modified CO2 in reducing the large volumes of organic and halogenated solvent waste streams released into our environment by solvent-intensive manufacturing and process industries.

Intercalibration of small-angle X-ray and neutron scattering data

Absolute calibration forms a valuable diagnostic tool in small-angle scattering experiments and allows the parameters of a given model to be restricted to the set which reproduces the observed intensity. General methods which are available for absolute scaling of small-angle X-ray scattering (SAXS) data are reviewed along with estimates of the degree of internal consistency which may be achieved between the various standards. In order to minimize the time devoted to calibration in a given experimental program, emphasis is placed on developing a set of precalibrated strongly scattering standards for the SAXS facilities of the National Center for Small-Angle Scattering Research (Oak Ridge). Similar standards have been developed previously for calibration of small-angle neutron scattering (SANS) data. Particular attention is given to standards which can be used for either SAXS or SANS experiments where each sample has been independently calibrated for both types of radiation. These calibrations have been tested via the theoretical relationships between the two cross sections. It has been found that specimens best suited for such intercalibration purposes are a glassy carbon specimen where the scattering arises from voids in a carbon matrix and a perdeuterated polyethylene where the scattering arises from periodic arrangement of the crystalline lamellae. In only these two cases could the identical specimen be used for both the neutron and X-ray scattering experiments.

Optimization of a Bonse–Hart Ultra-Small-Angle Neutron Scattering Facility by Elimination of the Rocking-Curve Wings

The ORNL Ultra-Small-Angle Neutron Scattering (USANS) facility at the High Flux Isotope Reactor (HIFR) has been recently upgraded, using the Bonse–Hart technique. Si(111) triple-bounce channel-cut single crystals have been used for both the monochromator and analyzer. The total width of the rocking curve of the analyzer is about 1.6′′ and the wavelength of the primary neutron beam is 2.59 A. It has been demonstrated that, owing to the low neutron absorption of silicon, the wings of the rocking curve are generally contaminated by neutrons propagating and diffracting inside the walls of channel-cut crystals. This parasitic intensity has been eliminated by the cutting of a groove in the long wall and the insertion of a cadmium absorber (0.6 mm thick). This modification effectively suppresses the wings of the rocking curve by over two orders of magnitude and thus dramatically improves the sensitivity of the diffractometer. The upgraded facility has been tested with several samples, including a polystyrene latex with a radius of 2.50 × 104 A as determined by optical microscopy. The average radius calculated from USANS data is 2.48 × 104 A, in excellent agreement with independently determined dimensions. The minimum accessible scattering vector of the upgraded USANS facility is Qmin ≃ 2 × 10−5 A−1, which corresponds to a maximum resolvable real-space dimension of 2π/Qmin ≃ 3 × 105 A (30 μm).

Reduction of parasitic scattering in small-angle X-ray scattering by a three-pinhole collimating system

A series of experiments have been undertaken on the Oak Ridge National Laboratory (ORNL) 10 m small-angle X-ray scattering (SAXS) camera to provide quantitative data on the level of background (parasitic) scattering generated by different types of bevelled collimating slits. The addition of a third (guard) slit, positioned close to the sample, resulted in a reduction of over an order of magnitude in the parasitic background generated by the best two-slit combination of collimating slits. This made it possible to reduce the size of the beamstop, permitting useful data to be collected down to a value of the scattering vector Q = 4πλ−1sinθ ≃ 3 × 10−3 A−1, where λ is the wavelength, and 2θ is the angle of scatter. This permits the resolution of distances d ~ 2π/Q up to 2000 A.

Thermodynamics of isotopic polymer mixtures: Significance of local structural symmetry

The dependence on composition (Φ) and degree of polymerization (N) of the effective segment–segment interaction parameter, χ=− 1/2 ∂2Γ/∂Φ2, has been determined for mixtures of perdeuterated and normal (protonated) poly(vinylethylene) and poly(ethylethylene) by small angle neutron scattering (SANS), where Γ represents the excess free energy of mixing. Owing to the small reduction in length and polarizability of carbon–hydrogen bonds with deuterium substitution, isotopic polymer mixtures are characterized by small positive χ parameters. However, deuterium substitution does not significantly influence the local structural symmetry (i.e., the liquid structure or reduced thermodynamic properties) of polymer isotopes, consistent with the assumptions of Flory–Huggins (FH) lattice theory. Nevertheless, we find that χ depends significantly on both Φ and N, contrary to the predictions of FH theory. A general theory, which corrects FH theory for monomer concentration fluctuations, is shown to closely account for these experimental results. In understanding the previous experimental investigations of χ for mixing chemically different species, the monomer concentration fluctuations discussed here must be addressed in addition to the effects due to local structural asymmetry.

