Birgit Fischer

Structure and short-time dynamics in concentrated suspensions of charged colloids

We report a comprehensive joint experimental-theoretical study of the equilibrium pair-structure and short-time diffusion in aqueous suspensions of highly charged poly-acrylate (PA) spheres in the colloidal fluid phase. Low-polydispersity PA sphere systems with two different hard-core radii, R(0) = 542 and 1117 Å, are explored over a wide range of concentrations and salinities using static and dynamic light scattering (DLS), small angle x-ray scattering, and x-ray photon correlation spectroscopy (XPCS). The measured static and dynamic scattering functions are analyzed using state-of-the-art theoretical methods. For all samples, the measured static structure factor, S(Q), is in good agreement with results by an analytical integral equation method for particles interacting by a repulsive screened Coulomb plus hard-core pair potential. In our DLS and XPCS measurements, we have determined the short-time diffusion function D(Q) = D(0) H(Q)∕S(Q), comprising the free diffusion coefficient D(0) and the hydrodynamic function H(Q). The latter is calculated analytically using a self-part corrected version of the δγ-scheme by Beenakker and Mazur which accounts approximately for many-body hydrodynamic interactions (HIs). Except for low-salinity systems at the highest investigated volume fraction φ ≈ 0.32, the theoretical predictions for H(Q) are in excellent agreement with the experimental data. In particular, the increase in the collective diffusion coefficient D(c) = D(Q → 0), and the decrease of the self-diffusion coefficient, D(s) = D(Q → ∞), with increasing φ is well described. In accord with the theoretical prediction, the peak value, H(Q(m)), of H(Q) relates to the nearest neighbor cage size ∼2π∕Q(m), for which concentration scaling relations are discussed. The peak values H(Q(m)) are globally bound from below by the corresponding neutral hard-spheres peak values, and from above by the limiting peak values for low-salinity charge-stabilized systems. HIs usually slow short-time diffusion on colloidal length scales, except for the cage diffusion coefficient, D(cge) = D(Q(m)), in dilute low-salinity systems where a speed up of the system dynamics and corresponding peak values of H(Q(m)) > 1 are observed experimentally and theoretically.

Highly charged inorganic-organic colloidal core-shell particles.

Highly defined, hybrid inorganic-organic colloidal core-shell particles consisting of a silica core and a shell of fluorinated acrylate are prepared in a two-step route. The core-shell structure of the particles is investigated by means of small-angle X-ray scattering (SAXS). Because of highly acidic sulfonic acid surface groups resulting from the radical initiator sodium peroxodisulfate at the organic shell, long-range electrostatic interactions lead to the formation of liquidlike mesostructures. Increasing the effective interaction by reducing the next-neighbor distances induces a freezing of the liquidlike structures, i.e., a transition to crystalline and glassy structures. Because of the high electron density in the core and the fluorinated polymer shell, these particles are strong X-ray scatterers. In combination with the large number of effective charges and the outstanding monodispersity, these core-shell particles are a promising model system for the investigation of the glass transition by photon correlation spectroscopy employing coherent X-rays.

Field induced anisotropy of charged magnetic colloids: A rescaled mean spherical approximation study

The liquidlike structure of colloidal suspensions with both electrostatic and magnetic interactions is investigated by means of small angle x-ray scattering (SAXS) dependent on an external magnetic field. For weak magnetic interactions, without external field, the magnetic dipoles are randomly oriented. Under this condition, isotropic structures are observed. In an external field, however, the magnetic momenta arrange parallel to the external field and induce anisotropic liquidlike structures. For weak magnetic interactions, the structure factor can be described within the framework of the rescaled mean spherical approximation. Due to the high experimental accuracy of synchrotron SAXS, from the anisotropic distortion of liquidlike structures, interparticle forces smaller than 10(-15) N can easily be detected.

Single shot speckle and coherence analysis of the hard X-ray free electron laser LCLS

The single shot based coherence properties of hard x-ray pulses from the Linac Coherent Light Source (LCLS) were measured by analyzing coherent diffraction patterns from nano-particles and gold nanopowder. The intensity histogram of the small angle x-ray scattering ring from nano-particles reveals the fully transversely coherent nature of the LCLS beam with a number of transverse mode 〈Ms〉 = 1.1. On the other hand, the speckle contrasts measured at a large wavevector yields information about the longitudinal coherence of the LCLS radiation after a silicon (111) monochromator. The quantitative agreement between our data and the simulation confirms a mean coherence time of 2.2 fs and a x-ray pulse duration of 29 fs. Finally the observed reduction of the speckle contrast generated by x-rays with pulse duration longer than 30 fs indicates ultrafast dynamics taking place at an atomic length scale prior to the permanent sample damage.

