Modulating shape transition in surfactant stabilized reverse microemulsions.

The formation of reverse microemulsions (RMs) of spherical shape in the oil/water/surfactant ternary mixture at high molar ratio of water to surfactant (ω) is well established. Using dynamic light scattering, small-angle X-ray and neutron scattering, we elucidate the formation of non-spherical reverse microemulsions stabilised by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at ω = 10 and volume fractions of the dispersed phase, Φ, ranging from 0.005 to 0.20. In addition, we propose a strategy to tune the aspect ratio of non-spherical droplets and colloidal interactions by (i) varying the volume fraction of the dispersed phase (ii) changing the temperature, and (iii) by substituting the aliphatic oil with a mixture of aliphatic and aromatic hydrocarbons. This tunability of anisotropy along with a precise control of the interactions in the RMs, their ability to form spontaneously and their thermodynamic stability is crucial to provide a handle on reaction kinetics, synthesis of anisotropic nanoparticles as well as for their application as lubricants and viscosity modifiers.

Investigation of Nanostructure and Interactions in Water-in-Xylene Microemulsions Using Small-Angle X-ray and Neutron Scattering.

The ability to modulate the size, the nanostructure, and the macroscopic properties of water-in-oil microemulsions is useful for a variety of technological scenarios. To date, diverse structures of water-in-alkane microemulsions stabilized by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) have been extensively studied. Even though the decisive parameter which dictates the phase behavior of micremulsions is the nature of the continuous phase, relatively very few reports are available on the structure and interactions in the microemulsions of aromatic oil. Here, we present a fundamental investigation on water-in-xylene microemulsions using small-angle neutron scattering (SANS) at a fixed molar ratio (ω) of water to AOT. We elucidate the microstructural changes in the water-AOT-xylene ternary system at dilute volume fractions (Φ = 0.005, 0.01, 0.03), where the droplet-droplet interactions are absent, to moderately concentrated systems (Φ = 0.05, 0.10, 0.15, and 0.20), where colloidal interactions become important. We also characterize the reverse microemulsions (RMs) for thermally induced microstructural changes at six different temperatures from 20 to 50 °C. Depending on the magnitude of Φ, the scattering data is found to be well described by considering the RMs as a dispersion of droplets (with a Schulz polydispersity) which interact as sticky hard spheres. We show that while the droplet diameter remains almost constant with increase in the volume fraction, the attractive interactions become prominent, much like the trends observed for water-in-alkane microemulsions. With increase in temperature, the RMs showed a marginal decrease in the droplet size but no pronounced dependence on the interactions was observed with the overall structure remaining intact. The fundamental study on a model system presented in this work is key to understanding the phase behavior of multiple component microemulsions as well as their design for applications at higher temperatures, where the structure of most RMs breaks down.

Magnetic nanoprecipitates and interfacial spin disorder in zero-field-annealed Ni50Mn45In5 Heusler alloys as seen by magnetic small-angle neutron scattering.

Shell ferromagnetism is a new functional property of certain off-stoichiometric Ni-Mn-In Heusler alloys, with a potential application in non-volatile magnetic memories and recording media. One key challenge in this field remains the determination of the structural and magnetic properties of the nanoprecipitates that are the result of an annealing-induced segregation process. Thanks to its unique mesoscopic length scale sensitivity, magnetic small-angle neutron scattering appears to be a powerful technique to disclose the microstructure of such annealing-induced nanoprecipitates. In this study, the microstructure of a zero-field-annealed off-stoichiometric Ni50Mn45In5 Heusler alloy is investigated by unpolarized magnetic small-angle neutron scattering. The neutron data analysis reveals a significant spin-misalignment scattering, which is mainly related to the formation of annealing-induced ferromagnetic nanoprecipitates in an antiferromagnetic matrix. These particles represent a source of perturbation which, due to dipolar stray fields, gives rise to canted spin moments in the surroundings of the particle-matrix interface. The presence of anticorrelations in the computed magnetic correlation function reflects the spatial perturbation of the magnetization vector around the nanoprecipitates. The magnetic field dependence of the zero crossing and the minima of the magnetic correlation function are qualitatively explained using the law of approach to ferromagnetic saturation for inhomogeneous spin states. More specifically, at remanence, the nanoprecipitates act magnetically as one superdefect with a correlation length that lies outside the experimental q range, whereas near saturation the magnetization distribution follows each individual nanoprecipitate. Analysis of the neutron data yields an estimated size of 30 nm for the spin-canted region and a value of about 75 nm for the magnetic core of the individual nanoprecipitates.

Probing the stability and magnetic properties of magnetosome chains in freeze-dried magnetotactic bacteria.

Magnetospirillum gryphiswaldense biosynthesize high-quality magnetite nanoparticles, called magnetosomes, and arrange them into a chain that behaves like a magnetic compass. Here we perform magnetometry and polarized small-angle neutron scattering (SANS) experiments on a powder of freeze-dried and immobilized M. gryphiswaldense. We confirm that the individual magnetosomes are single-domain nanoparticles and that an alignment of the particle moments along the magnetic field direction occurs exclusively by an internal, coherent rotation. Our magnetometry results of the bacteria powder indicate an absence of dipolar interactions between the particle chains and a dominant uniaxial magnetic anisotropy. Finally, we can verify by SANS that the chain structure within the immobilized, freeze-dried bacteria is preserved also after application of large magnetic fields up to 1 T.

Using the singular value decomposition to extract 2D correlation functions from scattering patterns.

The truncated singular value decomposition (TSVD) is applied to extract the underlying 2D correlation functions from small-angle scattering patterns. The approach is tested by transforming the simulated data of ellipsoidal particles and it is shown that also in the case of anisotropic patterns (i.e. aligned ellipsoids) the derived correlation functions correspond to the theoretically predicted profiles. Furthermore, the TSVD is used to analyze the small-angle X-ray scattering patterns of colloidal dispersions of hematite spindles and magnetotactic bacteria in the presence of magnetic fields, to verify that this approach can be applied to extract model-free the scattering profiles of anisotropic scatterers from noisy data.