R. Triolo

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.

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).

Simple electrolytes in the mean spherical approximation. III. A workable model for aqueous solutions

Some improvements in the primitive model of electrolyte solutions are discussed within the framework of the mean spherical approximation. Excellent agreement has been found between experimental and calculated osmotic and activity coefficients of 16 alkali halides in H2O at 25 °C, whenever a density dependent hard core ionic diameter is combined with an adjustable but not density dependent dielectric constant. Promising agreement is also found when Pauling crystallographic radii are used for the ions.

Study on the thermotropic properties of highly fluorinated 1,2,4-oxadiazolylpyridinium salts and their perspective applications as ionic liquid crystals

A new series of fluorinated salts, iodides and trifluoromethanesulfonates, was synthesized from perfluoroalkylated 1,2,4-oxadiazolylpyridines. Their thermotropic properties were investigated by combined temperature resolved small angle and wide angle X-ray scattering, differential scanning calorimetry and polarised optical microscopy. The UV–visible and photoluminescence properties were studied for all compounds. The results showed for two compounds the existence of an enantiotropic mesomorphic smectic liquid crystal phase. All iodides showed thermochromism phenomena suggesting prospective applications in optoelectronics.

Neutron scattering characterization of homopolymers and graft-copolymer micelles in supercritical carbon dioxide

Abstract Superficial fluids (SCF) are becoming an attractive alternative to the liquid solvents traditionally used as polymerization media [1]. As the synthesis proceeds, a wide range of colloidal aggregates form, but there has hitherto been no way to measure such structures directly. We have applied small-angle neutron scattering (SANS) to characterize such systems, and although SCF polymerizations are carried out at high pressures, the penetrating power of the neutron beam means that typical cell windows are virtually transparent. Systems studied include polymers soluble in CO 2 such as poly(1,1-dihydroperfluorooctyl acrylate) (PFOA), poly(hexafluoropropylene oxide) (PHFPO) and poly(dimethyl siloxane) (PDMS). PFOA has previously [2] been shown to exhibit a positive second virial coefficient ( A 2 ), though for PHFPO, A 2 appears to be close to zero ( 10 4 A 2 ⋍ 0 ± 0.2 cm 3 g −2 mol ). PDMS is soluble on the molecular level only in the limit of dilute solution and seems to form aggregates as the concentration increases (c > 0.01 g cm −3 ). Typical hydrocarbon polymers are insoluble in CO 2 , but it has been found that polymerizations may be accomplished via the use of a stabilizer [3]. For example, styrene has been polymerized in CO 2 by means of a polystyrene-b-PFOA block copolymer surfactant, which forms micelles in CO 2 and is also amenable to SANS characterization. Other amphiphilic surfactant molecules that form micelles include PFOA-g-poly(ethylene oxide) (PFOA-g-PEO) graft copolymers, which swell as the CO 2 medium is saturated with water. These systems have been characterized by SANS, by taking advantage of the different contrast options afforded by substituting D 2 O for H 2 O. This paper illustrates the utility of SANS to measure molecular dimensions, thermodynamic variables, molecular weights, micelle structures etc. in supercritical CO 2 .

Neutron tomography for archaeological investigations

Within the last decade neutron tomography and radiography significantly gained importance. Especially its application in non-destructive testing for industrial components can be underlined. A good example is the automotive and aviation industry, where a high contrast for the used lubricants and adhesive materials is required. In contrast to X-rays, neutrons are able to penetrate thick layers of metals and provide on the other hand a high sensitivity to hydrogen containing materials. In recent years, a large number of applications in other fields like biology, medicine, geology and especially archaeology have been reported. Here the potential of neutron tomography for investigations on archaeological samples shall be outlined and some recent examples will be presented.

Dynamic heterogeneity in polymer electrolytes. Comparison between QENS data and MD simulations

Abstract We have investigated the dynamics of poly(ethylene oxide) (PEO) lithium-based salt electrolytes (PEO–LiBETI) using quasi-elastic neutron scattering (QENS). Measurements were carried out on the spectrometer NEAT (HMI, Berlin) above the melting temperature of PEO ( T m ≈65°C). The experimental data fully support the Molecular Dynamics (MD)-derived model of a heterogeneous dynamics in dilute PEO-salt electrolytes. In agreement with MD simulations carried out on PEO–LiPF 6 , we find evidences for the existence of two dynamic processes: (a) a faster process that is described in terms of the pure PEO dynamics and (b) a second component which we identify with the slower motion of the PEO chains involved in PEO–Li + complexes. At this stage, due to the differences in PEO molecular weight and the anion chemical nature, the agreement with MD simulations is qualitative.

Fractal approach in petrology: combining ultra small angle, small angle and intermediate angle neutron scattering

Ultra small angle neutron scattering (USANS) instruments have recently covered the gap between the size resolution available with conventional intermediate angle neutron scattering and small angle neutron scattering (SANS) instruments on one side and optical microscopy on the other side. New fields of investigations are now open and important areas of material science (ceramics, glass fibers, natural materials) and fundamental physics (phase transition, phase separation and critical phenomena) can be studied in bulk samples with an accuracy previously unobtainable owing to a combination of favourable features of the neutron-matter interaction: high penetrability of neutrons, even cold neutrons, ability to easily manipulate local scattering amplitudes by means of isotopic substitution methods, small absorption for most nuclei and hardly any radiation damage. In particular, neutrons see rocks as two-phase systems, and therefore the data analysis is enormously simplified. Rocks showing fractal behavior in over two decades of momentum transfer and seven orders of magnitude of intensity are examined and fractal parameters are extracted from the combined USANS, SANS and intermediate angle neutron scattering curves.

Small‐angle neutron scattering from H2O/D2O solutions of a sodium alkylbenzenesulfonate having a branched alkyl group

Small‐angle neutron scattering results from 0.055 M sodium p(1‐pentylheptyl)benzenesulfonate solutions in D2O–H2O solutions of 57% to 100% volume percent D2O are reported. Strong interaction peaks indicate interparticle ordering. Good fits to the results can be obtained taking account of hard‐sphere and electrostatic repulsion with the mean spherical approximation. The model used is a spherical micelle, comprising a hydrocarbon core of 11.9 A radius surrounded by a shell. The thickness of the shell derived from the results is 8.9 A, implying a rough micellar surface. The measured geometry and change are essentially independent of the contrast between the micelles and the solvent.

Depolarized Raman scattering in normal and supercooled antimony trichloride

Depolarized low frequency Raman spectra obtained for molten antimony trichloride are discussed. Measurement has also been performed on the supercooled liquid down to 53.2° below the melting point (tm = 73.4 °C. The experimental results indicate that the spectral contributions, in the range 2–100 cm−1 of Stokes shift from the exciting line are due to: (1) a central Lorentzian line, whose width and intensity are connected to the dynamics of the break‐up of intermolecular chlorine bridges; and (2) a solidlike contribution that furnishes an effective density of states, which corresponds to a convolution of ’’acoustical’’ and ’’optical’’ modes in a coupled linear chain structure with middle range order.