256 research outputs found
Inelastic Neutron Scattering Analysis with Time-Dependent Gaussian-Field Models
Converting neutron scattering data to real-space time-dependent structures
can only be achieved through suitable models, which is particularly challenging
for geometrically disordered structures. We address this problem by introducing
time-dependent clipped Gaussian field models. General expressions are derived
for all space- and time-correlation functions relevant to coherent inelastic
neutron scattering, for multiphase systems and arbitrary scattering contrasts.
Various dynamic models are introduced that enable one to add time-dependence to
any given spatial statistics, as captured e.g. by small-angle scattering. In a
first approach, the Gaussian field is decomposed into localised waves that are
allowed to fluctuate in time or to move, either ballistically or diffusively.
In a second approach, a dispersion relation is used to make the spectral
components of the field time-dependent. The various models lead to
qualitatively different dynamics, which can be discriminated by neutron
scattering. The methods of the paper are illustrated with oil/water
microemulsion studied by small-angle scattering and neutron spin-echo. All
available data - in both film and bulk contrasts, over the entire range of
and - are analyzed jointly with a single model. The analysis points to
static large-scale structure of the oil and water domains, while the interfaces
are subject to thermal fluctuations. The fluctuations have an amplitude around
6 nm and contribute to 30 % of the total interface area.Comment: The following article has been accepted by Journal of Chemical
Physics. After it is published, it will be found at
https://aip.scitation.org/journal/jcp
Tunable viscosity modification with diluted particles: When particles decrease the viscosity of complex fluids
While spherical particles are the most studied viscosity modifiers, they are
well known only to increase viscosities, in particular at low concentrations.
Extended studies and theories on non-spherical particles find a more
complicated behavior, but still a steady increase. Involving platelets in
combination with complex fluids displays an even more complex scenario that we
analyze experimentally and theoretically as a function of platelet diameter, to
find the underlying concepts. Using a broad toolbox of different techniques we
were able to decrease the viscosity of crude oils although solid particles were
added. This apparent contradiction could lead to a wider range of applications.Comment: 13+7 pages, 6+7 figure
Influence of Ibuprofen on Phospholipid Membranes
Basic understanding of biological membranes is of paramount importance as
these membranes comprise the very building blocks of life itself. Cells depend
in their function on a range of properties of the membrane, which are important
for the stability and function of the cell, information and nutrient transport,
waste disposal and finally the admission of drugs into the cell and also the
deflection of bacteria and viruses.
We have investigated the influence of ibuprofen on the structure and dynamics
of L-alpha-phosphatidylcholine (SoyPC) membranes by means of grazing incidence
small-angle neutron scattering (GISANS), neutron reflectometry and grazing
incidence neutron spin echo spectroscopy (GINSES). From the results of these
experiments we were able to determine that ibuprofen induces a two-step
structuring behavior in the SoyPC films, where the structure evolves from the
purely lamellar phase for pure SoyPC over a superposition of two hexagonal
phases to a purely hexago- nal phase at high concentrations. Additionally,
introduction of ibuprofen stiffens the membranes. This behavior may be
instrumental in explaining the toxic behavior of ibuprofen in long-term
application.Comment: -Improved indexing in Fig. 4e) -changed concentrations to mol%
-improved arguments, however conclusions stay unchange
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Multi-scale processes of beech wood disintegration and pretreatment with 1-ethyl-3-methylimidazolium acetate/water mixtures
Background: The valorization of biomass for chemicals and fuels requires efficient pretreatment. One effective strategy involves the pretreatment with ionic liquids which enables enzymatic saccharification of wood within a few hours under mild conditions. This pretreatment strategy is, however, limited by water and the ionic liquids are rather expensive. The scarce understanding of the involved effects, however, challenges the design of alternative pretreatment concepts. This work investigates the multi length-scale effects of pretreatment of wood in 1-ethyl-3-methylimidazolium acetate (EMIMAc) in mixtures with water using spectroscopy, X-ray and neutron scattering. Results: The structure of beech wood is disintegrated in EMIMAc/water mixtures with a water content up to 8.6 wt%. Above 10.7 wt%, the pretreated wood is not disintegrated, but still much better digested enzymatically compared to native wood. In both regimes, component analysis of the solid after pretreatment shows an extraction of few percent of lignin and hemicellulose. In concentrated EMIMAc, xylan is extracted more efficiently and lignin is defunctionalized. Corresponding to the disintegration at macroscopic scale, SANS and XRD show isotropy and a loss of crystallinity in the pretreated wood, but without distinct reflections of type II cellulose. Hence, the microfibril assembly is decrystallized into rather amorphous cellulose within the cell wall. Conclusions: The molecular and structural changes elucidate the processes of wood pretreatment in EMIMAc/water mixtures. In the aqueous regime with >10.7 wt% water in EMIMAc, xyloglucan and lignin moieties are extracted, which leads to coalescence of fibrillary cellulose structures. Dilute EMIMAc/water mixtures thus resemble established aqueous pretreatment concepts. In concentrated EMIMAc, the swelling due to decrystallinization of cellulose, dissolution of cross-linking xylan, and defunctionalization of lignin releases the mechanical stress to result in macroscopic disintegration of cells. The remaining cell wall constituents of lignin and hemicellulose, however, limit a recrystallization of the solvated cellulose. These pretreatment mechanisms are beyond common pretreatment concepts and pave the way for a formulation of mechanistic requirements of pretreatment with simpler pretreatment liquors. © 2016 Viell et al
Structure and Dynamics of the Central Lipid Pool and Proteins of the Bacterial Holo-Translocon
The bacterial Sec translocon, SecYEG, associates with accessory proteins YidC and the SecDF-YajC subcom-plex to form the bacterial holo-translocon (HTL). The HTL is a dynamic and flexible protein transport machine capable of coor-dinating protein secretion across the membrane and efficient lateral insertion of nascent membrane proteins. It has been hypothesized that a central lipid core facilitates the controlled passage of membrane proteins into the bilayer, ensuring the efficient formation of their native state. By performing small-angle neutron scattering on protein solubilized in ‘‘match-out’’ deuterated detergent, we have been able to interrogate a ‘‘naked’’ HTL complex, with the scattering contribution of the sur-rounding detergent micelle rendered invisible. Such an approach has allowed the confirmation of a lipid core within the HTL, which accommodates between 8 and 29 lipids. Coarse-grained molecular dynamics simulations of the HTL also demon-strate a dynamic, central pool of lipids. An opening at this lipid-rich region between YidC and the SecY lateral gate may provide an exit gateway for newly synthesized, correctly oriented, membrane protein helices, or even small bundles of helices, to emerge from the HTL
Mutually Beneficial Combination of Molecular Dynamics Computer Simulations and Scattering Experiments
We showcase the combination of experimental neutron scattering data and molecular dynamics (MD) simulations for exemplary phospholipid membrane systems. Neutron and X-ray reflectometry and small-angle scattering measurements are determined by the scattering length density profile in real space, but it is not usually possible to retrieve this profile unambiguously from the data alone. MD simulations predict these density profiles, but they require experimental control. Both issues can be addressed simultaneously by cross-validating scattering data and MD results. The strengths and weaknesses of each technique are discussed in detail with the aim of optimizing the opportunities provided by this combination
Neutron Reflectivity and Grazing Incidence Methods Applied to Microemulsions Adjacent to Planar Walls
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