Neutron spin echo spectroscopy with a moving sample.

Neutron spin echo spectroscopy is a high resolution inelastic neutron scattering method probing nanosecond dynamics. It is well suited to study the atomistic motion in polymer systems and contributes to our understanding of viscoelasticity. However, for samples under shear, or moving samples in general, Doppler scattering has to be considered. We compare the measured phase shift and depolarisation due to Doppler scattering from a rotating graphite disk to numerical and analytical calculations and find excellent agreement. This allows to take into account Doppler scattering during the data processing and makes longer Fourier times as well as higher shear rates and Q ranges possible with neutron spin echo spectroscopy, enabling for example the study of polymers under high shear.

Thickness of the particle-free layer near charged interfaces in suspensions of like-charged nanoparticles.

When a suspension of charged nanoparticles is in contact with a like-charged water-solid interface, next to this interface a particle-free layer is formed. The present study provides reliable measurements of the thickness of this particle-free layer with three different techniques, namely optical reflectivity, quartz crystal microbalance (QCM), and direct force measurements with atomic force microscopy (AFM). Suspensions of negatively charged nanoparticles of different size and type are investigated. When the measured layer thickness is normalized to the particle size, one finds that this normalized thickness shows universal inverse square root dependence on the particle volume fraction. This universal dependence can be also derived from Poisson-Boltzmann theory for highly asymmetric electrolytes, whereby one has to assume that the nanoparticles represent the multivalent coions.

Structuring of colloidal silica nanoparticle suspensions near water-silica interfaces probed by specular neutron reflectivity.

Structuring of aqueous suspensions of colloidal silica nanoparticles near an isolated planar silica-water interface is studied by specular neutron reflectivity. The reflectivity data clearly show that the suspensions develop a damped, oscillatory concentration profile in the normal direction to the interface. The wavelengths of these oscillations agree well with those independently determined by direct force measurements in the slit-geometry. The reflectivity data further demonstrate that the oscillatory structure persists over several layers and that the first particle layer is separated from the interface by a particle-free region.

Capillary condensation and gelling of microemulsions with clay additives.

The capillary condensation in bicontinuous microemulsions takes place when two parallel surfaces are narrowed that result in a completely lamellar microemulsion. We expected that this phase transition is also observable when the amount of hydrophilic surfaces from clay particles is raised, because hydrophilic surfaces induce lamellar ordering locally. Using small angle neutron scattering, the structure of microemulsions was observed as a function of clay content. The critical concentration is indicated by discontinuous structural changes and depends on the platelet diameter and is explained by the free energy of the platelets competing with the fluctuating medium. The gel phase transition is observed in the spectroscopic measurements where the diffusion motion is widely suppressed in the gel phase, but otherwise superimposes with the membrane undulations.

Nanoscale rheology at solid-complex fluid interfaces.

Here we present an approach to measure dynamic membrane properties of phospholipid membranes close to an interface. As an example we show results of the membrane dynamics of a phospholipid membrane multilayer-stack on a solid substrate (silicon). On this sample we were able to measure local interaction and friction parameters using Grazing Incidence Neutron Spin Echo Spectroscopy (GINSES), where an evanescent neutron wave probes the fluctuations close to a rigid interface. With this method it is possible to access length scales in the nano to micrometer region as well as energies in the µeV range. Using a new neutron resonator structure we achieved the required intensity gain for this experiment. During our investigations we found an excitation mode of the phospholipid membrane that has not been reported previously and only became visible using the new methodology. We speculate that the energy transported by that undulation can also serve to distribute energy over a larger area of the membrane, stabilizing it. This new methodology has the capability to probe the viscoelastic effects of biological membranes, becoming a new tool for tribology on the nanoscale and has allowed the observation of the hitherto invisible property of phospholipid membranes using neutrons.

Study of chromatographic characteristics of inorganic analytes in biopartitioning micellar chromatography with ion-pair additives.

The chromatographic behavior of model ions of biomedical and environmental significance was investigated by using nonionic micellar mobile phases modified with ion-pair additives. The influence of concentrations of polyoxyethylene (23) lauryl ether and ornithine hydrochloride in the mobile phase on the retention factors of chromium (III), chromium (VI), iodide and bromide ions was studied. The possible mechanisms of retention of the mentioned ions in biopartitioning micellar chromatography with zwitter ion-pair additives were proposed.