Small angle neutron scattering signature of oil generation in artificially and naturally matured hydrocarbon source rocks

Abstract We have demonstrated for the first time the application of a small angle neutron scattering (SANS) technique for the precise determination of the onset of hydrocarbon transport (primary migration) in shaly source rocks. We used a sequence of rocks pyrolysed in the laboratory under nitrogen at temperatures in the range 310–370°C. These rocks contained several percent of dispersed marine Type II organic matter. Geochemical analysis indicated a peak of the hydrocarbon generation in the middle of the temperature range (at 340°C). We observed a sharp decrease of SANS intensity in a narrow maturity range within the geochemically determined region of the onset of hydrocarbon generation. This decrease was a direct consequence of the SANS contrast variation caused by the invasion of the pore space by bitumen during the primary migration of hydrocarbons. A similar phenomenon was observed for a natural maturity sequence of source rocks originating from the same location.

Small-angle neutron scattering in materials science : Recent practical applications

Modern materials science and engineering relies increasingly on detailed knowledge of the structure and interactions in “soft” and “hard” materials, but there have been surprisingly few microscopic techniques for probing the structures of bulk samples of these substances. Small-angle neutron scattering (SANS) was first recognized in Europe as a major technique for this purpose and, over the past several decades, has been a growth area in both academic and industrial materials research to provide structural information on length scales ∼10–1000A (or 1–100nm). The technique of ultrahigh resolution small-angle neutron scattering (USANS) raises the upper resolution limit for structural studies by more than two orders of magnitude and (up to ∼30μm) and hence overlaps with light scattering and microscopy. This review illustrates the ongoing vitality of SANS and USANS in materials research via a range of current practical applications from both soft and hard matter nanostructured systems.

Polyelectrolyte chain dimensions and concentration fluctuations near phase boundaries

We have measured the temperature (T) dependence of the correlation length (ξ) for concentration fluctuations in aqueous solutions of sodium–poly(styrene sulfonate) with a fixed level of added barium chloride salt. Apparent critical behavior is observed upon lowering the temperature to precipitation phase boundaries that complements our earlier work on salt-dependent behavior. We interpret experimental deviations from ξ−2 versus T−1 as crossover from the mean field to the Ising universality class. We also measured the radius of gyration (Rg) of labeled chains and ξ for semidilute polyelectrolyte solutions at low ionic strengths. We recovered the familiar result of ξ scaling with polymer concentration (Cp) and degree of polymerization (N), such that ξ=(73±9) N0Cp−0.48±0.03 [A], and using SANS high concentration labeling Rg=(400±28)Cp−0.24±0.01 [A] (for N=577) and Rg=(2.8±2.1)N0.6±0.1 [A] (for Cp=206 gL−1), respectively. The indices recovered are in agreement with theoretical predictions for low ionic streng...

Adsorption of supercritical CO2 in aerogels as studied by small-angle neutron scattering and neutron transmission techniques.

Small-angle neutron scattering (SANS) has been used to study the adsorption behavior of supercritical carbon dioxide (CO2) in porous Vycor glass and silica aerogels. Measurements were performed along two isotherms (T=35 and 80 degrees C) as a function of pressure (P) ranging from atmospheric up to 25 MPa, which corresponds to the bulk fluid densities ranging from rho(CO2) approximately 0 to 0.9 gcm3. The intensity of scattering from CO2-saturated Vycor porous glass can be described by a two-phase model which suggests that CO2 does not adsorb on the pore walls and fills the pore space uniformly. In CO2-saturated aerogels an adsorbed phase is formed with a density substantially higher that of the bulk fluid, and neutron transmission data were used to monitor the excess adsorption at different pressures. The results indicate that adsorption of CO2 is significantly stronger in aerogels than in activated carbons, zeolites, and xerogels due to the extremely high porosity and optimum pore size of these materials. SANS data revealed the existence of a compressed adsorbed phase with the average density approximately 1.07 gcm3, close to the density corresponding to closely packed van der Waals volume of CO2. A three-phase model [W. L. Wu, Polymer 23, 1907 (1982)] was used to estimate the volume fraction phi3 of the adsorbed phase as a function of the fluid density, and gave phi3 approximately 0.78 in the maximum adsorption regime around rho(CO2) approximately 0.374 gcm3. The results presented in this work demonstrate the utility of SANS combined with the transmission measurements to study the adsorption of supercritical fluids in porous materials.