W. Härtl

SIZE-CONTROLLED SYNTHESIS OF NANOCRYSTALLINE BATIO3 BY A SOL-GEL TYPE HYDROLYSIS IN MICROEMULSION-PROVIDED NANOREACTORS

Using the hydrolysis of appropriate alkoxide mixtures in water-in-oil microemulsions, nanocrystalline BaTiO 3 has been prepared in the form of nonaggregated, cube-shaped crystals at room temperature without any sintering process as is demonstrated by means of x-ray diffractograms and transmission electron micrographs. By variation of the length of the hydrophilic part of the surfactant molecules, the diameter of the water droplets in the microemulsions could be tuned to values between 8 and 55 nm as determined by dynamic light scattering. The size of the resulting nano-BaTiO 3 (6 nm ≤ 〈 d 〉 vol ≤ 17 nm) was evaluated from the line broadening of x-ray reflections and correlates to the droplet size. The particle size distribution is very narrow, and in some cases nearly monodisperse.

The glass transition of charged and hard sphere silica colloids

Dynamic and static light scattering is applied to concentrated suspensions of silica nanoparticles with surface functionalizations causing highly charged or hard sphere interaction potentials, respectively. The index of refraction of the dispersion medium was matched to that of the particles using a mixture of water/glycerol for the charged particles and toluene/ethanol for the hard spheres. The static structure factors correspond to the appropriate theoretical models, Percus–Yevick and rescaled mean spherical approximation. At volume fractions φ=0.18 a glass transition for the charged systems and at φ=0.53 for the hard spheres can be observed, as evident from the nondecaying components of the intermediate scattering functions. In the glassy state the experimental correlation functions agree with the predictions of the mode-coupling theory over several orders of magnitude in time. Using the fitted experimental structure factors as input for the mode-coupling theory we find good agreement between the theor...

Covalent Surface Functionalization and Self-Organization of Silica Nanoparticles

More than 10u2009000 carboxylic acid groups are covalently coated on monodisperse silica nanoparticles (see schematic diagram) prepared by a Stober synthesis. Colloidal super crystals form spontaneously on dissociation in aqueous dispersion with low ionic strength.

Determination of hydrodynamic properties in highly charged colloidal systems using static and dynamic light scattering

The structure and dynamics of highly charged colloidal silica systems has been investigated at low concentrations by means of light scattering. The index of refraction of the colloidal particles was matched using a mixture of water/glycerol. By simultaneously measuring the dynamic and static properties of the colloidal systems, we were able to extract the hydrodynamic function H(Q) at different concentrations. Our results are in excellent agreement with theoretical calculations over a large concentration range.

Size-controlled synthesis of nanoscaled aluminium spinels using heterobimetallic alkoxide precursors via water/oil microemulsions

Abstract Nanosized spinels of type MAl 2 O 4 (M = Mg, Co, Ni, Cu) were prepared by a sol-gel type hydrolysis of alkoxides in the inverse micelles of w/o microemulsions. Heterobimetallic alkoxides M[Al(OPr i ) 4 ] 2 containing both metallic elements in the desired stoichiometric ratio were employed as single-source precursors. The structure and thus metal stoichiometry in the molecules was established by single crystal X-ray diffraction analysis. By varying the hydrophilic chain length of the detergent, the diameter of the water droplets can be tuned in the nanometer range, as determined by dynamic light scattering. The size of the resulting spinel nano-particles as evaluated from XRD peak profile analysis, correlates to the droplet size. The results of the ceramic syntheses using the different types of alkoxide precursors were compared which reveal the advantage of a single source approach.

Self-diffusion and hydrodynamic interactions in highly charged colloids

Colloidal systems with a Yukawa interaction potential have been investigated by static and dynamic light scattering. By simultaneously measuring the dynamic and static properties of the colloidal systems we obtained the hydrodynamic function H (Q ) at different concentrations. Further the self-diffusion coefficient of highly ordered colloids was determined by tracer and FRAP measurements and compared with BD simulation and theoretical predictions.

