We describe the design and current performance of the backscattering silicon spectrometer (BASIS), a time-of-flight backscattering spectrometer built at the spallation neutron source (SNS) of the Oak Ridge National Laboratory (ORNL). BASIS is the first silicon-based backscattering spectrometer installed at a spallation neutron source. In addition to high intensity, it offers a high-energy resolution of about 3.5 μeV and a large and variable energy transfer range. These ensure an excellent overlap with the dynamic ranges accessible at other inelastic spectrometers at the SNS.

Motion of water molecules in Aerosol OT [sodium bis(2-ethylhexyl) sulfosuccinate, AOT] reverse micelles with water content w(0) ranging from 1 to 5 has been explored both experimentally through quasielastic neutron scattering (QENS) and with molecular dynamics (MD) simulations. The experiments were performed at the energy resolution of 85 microeV over the momentum transfer (Q) range of 0.36-2.53 A(-1) on samples in which the nonpolar phase (isooctane) and the AOT alkyl chains were deuterated, thereby suppressing their contribution to the QENS signal. QENS results were analyzed via a jump-diffusion/isotropic rotation model, which fits the results reasonably well despite the fact that confinement effects are not explicitly taken into account. This analysis indicates that in reverse micelles with low-water content (w(0)=1 and 2.5) translational diffusion rate is too slow to be detected, while for w(0)=5 the diffusion coefficient is much smaller than for bulk water. Rotational diffusion coefficients obtained from this analysis increase with w(0) and are smaller than for bulk water, but rotational mobility is less drastically reduced than translational mobility. Using the Faeder/Ladanyi model [J. Phys. Chem. B 104, 1033 (2000)] of reverse micelle interior, MD simulations were performed to calculate the self-intermediate scattering function F(S)(Q,t) for water hydrogens. Comparison of the time Fourier transform of this F(S)(Q,t) with the QENS dynamic structure factor S(Q,omega), shows good agreement between the model and experiment. Separate intermediate scattering functions F(S) (R)(Q,t) and F(S) (CM)(Q,t) were determined for rotational and translational motion. Consistent with the decoupling approximation used in the analysis of QENS data, the product of F(S) (R)(Q,t) and F(S) (CM)(Q,t) is a good approximation to the total F(S)(Q,t). We find that the decay of F(S) (CM)(Q,t) is nonexponential and our analysis of the MD data indicates that this behavior is due to lower water mobility close to the interface and to confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay. However, rotational mobility of O-H bond vectors in the interfacial region remains fairly high due to the lower density of water-water hydrogen bonds in the vicinity of the interface.

No abstract

A combined small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) study was conducted to investigate the structural characteristics of aqueous (D 2 O) solution of generation 7 and 8 (G7 & G8) polyamidoamine (PAMAM) dendrimer as a function of molecular protonation. A consequent change in the intramolecular dendrimer conformation was clearly quantified by a detailed data analysis separating intermolecular correlations from the intramolecular contribution. Our results unambiguously reveal both an increase in the molecular size and a continuous variation of the intramolecular density profile upon increasing molecular protonation. This observation is contrary to current understanding of high-generation polyelectrolyte dendrimers where steric crowding is supposed to stiffen the local motion of dendrimer segments hindering exploration of available intradendrimer free volume and thereby inhibiting electrostatic swelling. Our observation is relevant to the elucidation of the general microscopic picture of polyelectrolyte dendrimer structure, as well as the development of dendrimer-based packages based on the stimuli-responsive principle.SECTION Macromolecules, Soft Matter P olyamidoamine (PAMAM) dendrimers are man-made spherical-like polyelectrolyte macromolecules with a sophisticated hyperbranched organization. Their structural study is challenging from a theoretical standpoint because of the complexity arising from a single molecule having both polymeric and colloidal characteristics. 1 Moreover, when dissolved in acidified aqueous environments, the binding between free protons and the constituent amines of PAMAM dendrimer provides a molecular protonation tunable by simply adjusting the pH of the solution. The influence of this additional electrostatic interaction and the prospect of using this charge-stimulated conformational evolution to facilitate and enhance dendrimer applications as hosts for encapsulation of molecules for targeted drug carriers, gene therapy scaffolds, and water decontaminating agents have provided motivation for extensive studies. 1,2 Complemented by molecular dynamics (MD) simulation 3 and theoretical analysis within a framework provided by statistical mechanics, small-angle scattering techniques, including neutron (SANS) and X-ray (SAXS), prove to be an effective means to explore the structural evolution of polyelectrolyte dendrimers. 4 A quantitative SANS study of fourth-generation (G4) of PAMAM dendrimer dissolved in D 2 O solutions first revealed that, upon increasing the molecular protonation, the internal structural conformation evolved from a densecore configuration for the neutral state to a more uniform, outstretched distribution when fully charged. However, only a minor increase of the overall molecular size (less than 4% increase in the radius of gyration R G ) was observed, 5 in contradiction to the significant molecular swelling predicted by early computational studies. 3 With an improved force field, recent atomistic MD simulations carried out by Goddard and ...

The translational and rotational motions of water and dimethyl sulfoxide, [DMSO, (CH(3))(2)SO] have been investigated using quasi-elastic neutron scattering. Water-DMSO mixtures at five DMSO mole fractions, chi(DMSO), ranging from 0 to 0.75, were measured. Hydrogen-deuterium substitution was used to extract independently the water proton dynamics (d-DMSO-H(2)O), the DMSO methyl proton dynamics (h-DMSO-D(2)O) and to obtain background corrections (d-DMSO-D(2)O). The translational diffusion of water slows down significantly compared to bulk water at all chi(DMSO)>0. The rotational time constant for water exhibits a maximum at chi(DMSO)=0.33 that corresponds to the observed maximum of the viscosity of the mixture. Data for DMSO can be analyzed in terms of a relatively slow tumbling of the molecule about its center-of-mass in conjunction with random translational diffusion. The rotational time constant for this motion exhibits some dependence on chi(DMSO), while the translational diffusion constant shows no clear variation for chi(DMSO)>0. The results presented reinforce the idea that due to the stronger associative nature of DMSO, DMSO-water aggregates are formed over the whole composition range, disturbing the tetrahedral natural arrangement of the water molecules. As a consequence adding DMSO to water causes a drastic slowing down of the dynamics of the water molecule, and vice versa.

No abstract

No abstract

No abstract