Optical and electrochemical cations recognition and release from N-azacrown carbazoles

Two azacrown derivatives of carbazole have been prepared and their dipole moments in the excited state were estimated from solvatochromism. Their complexation constant K with calcium and sodium cations have been determined from the absorption spectra, the fluorescence spectra and, independently, from the first oxidation potential shift, as a function of the concentration in sodium and calcium perchlorates, when K is lower than 105. The fluorescence spectra of the calcium and sodium complexes in acetonitrile show two bands different from the fluorescence of the free probes, one from the excited complex similar to the fluorescence of the protonated probes and the other one from the nitrogen decoordinated cation complexes. These results are typical for moderatly polar crowned merocyanines with the ionophore acting as an electron-donor substituent. The stepwise decoordination rate of the calcium and sodium cations from the nitrogen atom of the fluorophore is competitive with the rate constant of fluorescence which accounts for the multiple fluorescence but does not allow a distant diffusion of the cations. From the study of the longer lived radical-cation of the azacrown carbazoles, generated electrochemically, the metal cations are completely released from the azacrown receptor as revealed by the unsensitivity of the second oxidation potential to the perchlorate salts. From the probe-cation pair with the higher association constant (log K > 6) the oxidation peak for the complex allows to estimate the decrease of K upon the oxidation of the probe to be a value of K •+/K = 8 × 10-4.

Influence of Solvent on the Structure of an Amphiphilic Block Copolymer in Solution and in Formation of an Integral Asymmetric Membrane

Nanoporous membranes with tailored size pores and multifunctionality derived from self-assembled block copolymers attract growing interest in ultrafiltration. The influence of the structure of block copolymer in the membrane casting solution on the formation of integral asymmetric isoporous block copolymer membranes using the nonsolvent induced phase separation process (NIPS) has been one of the long-standing questions in this research area. In this work we studied the principal role of the solvent on the micellization and self-assembly of asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers by using a combination of dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). Our results indicate a significant effect of the solvent selectivity on the optimal casting concentration and solution structure. In addition, morphological characterization of the resulting membranes demonstrates considerable influence of the solvent system on the ordering and uniformity of the pores and pore characteristics in the separation layer as well as porous substructure of the final membranes.

Sequential single shot X-ray photon correlation spectroscopy at the SACLA free electron laser

Hard X-ray free electron lasers allow for the first time to access dynamics of condensed matter samples ranging from femtoseconds to several hundred seconds. In particular, the exceptional large transverse coherence of the X-ray pulses and the high time-averaged flux promises to reach time and length scales that have not been accessible up to now with storage ring based sources. However, due to the fluctuations originating from the stochastic nature of the self-amplified spontaneous emission (SASE) process the application of well established techniques such as X-ray photon correlation spectroscopy (XPCS) is challenging. Here we demonstrate a single-shot based sequential XPCS study on a colloidal suspension with a relaxation time comparable to the SACLA free-electron laser pulse repetition rate. High quality correlation functions could be extracted without any indications for sample damage. This opens the way for systematic sequential XPCS experiments at FEL sources.

Tuning the relaxation behaviour by changing the content of cobalt in CoxFe3?xO4 ferrofluids

The frequency-dependent magnetic relaxation of CoxFe3−xO4 nanoparticles stabilized by N-methyloleamidoacetic acid is investigated in three apolar suspending media (toluene, cyclohexane and decane). Hereby, the dependence of the crossover from a Brownian to a Neelian relaxation mechanism on the cobalt content is observed. The experimental results obtained from the complex magnetic susceptibility confirm the theoretically expected crossover between both relaxation mechanisms. The topological characterization is performed by means of x-ray diffraction, photon correlation spectroscopy and—in the Brownian regime—by magnetic relaxometry, allowing the comparison of crystallographic and hydrodynamic particle sizes.

Colloidal crystallite suspensions studied by high pressure small angle x-ray scattering

We report on high pressure small angle x-ray scattering on suspensions of colloidal crystallites in water. The crystallites made out of charge-stabilized poly-acrylate particles exhibit a complex pressure dependence which is based on the specific pressure properties of the suspending medium water. The dominant effect is a compression of the crystallites caused by the compression of the water. In addition, we find indications that also the electrostatic properties of the system, i.e. the particle charge and the dissociation of ions, might play a role for the pressure dependence of the samples. The data further suggest that crystallites in a metastable state induced by shear-induced melting can relax to a similar structural state upon the application of pressure and dilution with water. X-ray cross correlation analysis of the two-dimensional scattering patterns indicates a pressure-dependent increase of the orientational order of the crystallites correlated with growth of these in the suspension. This study underlines the potential of pressure as a very relevant parameter to understand colloidal crystallite systems in aqueous suspension.

Direction-dependent freezing of diamagnetic colloidal tracers suspended in paramagnetic ionic liquids.

The dynamic behavior of an inverse ferrofluid consisting of diamagnetic, spherical silica particles suspended in the paramagnetic ionic liquid (EMIm)2[Co(NCS)4] is investigated by means of x-ray photon correlation spectroscopy in the presence of an external magnetic field. Dipole-dipole interactions between the diamagnetic holes in the paramagnetic continuum of the suspending medium induce a direction-dependence of the diffusive motion of the colloidal particles: due to a magnetic repulsion perpendicular to the direction of an external field the diffusive motion of the colloidal particles is selectively frozen in this direction.