Long time self-diffusion in suspensions of highly charged colloids: A comparison between pulsed field gradient NMR and Brownian dynamics

The long time self-diffusion of highly charged colloidal particles consisting of polybutylacrylate was studied employing pulsed field gradient NMR (PFGNMR) and Brownian dynamics (BD) simulations. The potential parameters for the Yukawa interaction potential used in the BD simulations were derived from the static structure factor measured by means of light scattering. The long time self-diffusion coefficients resulting from NMR and BD were compared for different ionic strengths, which cause a screening of the interaction potential due to stray ions. Increasing the ionic strength of the colloidal suspensions, we observed a phase transition from a crystalline to a liquidlike structure. Whereas at high salt concentrations both long time diffusion coefficients agree within a few percent, at intermediate ionic strengths the values measured via PFGNMR are by a factor of 2 higher than those predicted by the BD simulation. This deviation is explained by hydrodynamic interactions which are expected to enhance long ...

Structure and dynamics of electrostatically interacting magnetic nanoparticles in suspension

We investigate the structure and dynamics of charge-stabilized CoFe(2)O(4)-SiO(2) core-shell magnetic nanoparticles in suspensions. Small angle x-ray scattering and x-ray photon correlation spectroscopy allow us to analyze the intraparticle (core-shell) and interparticle structure of the suspension, as well as their dynamic and hydrodynamic behavior. Due to the weak magnetic interactions, the liquidlike structure is governed by screened Coulomb interactions. The hydrodynamic interactions of the measured systems are significantly stronger than predicted by current theories.

Dynamics in dense suspensions of charge-stabilized colloidal particles

Abstract.The dynamic behavior of charge-stabilized colloidal particles in suspension was studied by photon correlation spectroscopy with coherent X-rays (XPCS). The short-time diffusion coefficient, D(Q) , was measured for volume concentrations φ ⩽ 0.18 and compared to the free particle diffusion constant D0 and the static structure factor S(Q) . The data show that indirect, hydrodynamic interactions are relevant for the system and hydrodynamic functions were derived. The results are in striking contrast to the predictions of the PA (pairwise-additive approximation) model, but show features typical for a hard-sphere system. The observed mobility is however considerably smaller than the one of a respective hard-sphere system. The hydrodynamic functions can be modelled quantitatively if one allows for an increased effective viscosity relative to the hard-sphere case.

Self-diffusion of rodlike and spherical particles in a matrix of charged colloidal spheres: A comparison between fluorescence recovery after photobleaching and fluorescence correlation spectroscopy

The fluorescence recovery after photobleaching (FRAP) method and the fluorescence correlation spectroscopy (FCS) have been applied on suspensions of highly charged colloidal spheres with a small content of rod-shaped tobacco mosaic virus (TMV) particles. Since these methods only determine the self-diffusion coefficient of the fluorescently labeled species, D(S) of the rods and the spheres could independently be measured. The ionic strength of the dispersion medium has been varied to measure self-diffusion of rods and spheres in dependence on the degree of order of the matrix spheres. In contrast to FRAP, which allows the determination of the long-time self-diffusion coefficient D(S) (L), FCS measures self-diffusion on a shorter time scale. Thus a comparison of the results that were obtained by FCS and FRAP, in combination with Brownian Dynamics simulations, gives insight into the time dependence of the self-diffusion coefficient of an interacting colloidal system. As the mean interparticle distance of the matrix is of the same order of magnitude as the length of a TMV rod, the rotational motion is influenced by the assembly of spheres around a TMV particle. Since FCS is sensitive both to translational and rotational motion, whereas FRAP, which probes the diffusion at much larger length scales, is only sensitive to the translational motion of TMV, the comparison of diffusion coefficients measured employing FRAP and FCS can give some insights in the rotational diffusion: the experimental data indicate a slowing down of the rotational motion of a TMV rod with increasing structural order of the matrix spheres.